mkd

iTrader: (8)

I've spent a good bit of time reading about boost control, and while I know a lot more than I did a week ago, still have some big holes in my understanding.

Here is what I THINK I know:

Boost is controlled by two solenoids with corresponding vacuum lines: 1. wastegate control, 2. turbo precontrol

Wastegate control manages maximum boost pressure, Precontrol manages the spin-up of the secondary turbo. Lower pressure in the lines means more boost/more spin-up. If you raise the pressure in the precontrol, you will get more transition lag but minimize/remove any spikes, and vise-versa for lowering the pressure.

Using manual boost controllers, one would generally attach two -- one for wastegate control and one for precontrol, and gradually adjust until peak desired boost is achieved and max spool up without a spike at transition.

Am I correct so far?

Where I get confused is when electric boost controllers come into the picture. Mainly, even though some of them have fancy features that appear to manipulate precontrol, they all seem to come with ONE solenoid. How is it possible to control maximum boost and precontrol with only one solenoid?

In regards specifically to using the Power FC for boost control: does the Power FC actuate the control solenoids directory through the factory harness? If so, why is there a Power FC "kit" that comes with a new solenoid? What's so great about the new solenoid compared to the original(s)?

Also, I assume all of these EBCs come with a pressure sensor to read back the manifold pressure in real-time as they operate?

DaleClark

iTrader: (55)

You generally have it correct.

While it seems like it wouldn't work, T'ing the precontrol/wastegate lines together to run an aftermarket boost controller does work. I've had my car set up like this for 10-15 years. Amazing boost response.

The PowerFC does control the factory solenoids right out of the box. Big problem here is the factory solenoids were tuned to work with the restrictor pills in the lines. I messed with that some time back, you end up having to mess around with pill sizes as well as playing with the boost settings. That's just a mess. There are people using aftermarket solenoids (MAC valve) with some luck, though.

Yes, EBC's do have a MAP sensor as a feedback loop. Most boost controllers will also display boost on the screen as well.

Boost control is complicated and confusing in a regular setup, when you start talking about it in reference to the twins it's a real head-scratcher.

Dale

arghx

iTrader: (3)

read these threads

https://www.rx7club.com/3rd-generati...tified-841821/

https://www.rx7club.com/3rd-generati...-chart-952767/

arghx

iTrader: (3)

damn I just looked at the date I made those threads and they were years ago, which would make it less likely to show up in a search I think.

But FD's haven't changed since then, so go ahead and read them.

gracer7-rx7

iTrader: (16)

Some of the most informative threads on the forum

mkd

iTrader: (8)

Thank you for the reading material. arghx, that first one ("Why is this engine so damn complicated??") is definitely going to take a few passes...

Off topic... I read that as long as you can keep stock boost levels, you're safe with whatever mods you desire without tuning. Is that true? e.g. y-pipe, DP, high-flow cat, cat-back + stock ecu or PFC + 10-8-10 boost pattern = safe without extra fuel. I ask because I'm lumping some bolt-on installations together at once but won't be visiting the tuner for a month or two.

DaleClark

iTrader: (55)

Anytime you open up the intake and exhaust you're increasing flow and can increase boost. My car when it was near-stock with just a downpipe and cat-back (stock intake, stock cat-back) would spike on transition.

You can unplug the factory boost control solenoids which will bring boost down as well - the car may not be that fast, but it will be safe(r).

Or just drive the car with the mods and keep your foot out of it until you get it tuned and set up.

Dale

mkd

iTrader: (8)

Thanks Dale -- I understand that opening intake/exhaust up will cause spiking/increase boost. I'm saying, IF you can keep boost at or under 10psi via whatever sort of boost control mechanism, will the factory ECU add enough fuel to be safe (again, still only at 10psi), or would the mixture need to be manually richened up?

DaleClark

iTrader: (55)

Rule of thumb is if you can keep boost at 10psi with no spiking over that you're OK. But, every car is different and you may or may not be able to control the boost. You could have to port the waste gates to get good boost control.

But, regardless, modifying a car with a stock ECU is just not a good idea. Yes, there are ways you can get away with it but a modified car needs an aftermarket ECU, period. That's the whole reason the FD got a bad reputation for blowing motors in the 90's.

Dale

mkd

iTrader: (8)

Okay, I was looking for some validation on that "keep it at 10" rule of thumb. The plan is to limit boost to 10psi (or try to, anyway -- can always put factory cat back in if necessary) using PFC+MAC and richen up mixture in higher RPM ranges via PIM voltage until I can get the old lady some time with the tuner. Thanks again for your input.

scotty305

iTrader: (2)

The 'three mod rule' might have been common practice 10-20 years ago, but wideband gauges have gotten cheaper and IMHO there's no need to guess whether a car is running safe or lean AFR's in 2019. The fuel system and wiring harnesses on these cars are older and less likely to be in good shape, I would spend the time & money installing a wideband gauge and a boost gauge before doing anything to increase airflow in the intake or exhaust.

The stock ECU has a narrowband O2 sensor, it can't measure accurately enough to know if the engine is running safe or lean in boost. It can't even guess well like a 2JZ Supra that had a mass airflow sensor and could directly measure if the engine was breathing better and then add fuel to compensate. The stock RX-7 ECU can only measure boost and assume how well the engine breathes, plus there is some safety margin that was built in the original tune. For instance, my car with a downpipe (stock cat, stock muffler, stock airbox) was around 12psi with the stock ECU. My memory is fuzzy but I think AFR's were between 10.0-11.0 AFR with the stock ECU at 12psi, that was in 2007 or 2008. If your intake & exhaust let the engine breathe 10% better than my setup you might see 10% leaner (approximately 11.0-12.0 AFR). If your setup breathes 20% better you might see between 20% leaner (approx 12.0-13.0 AFR), assuming the OEM fuel system can keep up with the demand.

Spoiler alert, a completely stock 25-year-old fuel system will probably not be able to keep up with 20% increased demand. I have a weird combination of parts on my RX7, mostly stock in terms of airflow and power but lots of electronics including standalone ECU and wideband and fuel pressure sensors. I've got datalogs showing the OEM fuel pump (with stock wiring) is barely moving enough fuel to maintain pressure at high RPM with pretty tame power levels, around 230-250hp at the wheels. This is 10psi with stock twin turbos, stock airbox, stock muffler (aftermarket cat & downpipe), and the fuel pressure isn't keeping up above 5000 RPM. Next on my to-do list is upgrade the wiring for the OEM fuel pump, then upgrade the pump if needed. For now I'm keeping the boost below 10psi, ECU is already compensating by increasing the injector duty when fuel pressure goes away, and the ECU's lean protection is configured to kick in if the engine runs leaner than I like to see.

TomU

iTrader: (11)

This is a good point. More air means more fuel if you want to maintain your target AFR. For bolt-ons, you don't need much. A Supra pump (direct replacement) should be more than adequate. Check your grounds to ensure good voltage. Some direct wire the pump, but this may be overkill (but a direct ground close to the pump doesn't hurt). The stock wiring runs through the ignition switch. There's a mod to bypass the switch which will give the pump an extra volt or two. Also, the fuel pump connection at the fuel hat has a tendency to melt with more pump output. This should be rewired from the fuel hat to the pump.

As for boost control, porting the wastegate is highly recommended if you add a bunch of mods (you can't control what you can't vent)

mkd

iTrader: (8)

That sounds like good advice -- I also came to the conclusion that investing in a wideband and datalogic would probably be a wise use of money -- if not only for safety but also learning potential.

I have read a bit about the fuel pressure drop and various re-wiring techniques. I actually have a Rotary Performance fuel pump that I'm glad you reminded me to drop in BEFORE doing the cat/ypipe/pfc. I'm reluctant to do any rewiring at this point, though, as I've heard that a high flow pump combined with improved wiring will overwhelm the stock fuel pressure regulator... Since that presumably means it will make the car run rich with factory ECU installed, it also means it's one more thing I have to take out and put back in every two years for CA smog...