"Why is this engine so damn complicated??" Part 1: Sequential turbos demystified
 

Key words: vacuum, routing, simplified, sequential, boost, control, turbo

Introduction

We all know the flaws in the sequential turbo system. The biggest problems are that the components cannot stand up to the heat, and that the factory boost control system was designed for a completely stock car and needs a lot of tweaking to work right with mods.

But did Mazda sit down and say "ok, I want a sequential turbo system [that's a can of worms in itself]. How can I make it as unreliable of a clusterfuck as possible?" I doubt it. Every engineer must build a system to operate within certain tolerances (range of acceptable values) given their budget and level of technology. Mazda modified older designs from the second generation turbo cars and then put it all together. I can't defend the physical quality of the components. But I would like to present here Mazda's overall system design, first with a focus on the function of each actuator/solenoid and then a discussion of how the air moves in the vacuum routing system. I would appreciate any constructive criticism or corrections.

Mazda left us a host of clues as to why they designed particular aspects of the FD. We use as our sources the vacuum routing diagrams, the workshop manual, and the service highlights document.

Solenoids and Actuators

The sequential system consists of the following solenoids and actuators:

Precontrol
Turbo Control (2 solenoids, vacuum and boost)
Wastegate
Charge Control
Charge Relief


But why did Mazda feel they needed all that stuff for a sequential turbo system?

Design hurdles

Mazda had to overcome a series of problems/hurdles when it decide to design a sequential turbo system. Here are the two big overall issues:

1. How do we control the exhaust gases to the secondary turbo before it comes online?

For this, Mazda needed to bleed exhaust away from the primary turbo in a controlled manner in order to maintain a set boost pressure and spool up the secondary turbo--a function equivalent to a wastegate. That's what the "precontrol" system is for. They also need to make sure that both turbos get as much flow as possible as quickly as possible once it is time for both turbos to be online (after transition). That's what the "turbo control" system is for. The 87-88 turbo Rx-7's also had a type of turbo control valve, to open two different passageways in the turbine housing based on rpm. You can see from this old marketing literature for the 87 turbo models that there is nothing new about this type of design at all:

https://www.rx7club.com/attachment.p...1&d=1243179870

Mazda felt that the turbo control valve needed to open as quickly as possible. When you are moving a couch out of your hose, you usually have one guy pick up one end and the other guy pick up the other. One guy is applying a pulling force and one guy is applying a pushing force. That is exactly how the turbo control valve works. The guy pulling the couch is a vacuum force, and the guy pushing the couch is the pressure force. An engine does not produce vacuum when it is producing boost, so now we need a vacuum chamber to store that vacuum for use later. And if we want the level of pressure to be relatively predictable every time, we store that force in a carefully designed pressure chamber.

https://www.rx7club.com/attachment.p...1&d=1243180104

2. What do we do with the compressed air from the secondary turbo before the turbo comes fully online?

As the secondary turbo starts spinning up (but before it is ready to come online), the air it compresses can either go into the engine or not go into the engine (duh). Mazda chose to keep it out of the engine, perhaps because it minimized the amount of calculations that their 8-bit (8-bit like an NES with Duck Hunt and everything) ECU would have to perform. To keep secondary turbo's output from the engine, they need to:

1) make sure it does not combine with the air from the primary turbo. That's what the "charge control" valve is for. The charge control valve is a butterfly valve just like a throttle plate. They took the same type of throttle plate you have in your TB, modified it a bit, and put it in the Y pipe. That valve must not leak or crack open in any way, so Mazda needed to apply vacuum to it to keep it shut when the secondary turbo is not online. An engine does not produce vacuum when it is producing boost, so now we need a vacuum chamber to store that vacuum for use later. When it is time for the secondary turbo to come online, pressure is applied to the butterfly valve and it opens.

2) Since the air isn't going into the engine, it has to be vented back to the air cleaner. That's what the "charge relief" valve is for. The charge relief valve is basically just a blowoff valve. Blowoff valves open when vacuum is applied. An engine does not produce vacuum when it is producing boost, so now we need a vacuum chamber to store that vacuum for use later.

While the charge relief valve vents during the whole prespool process, the turbo control and charge control open at about the same time.

https://www.rx7club.com/attachment.p...1&d=1243182460

So at that point the engine sends more exhaust from the engine to the secondary turbo and the secondary turbo now combines its air with the primary turbo. Perhaps this sudden snapping-open effect of the two valves causes such a pressure drop during transition. Why don't they open gradually and progessively, which I believe is what the Mark IV Supra does with these functions? Well, if your computer was as powerful as the one that gave us such classics as Burger Time and Contra, that pressure drop would be... good enough.

Note that the RPM of transition from just the primary spinning to both turbos spinning varies with throttle position. This rpm value is actually adjustable in the PFC with a Datalogit. On deceleration, the rpm at which it returns to sequential operation (just the primary turbo active) is also adjustable with a datalogit. The PFC indicates that that RPM does not vary with throttle position, unless you program it to do so.

https://www.rx7club.com/attachment.p...1&d=1243187864

See where this is going? It's all very logical, albeit unreliable due to the quality of the components that were used.


Boost Control system

What is the point of these damn restricter pills?

The amount of air entering the wastegate actuator can vary based on a number of conditions--just like the amount of water flowing through a river can vary with the seasons. So what's one way we can predict how much water is going to flow through a given point on a river at any given time? We dam it up! Then we can build an entire hydraulic system around a predictable flow of water. The same principle applies to boost control systems. Restricter pills are used in lots of factory boost control systems, for example the WRX. Why spend money and time making an insanely complicated model of airflow (calculated on your Super Mario Bros. 8-bit ECU) through the actuator under every condition, when you can just put a half-cent little piece of plastic in there?

Now consider a tea pot. For purposes of our discussion, when pressure builds up, it whistles. Think of the wastegate and precontrol actuators as tea pots then. When the teapot whistles, the wastegate cracks open. So what if I didn't want it to whistle, or I wanted it to whistle at a time of my choosing? I'd open the lid and vent the pressure out in a controlled manner. That's exactly what the factory wastegate and precontrol solenoids do. With the restrictor pills in there it's a very simple and predictable design.

The precontrol and wastegate solenoids are simply passageways that open and close on command. All the ECU can do is tell it to turn on or off (by switching ground), usually in a rapid manner like flicking a light switch repeatedly. If we can predict how much air is coming in (restricter pills), then we will always know how often to open and shut the passage to get the amount of air we want. Think about flicking a light switch very rapidly but in a controlled manner so that it appears that the light is on almost continuously.

There's nothing new about this design. Mazda modified the boost control system of the series 5 turbo Rx-7. In that case, air was vented before it reached the actuator (which had one nipple on it instead of two) instead of the air being vented after it entered a two-port actuator. No restricter pills were used in the series 5 boost control system to my knowledge, but boost was only 8ish psi on that car.

Vacuum Routing

I have modified the simplified sequential diagram to illustrate how all the air flows. The charge control and charge relief solenoids switch back and forth from a barometric pressure (or boost pressure depending on engine condition) source to a vacuum source. By default, the charge control solenoid is applying vacuum (to keep the butterfly closed), and then switches to pressure when the ECU engages the solenoid (to open it up). By default, the charge relief solenoid is applying baro/boost and then applies vacuum when it is time to vent out air. The turbo control solenoids presumably switch their respective vacuum or pressure sources at about the same time based on the FSM pages I have posted here.

The wastegate and precontrol solenoids open and close rapidly (duty control) to vent air out of the wastegate and precontrol actuators. The less air is vented, the lower the boost will be (but no lower than the spring pressure allows):

https://www.rx7club.com/attachment.p...1&d=1243186262

All these solenoids are really the vacuum-routing equivalent of relays. Certain "passageways" in a relay open or close based on whether power and ground are applied to the switch (aka the coil) inside. The same principle applies to an air solenoid like those used in the FD sequential turbo system.

https://www.rx7club.com/attachment.p...1&d=1243185797


Conclusion

I'm not saying the sequential turbo system is reliable. But if I had to design one given their development budget and level of affordable computing power, I'm not sure I would have done it any differently, besides using components that could stand up to heat better over time.

Part two of this article will be a similar discussion of all the emissions control systems on the vehicle.

Appendix:

https://www.rx7club.com/attachment.p...1&d=1243187323
Another diagram of all the sequential actuators

https://www.rx7club.com/attachment.p...1&d=1243186414
Courtesy AVC-R manual

https://www.rx7club.com/attachment.p...1&d=1243186655
Series 5 Rx-7 turbo boost control system

PFC boost control behavior:

Boost settings:

Pr 1.00 boost 70% duty
Sc .90 boost 86% duty

https://www.rx7club.com/attachment.p...1&d=1242624130

Nice info, I especially like the analogies of the teapot whistling and two people pushing&pulling a couch to move it.

In the section on boost control:

There's a typo here. "more" should read "less" . higher wastegate/precontrol duty = more air vented = higher boost . lower duty = less air vented = lower boost

nice write up

Fixed.

I agree. While I believe the underhood temperatures and/or embrittling underhood components (same problem, just how you look at it) is a major mess-up by Mazda, a lot of people get upset that the stock boost control system is not adjustable. For a fixed-level system this is a very basic and sensible approach.

So, it seems like the primary and secondary PFC boost numbers do not actually control boost - the duty numbers do. (The secondary boost number does control fuel cut; the primary is ignored.)

Are there any rules of thumb when setting these duties?

For example, on a stock FD what PC and WG duty numbers are necessary to produce a 10-8-10 psi pattern?

On an FD with the standard bolt-ons (intake, ic, downpipe, exhaust etc.), how do these numbers change?

If I start with a pattern of 10-8-10 psi how do I get to, say, 12-10-12 psi?

Is a lot of experimentation required or is there a fairly direct relationship between duty and boost for a particular configuration?

Informative write up, thanks.

I will begin by saying that my analysis of the boost control system is based on 3 things:

1) a limited amount of empirical testing on an FD that I don't even own. All my logs are based on tuning a friend's car. I own a single turbo series 4 FC with the Banzai Racing PFC adapter kit. After the MAP sensor line blew off on my T2 I rebuilt the engine on it and I've been working steadily towards getting it on the road, a point which I will hopefully be reaching soon. So I can't do tests on a stockish FD and I can't do test. I can only occasionally do tests on this modded street ported FD with FMIC, 3" exhaust with no cat, a ported wastegate, and stock twins at 14psi (which will not hold to redline even with boost control duty maxed, probably due to mechanical reasons).

2) My working knowledge of control theory (don't ask me to do any calculus!). In any control system there must be a target or set point. The set point in the factory fuel feedback system is 14.7:1 (Lambda = 1), or .5v on a narrowband sensor. To say that boost is controlled by the set duty cycle of the solenoid is like saying that air/fuel ratio is controlled by the injector pulsewidth in a fuel map: it is based on a limited understanding of how feedback systems work. How would the ECU know whether the mixture is lean or rich if it didn't have .5V as the target value?

Continuing with the analogy, the set point in the fuel feedback system is .5V on the O2 sensor. The injector firing time/on-time/duty/pulsewidth, however you want to consider it, is first calculated by taking a base value (after temperature correction etc). Then the injector is fired. Then the mixture is measured as rich (greater than .5V) or lean (less than .5v. or did I get that reversed?). The amount the O2 sensor reading deviates from the target is then calculated (called an "error" calculation. Then an adjusted pulsewidth is calculated in an effort to reach the target.

Basic Injector pulsewidth --> resulting O2 sensor voltage compared to target voltage (error calculation)--> base pulsewidth is recalculated and then further corrected to get closer to target voltage --> o2 sensor voltage is compared to target again

This is a feedback system. For purposes of this discussion, the amount of correction could be calculated in two basic ways. There are basic logic statements: "IF mixture is too rich, THEN reduce fuel." This is called "Fuzzy logic" and is the control method employed by most aftermarket boost controllers, including the AVC-R and presumably the Power FC.

The second basic method is called PID (Proportional, Integral, Derivative) control. In the PID system of control the "Proportional" factor is akin to the "gain" setting on a lot of aftermarket boost controllers, which adjusts how responsive the self-correcton ability is. Too much "gain" and whatever is being controlled will overshoot the target, and too little gain will result in undershooting the target. I'd go into the mechanics of it further but it's not too relevant here, and it relies on calculus that's beyond me. Some variation of PID control is probably what's used in the factory fuel feedback system.

https://www.rx7club.com/attachment.p...1&d=1243321494

We know that Mazda employed a feedback system in the factory boost control logic:

https://www.rx7club.com/attachment.p...1&d=1243321494

It would be reasonable to assume that such a feedback system is employed by the Power FC. It would go something like this:

Basic boost control duty --> resulting manifold pressure compared to target pressure (error calculation)--> base duty recalculated and then further corrected to get closer to target pressure --> pressure is compared to target again


Look back at that boost control graph at the end of post #2. Why else would the duty cycle be varying? How would the Power FC know how to change the wastegate/precontrol duty cycle if it didn't know what to shoot for? From my testing, that target value appears to be the Secondary boost control value. Look at the blue line on the bottom of the graph (with yellow text I put in there). The measured peak boost before and after transition was about .90 kg/cm^2, 12.8psi according to the inaccurate PFC MAP sensor calibration and 14psi on an autometer boost gauge. Is it just coincidence that the boost value actually reached corresponds with the set secondary boost control value?

3) We can get some clues as to how the PFC boost control works based on reading through the documentation for the AVC-R, which also has a set boost value, a set duty value, a self-learning mode to correct the duty, and a gain setting to adjust the response of the self-correction logic (the PFC has no adjustable gain setting from what I can see).

https://www.rx7club.com/attachment.p...1&d=1243323190
https://www.rx7club.com/attachment.p...1&d=1243323190
https://www.rx7club.com/attachment.p...1&d=1243323190

I can only speculate here. All this academic discussion doesn't always translate into real-life results, especially as everyone's setup is so different and the MAP sensor calibration issue coupled with the somewhat undersized factory wastegate makes things more complicated.

A possible method for setting PFC boost control duty

1. Set both boost and duty cycle values the same. If we wanted 10psi, we would set it to say .65 kg/cm^2 or 9.25psi. By default, the setting is .70 (10psi), but the factory MAP sensor calibration reads lower than a mechanical gauge.

2. Adjust both duty values until the boost after transition reaches the target (or maybe it would hit the target value before and after transition if you're lucky).

3. If boost after transition is where you want it to be but boost before transition is not, adjust the "primary" duty value if boost is too high (subtract duty) or low (add duty) before transition. Keep the secondary boost the same. But that boost level after transition may change some anyway, in which case you would have to keep fiddling with it until it reaches a level that you can live with. There are good reasons why people throw in the towel and ditch the PFC for boost control and I'm not knocking that.

Example of tuning the PFC boost control:

Default settings:

Pr .80 56%
Sc .70 64%

Let's say we are overshooting some. So we'd try:

Pr .65 56%
Sc .65 56%

This might work (maybe it would take a couple pulls for the PFC to learn?), because the PFC can make adjustments as I've said. I've seen the PFC vary primary duty by 3-4% and secondary by 2-3% maybe.

But for the sake of example let's say the pattern is now 9-8-10 or something for whatever reason. So we need to increase the primary duty value some (Datalogit logs would help a lot here so you can see the whole duty curve). This is just an example I am pulling out of thin air, don't go putting these numbers in and expect a particular result! We could try:

Pr .65 64%
Sc .65 56%

Max acceptable duty value appears to be capped at 89%, at which point it will hit a ceiling and the PFC won't allow it go any higher. Maybe that would help. Maybe not. I don't even own an FD man, give me a break :dunno:

It would be the same basic tuning process. If you have the stock inaccurate MAP sensor calibration, you could try a target of say .75 kg/cm^2 (10.665 psi) or .80 g/cm^2 (11.376 psi). The factory ECU corrects the MAP sensor reading based on barometric pressure (altitude basically), the PFC does not. That's why I can't give people hard and fast values for anything, nor guarantee any results. Engineering is all about tolerances (acceptable range of values), and aftermarket ECU's (like most aftermarket components) do not have the tight tolerances of a factory component that was engineered as part of a complete system.

As far as how much the boost drops during transition, I'm not sure what to tell you about that. I'm not sure how you would adjust it, or if it is even possible due to the fact that the turbo control valve (which feeds exhaust to the secondary turbo) and the charge control valve (which feeds secondary turbo boost to the engine) snap open. You might able to adjust the "high" value under the turbo transition settings, but that may not really do anything. On my friend's car, it did 14-10-14, and for the life of me I couldn't make it 14-12-14, nor could I tell you exactly what determined how much the dropoff would be.

I hope I didn't lose too many people on that exhaustive discussion.

My understanding is that in the PFC, the duty % you set are the initial duty values only; the closed control system then adjusts the duty of the bleeding solenoid valves to meet the target. With higher initial duty you bleed more air at the beginning, i.e. higher/quicker boost.

- Sandro

Correct. From the logs I do have, once the actuators begin to open the boost control system will vary pre control duty ("primary" duty) by up to maybe +/- 4% and wastegate duty ("secondary" duty) by maybe 3%. Once the actuator opens, the PFC isn't going to change duty by 10 or 20% in either direction from what I can see, although I welcome any logs someone may have that indicates otherwise.

This is partially true. During the very beginning of spoolup, the wastegate and precontrol solenoids run at a high duty value that does not vary and cannot be adjusted. This bleeds off most/all pressure that would normally crack the actuators open, and effectively closes the respective flappers completely. The high fixed duty value continues until the PFC feels it is time to bleed off less air (drop to lower duty) first so that the precontrol may open. When the precontrol opens (duty drops) is probably determined in part by the set secondary boost level and a calculation of how fast boost has already been building so far.

The wastegate is held shut until right around the point of transition. This behavior is consistent with the factory boost control system:

https://www.rx7club.com/attachment.p...1&d=1243381431

Look at the flat PC and WG duty lines in the graph I posted at the end of the 2nd post, the part of the graph marked "WG shut." You will see that overall, the PFC boost control logic is mostly consistent with the way Mazda did it from the factory.

First, thank you very much for compiling your notes. I finally found the time to read them diligently and really appreciate your effort.

I went back to my logs and would like to ask your advice.

The attached log shows a typical start of an autox run. 1st-2nd gear shift, rpm ramp up, primary-secondary transition. As you can see, I have a very large drop in boost during transition (all parameters shown are scaled to multiples of 10 for easy reading).

Map monitor shows a drop from 0.74 Kg/cm2 to 0.42Kg/cm2. Questions:

1. Do you think such a large drop may be indicative of some problem in the control system?
2. What would you suggest for improving the transition? Note, in my application it only affects the start of the run, after that rpm usually stays above 3,000 and control system does not switch back to sequential.

Stock ports, stock turbos and (unported) wastegate, pills, DP, muffled MP, high flowing catback, Adam's intake, stock IC.

PFC boost settings are attached.

Note, my older logs show a relatively similar boost drop during transition even when the cat was still on.

Thanks,

Sandro

Great thread! Discussions like this is what this forum is all about.

Very nice. Now I want to chart mine . . .

Thanks,
:-) neil

i smell sticky!

Somehow related...As I was searching for info on how to improve transition I found this amazing old thread

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/weird-boost-issue-341534/page3/

Note solenoids may get stuck open if pressure above 10 psi

- Sandro

You need to post up a log, I'm hesitant to make any comment based on the graph you posted. I need to see the precontrol and wastegate duty cycles and I need to see them in their own separately scaled graph. Those are "advance" parameters but you have posted up a log of "basic" parameters, the ones that are viewable on the commander. Make sure that you are logging "advance" under the monitor window, but also check "sensors" but only "CCN" (charge control solenoid) and "TCN" (turbo control solenoid). Then we can see exactly when those solenoids are kicking in.

So under your monitor window, the following should be checked:

in the bottom right corner,

Advanced
Sensors

Under the sensors section,

Sensors --> TCN
Sensors --> CCN

and nothing else

duck hunt
 

Why you knockin' on 8-bit duck hunt? I loved that game :) j/k, thanks again for compiling this kind of info for us, again :D

I don't have historic advanced logs to post at this time. I will come back after logging the parameters you indicated.

Thanks,

Sandro

So how does Mazda's design compare to say, Toyota's twin turbo set up? The 1j/2j system seems to be reliable. Anyone know how they work, compared to ours?

^ there has been a thread or two comparing them...

They are very similar. I believe the difference is in the transition - the Supra has one less valve in the prespool.

I think it boils down to the heat in the RX-7 engine bay is much higher. It causes the hoses to harden and loosen, solenoids to fail, wiring to get brittle, and actuators to leak.

Dave

First off, EXCELLENT post. This is a grey area that a lot of people have. If you truly understand something, then you can really work with it.

I have a few points to make from my own experience.

The stock boost control. You have to remember, like Arghx said, the parameters Mazda engineers were working with. Back in the late '80s/early '90s, they hammered out this system based on a STOCK car. Precat, big muffler, restrictive airbox, all the good stuff. With a stock car, the boost control solenoids and pills work quite well. But, the factory ECU has a VERY limited set of parameters to work with and, as many blown motors can attest, can't deal well with mods.

When you change the flow characteristics of the motor by opening the intake and exhaust up, the factory boost control CANNOT compensate. Mind you, the sequential control system is doing just fine with the turbo control door and what have you, it's the wastegate control and turbo precontrol solenoids and pills we're talking about. When you open things up, you start getting creeps and spikes. The engineers didn't account for dramatic changes in flow.

Anyhow, this is a long roundabout way in saying the stock boost control is good for a STOCK or NEAR STOCK car. Once you modify the car, the system has to go. Would you put a single turbo and big fuel system on a car then pump that all through the stock intercooler? No. You have to realize the limitations of stock parts and know when they need to move on.

Even with the PowerFC being able to actually control the duty cycle of the stock solenoids, you are still limited by the pill design. You will soon have to drill out pills/swap pills out, and that ain't boost control, that's voodoo. You can pick up a used electronic boost controller very reasonably that will do the job better than the factory setup and keep your boost where it should be.

The object of boost control is in the name - to CONTROL BOOST. If you can't, time to do something about it.

Dale

^ I have used the PFC boost control on a friend's significantly modded sequential car with some success (14psi, street port, FMIC, ported wastegate). I presume he has no restrictor pills but I'm not sure as it's not my car and I didn't do the vac job. I think the jury is still out on this matter, although most people have understandably ditched factory boost control solenoids and not looked back. That's why I say that if you want to play around with factory/PFC boost control, do it, but if you don't have the nerves for it just go aftermarket.

One of my latest projects has been trying to figure out if it's possible to control boost on a single turbo car using a factory FD wastegate solenoid and sequential turbo control enabled. This is utter madness in most people's minds I'm sure. I did a bunch of testing tonight, and if I actually get anywhere I will let everybody know.

I am attaching the log of a recent run (National Tour in Seneca, NY last Sunday).
As suggested, I logged advanced and sensors.

The course had several turns at the start. Could not stretch the rpm at the start, so this run is not that indicative of the turbos transition pattern.

However, I have another question as I do not understand the following.

You have know that my last PIM row (19600) is set to stop boost creep by pulling the timing to zero deg. As you can see this works well for my needs - although for some reason the IGL does not drop to zero but sets at 4 deg - any clue on why?

My main question is the following: if you look at the overboost conditions, e.g. at around 350 sec, the WG duty is only around 65% (@160-170 range divided by 255, by the way, why 255?). Note that during the run, it can be as high as 88% (@225). Why? I would expect the PFC to bleed the maz amount of air when the pressure error is the highest. i.e. in those overboost conditions. I had set Pr and Sec boost targets both at 0.90 Kg/cm2 (approx 18900 PIM I believe), with initial duty of 62% and 76%, for Pr. and Sec., respectively.

I am attaching an excel spreadsheet were I have hidden non-relevant rows (a lot of time spent on idle) and columns, as well as the raw log (with non relevant information deleted).

Thanks for looking,

Sandro

Computers "think" in binary--On or off (0 or 1), technically +3V or ground on many chips. Everything is in powers of 2. 255 is actually out of 256 technically (255 + the value 0 ), or 2^8 power. 2^8 is 256. Some of the other settings in the PFC are out of 255/256, such as INJ vs accel TPS 1 in settings 2.

Remember that the more air bled from the actuator (higher duty value), the HIGHER the boost will be, and the opposite is also true: lower duty correlates with lower boost. Don't get the relationship confused. To some extent the PFC will drop duty in an overboost situation. Your target secondary duty value is 76%. The PFC dropped it to 65% (and in some cases 60% in this log) in an attempt to correct the overboost. One thing i've noticed lately is that in some cases the PFC will "hone in" on a boost value between .05 and .10 kg/cm^2 higher than what you set it at for reasons unknown to me. There are times when you need to set the target boost lower. It's not a perfect system unfortunately.

Also, any closed loop control logic will have what's called a "controller authority," or the amount the self-correcting logic is allowed to deviate from the set duty value given certain conditions. This "controller authority" is actually adjustable in the fuel correction closed-loop circuit of the Megasquirt standalones for example. We have no control over the PFC's controller authority for boost control. I usually see it deviate up to 5-7% or so from the set secondary duty value. The fact that in your case it is dropping 11-16% from the set value indicates that it is doing the best it can and it is probably functioning within its design parameters. If your restrictor pill is removed but your wastegate doesn't flow enough, you need to correct that mechanical aspect of the system, or if there is some other sequential problem you need to fix that.

Before the "high" transition point in the PFC, the wastegate is held at 95% duty (or 243/255). This is exactly what the FSM specifies for pin 4U, page F-162. Once the transition point has been reached and the turbo control actuator ("TCN" switch) is engaged, the max allowable duty appears to be 225/255 (89% or so) for whatever reason. If you set your WG duty (Sc duty) to 95% in the PFC it will still be capped at 89% once the TCN has been switched. I've seen this during dyno runs where a mechanical problem kept the PFC from holding boost up top. I tried to correct the boost drop with more boost control duty but it capped at 89% anyway.

Right after transition the PFC may keep the wastegate duty at 89% in order to improve spool, and then drop down when its control logic dictates that the wastegate should be opened. Remember that the lower the duty, the more the wastegate is opening and the less pressure is vented from the actuator (lower boost). The more pressure an internal wastegate actuator sees (less duty, less venting), the more it will stay closed. FYI, I tend to disregard the primary duty value. If it does have a purpose, I can't figure out how it is used yet.

I hope that helps some. I'm not claiming the PFC is perfect, nor do I claim to understand everything about it. I am still testing it in my custom single turbo boost control project which I have discussed in another thread ( https://www.rx7club.com/3rd-generation-specific-1993-2002-16/free-power-fc-single-turbo-probably-nonsequential-boost-control-846883/ for anyone interested ).

Thank you very much for your exhaustive explanation.

Clearly, I got confused regarding the WG solenoid duty cycle.
Now, it appears then that the "inability" of the PFC to deviate much from the "initial" duty cycle set (76% in my case for the secondary) is of no help in mitigating boost creep, but actually would make it worse.
I will try lowering the setting of the initial duty and see what happen under those conditions. I still have the pills, so maybe that even with lower duty at the bleeding valve, the WG would not open quickly on boost build up, we'll see it.

Now the question is, could an AVCR be more effective? I have one still in the box, which I bought after reading Dale Clark's notes a couple of years ago. I never installed it because the boost control system in the PFC appeared to be sufficient (although - as indicated in my previous post - I still have to investigate the reason for the large boost drop while transitioning from primary to parallel turbos operation). In case of overboost (like my boost creep), would the AVCR operate the feed solenoid valve at its max duty (AVCR solenoid valve in the feed line and bleed line plugged)? If that is the case, I would install it for this reason alone. If the WG could be forced full open by the control system, I might perhaps not experience boost creep at all, or would certainly mitigate it more effectively than with the PFC

Thanks,

Sandro

https://www.rx7club.com/attachment.p...1&d=1245422564

It can drastically deviate from the set value in some cases. Here is a log from my other thread of me controlling a single turbo with the PFC and a factory FD wastegate solenoid. Duty was set to 20%, far too low. The PFC made drastic corrections to slow the boost drop, but it can only do so much. If the duty value actually needs massive oscillations to keep the boost consistent, something in the system isn't right.

If your wastegate doesn't flow enough, your wastegate doesn't flow enough. Exclusively electronic solutions can't fix mechanical problems. Try this: remove the restrictor pill if you haven't already. The lines that go from the actuators to the precontrol and wastegate solenoids, cap off both ends of those. You should now be running spring pressure on both actuators and the wastegate is effectively "full open." Do a few WOT pulls and post the results. In my experience with internal wastegates (series 5 single turbo, internal wastegate), it will creep under higher gears even at just spring pressure. So put that bitch in 3rd and 4th on the highway and log the boost curve. Factory spring pressure is like 7psi right?

Boost creep is very common on the factory 2nd gen turbos. A boost controller cannot increase the flow rate of a wastegate. When people buy aftermarket boost controllers they are removing the restrictor pills and introducing new control logic to the system. But if the wastegate is undersized it's undersized. For example, I tried a lot of different things on my old 40mm HKS external wastegate, but it just didn't flow enough. The boost pattern was inconsistent with the weather, with or without a boost controller installed.

https://www.rx7club.com/attachment.p...1&d=1245512600

Here is an old log of me with that undersized wastegate setup. All I had was pressure going from the compressor housing directly to the side port of the wastegate to force it open as early as possible with no boost controllers involved. In purple I have drawn a line showing where the boost "should" reach, with maybe a few slight oscillations around that point. But because the wastegate was undersized, it opened completely but boost kept climbing, albeit slower.

The "inflection point" in the boost curve is where the boost climbs much more slowly due to the valve completely opening (second derivative is negative if you know anything about calculus). FYI velocity is the first derivative (derivative = rate of change) of an object's position at a particular point in time, and acceleration is the second derivative of an object's position--a measurement of how fast the velocity is changing. But I digress.

The area between our achieved peak boost value and our spring pressure is what I will call our "boost creep integral," the net amount of overboost over a given period of time. An integral is just calculus terminology for the area under a curve.

I hope that clarifies a bit the difference between improper boost control settings and insufficient wastegate flow. Always remember that boost control is a complete system, not just a discrete device.

However, my situation appears to be the reverse of what shown in the chart you posted, if i understand it right. Although the the boost pressure is well above the set pressure during boost creep, the bleed solenoid duty is low. My rationale/question is that, in addition to the "physical" constrains represented by the unported WG, any bleed duty present would certainly be of no help and likely make things worse. If the control system of the PFC were able keep the bleed line shut, all the the pressure in the WG actuator chamber would force the WG full open. If bleed duty is present (like my 65%), it may be that the WG would only be partially opened. That is what I meant when I hypotized that with a different control system (AVCR) I may not experience boost creep at all. Keep also in mind that my runs are in 2nd gear, so the load is lower than 4th or 5th, where normally boost creep may become unavoidable, as I understand.

That prompted my question if an AVCR (i.e. just a device) which utilizes a different control system (by controlling the feed, nor the bleed) could work better against boost creep.

Clearly, the test you suggested would prove whether physical constrain (too small WG) is present, which could not be solved by any control system. For me it is a little difficult to perform it because I only use my car for autox. I'll see if I can do it in some test and tune session one day.

Thanks again.

- Sandro

The diagram I posted was to illustrate that the PFC can make big adjustments, but it shouldn't have to. In most cases the results will likely be disappointing. One reason is because there is always a time delay when you make a drastic change to boost control solenoid duty. Remember that there are hoses and pressure waves involved, unlike fuel injectors (which are duty controlled) and which sprays right into the intake charge.

The second reason is this: if the correction logic is sensitive enough (on other EBC's, gain setting would be too high) in the system, there will be constant overcompensation and undesireable oscillations. It's the same reason why cruise control computers tend to undershoot the target speed when the car is climbing a hill. The factory engineers could have given the cruise control system more authority to respond more quickly (more gain) and apply more throttle (more controller authority), but there is too much of a risk of cyclical overcompensation (sinusoidal curve). There is nothing more unpleasant to ordinary Joe Driver than a perception of hunting or surging in any aspect of vehicle operation. But even performance oriented drivers don't like hunting either.

That's why engineers build systems that don't require too much second-by-second closed-loop correction. On modern cars they have layers of fuel correction logic and self-learning. There is the immediate correction to the fuel mixture (short term fuel trim). When that level of immediate correction is significant (usually over 5% richening up or leaning out), the ECU builds long term fuel trim. The long term correction tells the ECU to make corrections before it even starts the whole second-by-second adjustments. Subaru is using this type of logic on their ignition advance tables and boost control systems.

Again, closed loop systems are not designed for drastic changes. Why do you think these cars have restrictor pills installed from the factory? If the amount of pressure entering the wastegate is predictable within a tight range, drastic changes in solenoid duty will not be required.

From the AVC-R instruction manual:

https://www.rx7club.com/attachment.p...1&d=1245543855

Even the AVC-R is not designed for making massive changes.

I'll put up Sandro's log to illustrate a few things. He goes through transition one time and then stays non sequential for the rest of the run. First, take a look at this description of how the solenoids should operate from the factory:

https://www.rx7club.com/attachment.p...1&d=1245600017

Now here's an excerpt from Sandro's log. Sorry there's so much crap on the screen, I'm trying to get a lot of points across with one screenshot. I am assuming he is using default turbo transition settings.

https://www.rx7club.com/attachment.p...1&d=1245600017

symbols:

TCN ---> Turbo Control solenoid. A value of "1" means it is energized, "0" means it is not.

CCN ---> Charge control solenoid. A value of "1" means it is energized, "0" means it is not.

PIM ---> Manifold pressure value. 10000 is atmospheric pressure, 20000 is 1.0 kg/cm^2 boost, and any values in between

PC% ---> Precontrol solenoid duty. More duty = actuator more closed (higher boost)

WG% ---> Wastegate solenoid duty. More duty = actuator more closed (higher boost)

TPS V ---> Throttle position sensor voltage. Technically 100% throttle is somewhere between 4.2 and 4.6 volts

Raymond, my settings were slightly different, although they did not impact the explanation of the operational steps that you summarized so clearly. For your reference, relevant settings in the run were:

- turbo transition

TPS(%) Low High RPM
60 2800 3600
40 2800 3760
20 2800 5120

I am using 2800 Low to make sure the turbo operation remains non-sequential for the entire run (after the first transition at the start)


- Boost control

Boost(Kg/cm2) Duty
Primary 0.90 62%
Secondary 0.90 76%

Thanks,

Sandro

another log to illustrate turbo transition:

https://www.rx7club.com/attachment.p...1&d=1245787926

settings for that test run:

https://www.rx7club.com/attachment.p...1&d=1245787926

Raymond,

please take a look at these two charts.

The first chart "high duty" is with these settings
Primary 0.90 Kg/cm2 62% initial duty
Secondary 0.90 Kg/cm2 76% initial duty

The second carts "low duty" is with the same settings but the initial duty
Primary 30%
Secondary 30%

Note my map is scaled as follows
P18 18930 PIM
P19 19589
P20 19600

The P20 row is a safety row I use to stop boost creep by pulling the IGL to zero deg

As you can see, with the "high duty" setting, I overshoot boost setting and WG% stays high at 62.5% - which confirms your observation that the PFC is keeping the closed loop operation of the bleed solenoid withing a relatively close range of the initial duty setting. Therefore, under this conditions, given the high bleed duty, I suspect that the WG is just prevented from opening fully.

On the other hand, with the "low duty" setting, WG% is 27% (given the low initial 30% setting) and PIM is limited to 17445.

However, the problem with 30% is that the boost builds up very slowly and during the run never reaches the desired setting of 0.90 Kg/cm2 (approximately 18900 PIM). The max I got during the run was 17200 during the initial transition primary/secondary turbo with the WG "closed", i.e. WG=243 and PC=75

- Sandro

FYI, try viewing these logs with a bunch of separately scaled charts so you can see the numerical range of each trace better.

I'm not sure if you looked through my thread about using the sequential turbo control system to actually control a single turbo: https://www.rx7club.com/showthread.php?t=846883. In that setup the factory WG solenoid is controlling flow to an external wastegate diaphragm which pushes the gate shut. So more duty results in higher boost just as in the stock plumbing for a sequential setup.

I have also tested using low duty cycle numbers and have seen somewhat similar results:

https://www.rx7club.com/attachment.p...1&d=1245422564

But you see here that raising the duty up eventually resulting in a boost curve that is almost flat:

https://www.rx7club.com/attachment.p...1&d=1245422564

Since I made those logs I have still been playing around with the car and I hope to have more logs soon, where the boost doesn't really drop off at all. Assuming your system can mechanically control the boost well enough (plumbing and wastegate flow capability), you should try to start with low duty and gradually increase. Crank up both solenoid duty (primary and secondary duty values) working up from that 20% value, first in 10% increments and then maybe less than that as you get closer to an acceptable boost curve.

I'd ramp up both at first to a point, then adjust primary duty only until it's pretty good. Then adjust secondary, but you may have to jump back to the primary value again after that. Once you start getting close you will have a lot of fiddling back and forth among all the settings, including the boost value. You may have to increase or decrease the boost value by a small increment to get near where you want. The duty curve we see is a function of the target boost, the initial duty, and the feedback logic that we are grappling with right now which is based on the PFC observing the boost curve.

Since those logs I posted here, I now run .95 boost and 50% duty on my single turbo setup. I'm still fiddling with it some, but with a 13psi spring pressure (.91 kg/cm^2), I get right at .95 in first and then a value of about 1.05-1.09 in 2nd and 3rd, which is actually what I want anyway. I haven't logged my boost curve recently, I've just been looking at peak values in the Commander. The turbo I am running is a T04R (bigger than a GT35) which is going to be sensitive to the load differences in each gear.

This may seem odd... but here is an everyday application of a restricter pill:

https://www.rx7club.com/attachment.p...1&d=1249842017

Yes, that's a bottle of barbecue sauce. Now think about it for a second. When you squeeze a bottle of sauce with a restricter in it, it comes out in a much more predictable manner than just taking off the whole lid. There's only so much sauce that can come out of that orifice no matter what's in the bottle or how hard you squeeze. In the same way, restricter pills on OEM boost control systems allow the factory engineers to always have a pretty good idea of how much air is flowing into a wastegate actuator.

Raymond, I eventually was able to put the car on the road and do some logs last weekend.
I will post the results once I'll have the time to sort them out - which will take some time.
Pulls were mostly by sharply applying full throttle in 1st and 2nd gear, to simulate autox conditions. A few points after taking a first look at them:
1. Confirm that PC opens at 225 and stays there for a while and WG opens at 243 and stays there until transition
2. After opening at 225 PC duty undershoots then tapes off at the Primary setting duty while still within the 255 WP duty "envelop". It appears therefore that - at least for these kind of sharp transients - the PC duty rapidly approaches the Primary setting duty.
3. At the start of the transition, PC duty goes to zero and WG% goes to the "initial" duty value as set in Secondary duty
4. Not clear from my sharp and quick pulls how does the P and S boost settings play; anyhow duty goes up and down like trying to hunt a target (boost setting or % duty, or a combination?) while the boost builds up
5. Now, about my boost creeping: If I use low S duty setting I generally don't have any because the WG solenoid has the time to close enough to compensate - it appears there is an exception to this in the 8500-9000 rpm range but those are quite extreme conditions. On the other hand, as I had noticed already, if the S duty setting is high, the control system appears just incapable to open the WG as it can move only within a relative small % from the "initial" duty.
6. Another "extreme" case (but I only have one or two logs about this) is that if I pull sharply and full throttle in 3rd gear (higher load) I get overboosted because the WG duty is still in the 243 "initial" duty period (independently from the duty % setting).

In the end, by focusing on the 2nd gear pulls only, I seem to have found a workable compromise by setting the P and S duties at 67% and 73%. That takes me quickly to around 13 psi without hitting my safety row I set at 19600 PIM. But it appears though that if I wished to run - say - 14 or 15 psi the PFC control system could not handle it with my open exhaust (catless and N1 dual). And, also clearly, this has noting to do with my unported wastegate; it's just that the PFC does not open the WG completely and quickly enough.

Question: how to copy the PFC charts and post the full image as you do? In the past I made screenshots and pasted them onto Paint, saved as jpeg then attached to the message. Is there a more efficient way of doing it and how to make appear the chart images in full size within the message rather than just attachments?

Thanks,

Sandro

RYes, I too have confirmed that the Primary duty value is used to control boost before "Turbo Transition high" rpm is reached.

Yes, if it doesn't reach the exact secondary duty value it gets pretty close.

I have been doing some testing on this. I still can't figure out what the primary boost value does, except that it might have a minor effect on when the precontrol duty drops from 225 to near the target primary value. Some level of boost oscillation is inevitable; that is the nature of PID (proportional, integral, derivative) control or whatever variation Apex'i uses. It's more a matter of getting the system to operate within an acceptable tolerance than trying to eliminate fluctuation altogether.

If you post the logs I can offer further opinion/analysis.

This is a limitation in any closed loop system. There is only so much variation that the system can correct for before overshoot or instability occurs. On any boost controller, if the duty values are set too high some kind of overboost will likely occur. Factory boost control systems are always tuned so that the base duty cycle value without any kind of closed loop feedback would result in underboost. Then the control logic/coefficients raise the boost closer as close to the target as possible, but the sensitivity of that control is set such that the duty and observed boost will not reach an unstable state. Consider these factory boost control tables from a 2005 Subaru Legacy 2.5GT, which is basically an STi engine with a smaller turbo:

https://www.rx7club.com/attachment.p...1&d=1254781239

3 dimensional table of target boost (measured in bar) vs rpm and accelerator pedal position

3 dimensional table of base wastegate duty cycle (before closed loop correction) vs rpm and accelerator pedal position

All the Subaru engines use single port internal wastegate actuators with a two-way bleeder solenoid and a restrictor pill, functionally the same as the FD's. So here we have a lookup tables of what boost the ECU is trying to achieve, and the base duty value that will be used to get us there.

https://www.rx7club.com/attachment.p...1&d=1254781239

Here are several two dimensional tables which adjust the boost control duty. The first is air temperature correction in celcius vs the amount the duty cycle will change. The second is some kind of duty table vs rpm, to reduce the chance of overboosting or underboosting because of changes in rpm. The turbo dynamics (Fine Gain) is the proportional coefficient for the closed loop control. The turbo dynamics (Coarse Gain) is the integral coefficient. Derivative coefficients are usually set to 0 in most boost control setups.

https://www.rx7club.com/attachment.p...1&d=1254781960

This graph I modified off a thread in the Haltech forum indicates which PID coefficient predominantly affects which areas of the boost curve. These types of gains would be set in any other ECU's boost control system (Haltech, AEM, etc). The PFC picks these numbers for us, sacrificing adjustability for an easier learning curve.

One method to determine if there is a mechanical limitation to your boost control would be to cap off the line from the WG and PC actuators to their solenoids, then remove the restrictor pills. It should be able to hold spring pressure. I have not seen any logs of the creep/overboost you were experiencing so I will have to withhold judgment on that.

What I will say is this: At any given target boost level setting, the duty values appear to correspond to different ranges of peak boost. I am currently controlling a single turbo with the sequential boost control logic. A target boost of .95 with duty of 50 yields observed peak boost values of 1.07 +/- .03 . A target boost of 1.00 with duty of 48 yields observed peak boost of 1.15 +/- .02 . I need to take some more logs to observe the duty curve a little more. You may be able to try lowering your target secondary boost by .05 or .10 kg/cm^2 and raising the duty by some percentage.

http://download.cnet.com/Screen-Prin...-10135610.html

Screen printing utility, I think that's what I'm using. If you set it up right it will let you select an area of the screen to capture. Then after you attach it, preview the post. Right click and click either "copy image location" or "properties" and copy the location from that.


Thanks,

Sandro[/QUOTE]

Thanks Raymond. Here is a log.

This is from a combination that works:
P 0.95/65% S 0.95/73
2nd gear pull
throttle pedal controlled not to exceed 8,000 rpm
Note:
To start with, WG stays at 149 @ 95% for the entire period before transition
PC starts at 225 @ 88% duty for less than one sec, then undershoots a bit but rapidly recovers to reach the set Primary duty in less than one sec (not bad!)
PC closes at 168 @ 65.6% right on the money with the P set duty of 65%
after transition WG is 189 @ 73.8% right on the money with the "initial" S duty
Primary max boost before transition is 18597 or about 12.5+ psi
Boost after transition is 18500-19000 or about 13 psi
Note when I tape off the throttle (at 7 sec on) the WG duty tops 225 (one of those "fixed" numbers) or 88% duty, attempting to keep the boost up
https://www.rx7club.com/attachment.p...1&d=1254789358

- Sandro

attachment doesn't work

I must be retarded but this is the best I could do. When I copy "location" it doesn't let me paste it into the message...

https://www.rx7club.com/attachment.p...1&d=1254827156

Anyhow, the thumbnail of the image has loaded (I replaced the previous .pdf you could not open with a .jpeg). I am also attaching the source log.

Copied below are my previous comments with a minor correction on the WG value before transaction, 243 not 149 (typo)

This is from a combination that works:
P 0.95/65% S 0.95/73
2nd gear pull
throttle pedal controlled not to exceed 8,000 rpm
Note:
To start with, WG stays at 243 @ 95% for the entire period before transition
PC starts at 225 @ 88% duty for less than one sec, then undershoots a bit but rapidly recovers to reach the set Primary duty in less than one sec (not bad!)
PC closes at 168 @ 65.6% right on the money with the P set duty of 65%
after transition WG is 189 @ 73.8% right on the money with the "initial" S duty
Primary max boost before transition is 18597 or about 12.5+ psi
Boost after transition is 18500-19000 or about 13 psi
Note when I tape off the throttle (7 sec on) the WG duty tops 225 (one of those "fixed" numbers) or 88% duty, attempting to keep the boost up



Thanks

- Sandro

I took a look at the logs. That first dip in boost after transition is something that every sequential car has to deal with. It's a mechanical limitation of the system. The turbo control and charge control actuators were not optimized for people cranking up the boost, and there is no easy way to adjust them. They come on abruptly at the transition point (turbo transition high setting). I noticed that you were at part throttle in this log. I think your Turbo Transition High - 40% was at play. You could attempt to play with that setting a little bit to see if switching the charge control earlier or later affects the boost dip.

Based on this log I don't see anything major here that could be optimized. If you were using an aftermarket EBC and tuning it with an external boost gauge, a similar behavior would likely emerge after transition. There are two differences though:

1) an aftermarket EBC does not let you easily log duty cycle in a readable way. So you'd have no idea that the duty had been fluctuating in an effort to stabilize boost.

2) external boost gauges usually smooth the boost signal a little bit so that minor fluctuations are harder to notice. In contrast, logging an appropriately calibrated pressure sensor through the Datalogit shows you all the fluctuations. I also see that you have a pretty good sample rate here (around 25 samples/second). That's going to make all the minor fluctuations more visible.


Also, about the screenshots. In internet explorer:

https://www.rx7club.com/attachment.p...1&d=1254840286

here I clicked on the thumbnail (do it in the preview post screen). Then I went to properties and copied the link.

In firefox:

https://www.rx7club.com/attachment.p...1&d=1254840286

Here I opened the thumbnail into a new tab (the middle button/scroller thingie on my mouse does that automatically). Then I right clicked and used "copy image location."

Thanks Raymond. As in the premises, this combination looks like a good compromise - and I do not plan on increasing the boost any further right now, so the PFC boost control system seems OK for my current targets. With higher boost settings, i.e. higher secondary duties, I am faced with the "boost creep" issue I described earlier already - for example see please my previous post #25; the sec. setting on those runs were at 76%

I have been thinking more at the PFC approach of controlling the actuators by acting on the bleed side. What puzzles me is that to achieve a high rate of boost build up, the PFC "locks" the actuators "close" by bleeding air from the WG actuator - for the entire time before transition, and for a good portion of the time from the PC actuator as well. Incidentally, with this type of control, qualitatively it seems to me "better" keeping the pills (which acts like a "I" in your PID comparison). With no pills the pressure would build up faster on the PC and WG actuators and "push" the actuators to open. Anyhow, this is just from a qualitatively point of view. Maybe the "imposed" 95% or 88% initial duties are high enough to bleed the excess air even after the pills are removed, and the resulting pressure on the actuators would still be insufficient to overcome the actuator springs, don't know...

I was just curious though, what is your comparative opinion between PFC control (bleed) and "feeed" type control, like EBC or check valves, Hallman type? Based upon what I have been logging and learning, it seems to me that if I ever want to run at - say - 15 psi, an upgrade to a "feed" control type might be necessary.

Thanks.

- Sandro

Since I mentioned my previous post #25, here is an image of the chart. Sorry I still can't show the image. I followed religiously your detailed and clear instructions - with Firefox - but when I "Copy Image Location" onto the message are I only get the link transferred, not the image. The only way I seem able to do it is to import it into my Album and then copy and paste the link which gets then bracketed by [IMG]....[/IMG] too many passages...

https://www.rx7club.com/attachment.p...4&d=1254851120

In any event, please refer to the attached thumbnail image.
Run is in 2nd gear with Secondary set at 0.90 and 76% duty
Between 328.0 and 328.5 sec I get boosted to my safety row and the IGL gets pulled.
Max PIM is 20165 when I start easing on the throttle (starting with TPS V 4.248); my safety row 20 is set at 19600 and row 19 (normal IGL) is at 19589. WG is at 185 (before starting decreasing) after the safety row is hit. My theory is that although the boost increased beyond the .90 target, the duty on the WG bleed solenoid did not drop quickly or widely enough to compensate for it.
Nothing to do with my un-ported WG, just that the WG does not open enough because the bleed duty is too high.

Note that the run I posted earlier at 73% set duty shows no problem despite a higher sec. boost target of 0.95 and a higher rpm (8,000). Which shows how sensitive this control system is even to such small variations to the duty cycles set.

I have other runs I can post later with low duties, which show there are no mechanical limitations (a kind of equivalent to the procedure you suggested to cap off the bleed lines and check that the 7 psi spring rate holds), unless perhaps for very high rpm, beyond 8,000 (more later).

- Sandro

This seems a bit abrupt to me. Maybe you should give the boost a little more leeway to fluctuate. It's your call though. My overboost protection is the PFC overboost fuel cut (I didn't want to rescale P rows), and I have everything set up such that if boost rises about .05 kg/cm^2 more than I want, it will fuel cut. The PFC is supposed to cut fuel when boost is .25 kg/cm^2 more than the target boost value, but I think it's more like .22 . My boost is set to 1.00 kg/cm^2 and I actually run about 1.16 with my current duty settings. So if boost hits around 1.22 it will cut fuel just like the overrev cut.

Hit the insert image button above ^ and paste it into there, that'll populate the [IMG] tags. That's what I do. Or you can type them in manually.


Running a high duty cycle during spool is the proper approach regardless of plumbing, and that control strategy is utilized by all the OEM's. Look at the OEM Subaru target boost and duty cycle table again.

https://www.rx7club.com/attachment.p...1&d=1254781239

The wastegate is completely shut (max pressure bleed on the internal wastegate actuator, or it would be full pressure applied to top port of am external wastegate) until right before the target of .94 bar is reached. This is the same as the "START BOOST" function in the Greddy Profec Spec II, the "spring" function in the AEM Tru Boost, and the START DUTY function in the Apex'i AVC-R. The "I" portion of the PID control graph I posted (which I didn't make originally) is kind of an oversimplification of how those systems work. Both the proportional and integral coefficients work together with the base duty value tables. We just can't adjust them in the PFC.

I'm not completely sure what the answer is to this, but I suspect the restricter pill is usually needed for higher boost, although "higher" is a relative term and depends on the design of the rest of hte system. What I will do though is post several OEM boost control routing diagrams for you to look at.

https://www.rx7club.com/attachment.p...1&d=1254862363
Rx-7 S5 Turbo II, bleeder valve tee'd off without a restrictor pill

https://www.rx7club.com/attachment.p...1&d=1254862363
Factory STi/Evo boost control plumbing (top) and then a 3 way solenoid installed (bottom) with the NC port rerouted to the intake to eliminate noise.

https://www.rx7club.com/attachment.p...1&d=1254862363

Porsche 944 with factory 3-way interrupt type boost control solenoid. The solenoid interrupts the signal to a single port external wastegate. The single port external wastegate is controlled with a single diaphragm chamber just like an internal gate

https://www.rx7club.com/attachment.p...1&d=1254862363
Vr-4/Stealth . The restrictor pill is actually on the inlet to the bleeder solenoid to keep boost down. The pill was removed in the 94+ models which explains the increase in rated horsepower from 300 to 320.

https://www.rx7club.com/attachment.p...1&d=1254862363
Nissan S14 SR20DET motor, notice two restrictor pills ("orifice"). These combine the Subaru design (restrictor to wastegate inlet to raise boost) with the Mitsu design (restrictor to solenoid inlet to lower boost)

If it were me, I would experiment with running a 3-way solenoid on the wastegate and keep the bleed type on the precontrol as it seems to be doing a good enough job. Remember that the PFC supplies a duty signal and that's all it does. It can work with whatever plumbing you want. I do know that you will likely need to lower your duty values if you go with a 3-way solenoid. The easiest solenoid to use would be the 3-way solenoid that comes with the boost control kit:
http://www.2kracing.com/images/produ...xi499-x003.jpg
You can buy that separately from the kit here: http://www.2kracing.com/Product/321.html . To install that you would cap off the line that goes from the WG actuator to the WG solenoid. Then you'd run a pressure source to the NC port and run the remaining wastegate line to the COM port. The solenoid should plug right into the factory harness I believe.

Thank you very much for all this valuable information Raymond. I really appreciate it and will study into it.

Just to clarify some of my thoughts/questions/choices

On the protection row at 20. Note my target is to keep the boost within - not at - 19589 (row 19). That is why have been attempting to optimize the boost in 2nd gear just to avoid hitting row 20. My target is actually to move not too much around my row 18 which I set at 18930 (about 13 psi). with my scaling there is about one psi difference between row 18 (my - hopefully - operative row) and row 19 (my limit). My idea of row 20 is actually a safety row as suggested by Chuck, hopefully not to be ever hit under normal conditions. But clearly it remain to be seen if this will actually be possible with the PFC.

On the initial "locking" of the duty cycle. I understand clearly this is what needs to be done with a control system based on controlling the bleeding. My question is more about learning if the control system that control the feed, rather then the bleed, like the EBCs with solenoid valves in the feed line (and no bleed) or the spring/ball check valves may actually work better. Please keep in mind that I have zero experience on those. My car was stock until last year and it is only recently that I have been started playing with the PFC with regard to the boost control. So far I had been focusing on the map and I am OK with that now.

Thank you again for sharing your experience.

- Sandro

Feed vs bleed--the main difference is the required duty cycle to achieve a particular boost curve. The "control system" is the same: a basic duty value plus various corrections. Duty is high during spool, drops to flatten out the boost, then fluctuates to keep the boost within a stable range. That's in any type of boost control system, whether it is bleeding air from an internal wastegate actuator, blocking the pressure signal to an internal wastegate actuator, or feeding pressurized air to the top port of an external wastegate. For what it's worth, Subaru owners frequently ditch the bleed style solenoids when they want to run a bigger turbo. They install a 3-way type solenoid ("feed" solenoid) and plug it into the factory harness. Then they adjust the duty cycle and gain values in the factory ECU using reflasher software like I have already shown.

Plumbing (restrictor pill, actuator type, solenoid type, etc) and control logic (base duty curve, closed loop control) are two separate but related parts of the overall boost control system. That's why aftermarket EBC's all work with one solenoid in different plumbing configurations.

Here is another log - eureka! I was finally able to embed the image...

https://www.rx7club.com/attachment.p...8&d=1255488264

Primary 1.00 / 67%
Secondary 0.90 / 65%

from the top chart you can see 4 legs
- smooth full throttle
- throttle partial lift off
- short full throttle application
- throttle full lift off

Comments
1. primary / sec transition, PC% and WG% are very close to their 67% and 65% settings - as already noticed in my previous logs
2. primary boost quickly reaches and stays around PIM 19500, except for a few msec at around 126.3 sec when my safety row (19600) is very briefly hit (recognizable by the dip in the IGL)
3. at 127 sec, WG% momentary drops to 87 (34%)! Looks like some strong derivative gain comes to play, reacting to the short but steep increase in boost after the sec turbo gets on line
4. however, immediately after, WG% stabilizes at around 162 (63%), and does not react at all to the slow but continuous increase in boost; eventually the safety row is hit at about 19600 - even though the 0.90 Kg/cm2 set-point should be like 18900, I believe; so, while the WG% reacts quickly and effectively to rapid changes in boost, slow changes do not seem to cause any reaction in this phase, and the duty stays close to the set duty
5. it is only after time=128 sec that he WG% starts to slowly decreases, down to 128 (50%), but not quickly enough to stop the "boost creep", causing the PIM to exceed 20000.
6. but when the throttle is momentary lifted off at 128.5 sec then resumed, note how the WG% reacts quickly and effectively to counteract for the derivative of the boost changes

- Sandro

hate to burst your bubble, but I don't see any image. sometimes if you upload an attachment, then walk away from the computer for a while before submitting the post, the attachment and embedded image will disappear.

There must be a curse of some sort... because I am now in the thread and I can see the image of my post #48. I also tried and connected as a guest - without logging in - and still can see the image... Anyhow, I am re-uploading the image to this post and attach to it. Hope this works...