The "perfect 10-8-10 boost pattern" is a myth!
12-15-10, 10:00 AM
#1
The "perfect 10-8-10 boost pattern" is a myth!
Ok let me preface by saying that we don't 100% understand every bit of the design of the sequential turbos' design. I have been studying it carefully because that kind of thing interests me. Recently I made a thread which uncovered new information about the way the primary and secondary turbos behave during the turbo transition. https://www.rx7club.com/3rd-generation-specific-1993-2002-16/mazda-fds-compressor-surge-factory-933909/ A while back I also put together a long explanation of the sequential turbo system which I now realize is in need of revision.
There are still a few FD's out there that are running properly and are barely modified. Some of these owners have posted on here worrying about their boost hitting 12psi in the mid rpm range. I have in the past told them not to worry about it and in light of this information I will continue to say so. Now, the idea that a healthy stock series 6 FD boosts with a 10-8-10 pattern has been around since before my time and I'm not entirely sure where it came from. I've been questioning this lately though based on documentation we have from Mazda.
According to Tashima et. al, "Sequential Twin Turbocharged Rotary Engine of the Latest Rx-7" 1994, a stock Series 6 REW hits nearly 12psi in Mazda's own testing. So here is a proper boost curve for a series 13B-REW according to Mazda engineers:
The solid line shows a boost pressure pattern on the production series 6 REW with sequential twin turbos. We don't know the exact test conditions, but in the graph the peak boost reaches or even exceeds 80 kPA (11.5psi). The hashed line shows the boost pattern on a prototype engine with a single turbo, probably a modification of the twin scroll/divided design used in Series 5 FC3S turbo engines. So for those of you wondering why they didn't use a single turbo (given the design/budget constraints on the engine), there's your answer.
This control logic allowed Mazda to achieve the following power and torque curve:
solid line is production series 6 REW with sequential twin turbos, hashed line is series 5 turbo engine with divided twin scroll single turbo
There are still a few FD's out there that are running properly and are barely modified. Some of these owners have posted on here worrying about their boost hitting 12psi in the mid rpm range. I have in the past told them not to worry about it and in light of this information I will continue to say so. Now, the idea that a healthy stock series 6 FD boosts with a 10-8-10 pattern has been around since before my time and I'm not entirely sure where it came from. I've been questioning this lately though based on documentation we have from Mazda.
According to Tashima et. al, "Sequential Twin Turbocharged Rotary Engine of the Latest Rx-7" 1994, a stock Series 6 REW hits nearly 12psi in Mazda's own testing. So here is a proper boost curve for a series 13B-REW according to Mazda engineers:
The solid line shows a boost pressure pattern on the production series 6 REW with sequential twin turbos. We don't know the exact test conditions, but in the graph the peak boost reaches or even exceeds 80 kPA (11.5psi). The hashed line shows the boost pattern on a prototype engine with a single turbo, probably a modification of the twin scroll/divided design used in Series 5 FC3S turbo engines. So for those of you wondering why they didn't use a single turbo (given the design/budget constraints on the engine), there's your answer.
This control logic allowed Mazda to achieve the following power and torque curve:
solid line is production series 6 REW with sequential twin turbos, hashed line is series 5 turbo engine with divided twin scroll single turbo
12-15-10, 10:01 AM
#2
Here's another major piece of evidence showing that 10-8-10 is not always the only "correct" boost pattern:
This is a chart from the workshop manual which among other things shows the overboost fuel cut logic. The chart could be interpreted in a number of ways, but one thing is indisputable--the engine is allowed to exceed 182.5 kPA absolute pressure (12psi). Now if your car boosts 10-8-10, that necessarily doesn't mean there's something wrong with it. I won't pretend to understand every little thing about the control logic for the sequential twins--it's more complex than anyone realized. We know that the control logic can vary with gear, throttle, and rpm:
But if your stock mildly modified engine hits 12psi and tapers downward, that doesn't automatically mean there's anything wrong with it either. It may be working just fine.
This is a chart from the workshop manual which among other things shows the overboost fuel cut logic. The chart could be interpreted in a number of ways, but one thing is indisputable--the engine is allowed to exceed 182.5 kPA absolute pressure (12psi). Now if your car boosts 10-8-10, that necessarily doesn't mean there's something wrong with it. I won't pretend to understand every little thing about the control logic for the sequential twins--it's more complex than anyone realized. We know that the control logic can vary with gear, throttle, and rpm:
But if your stock mildly modified engine hits 12psi and tapers downward, that doesn't automatically mean there's anything wrong with it either. It may be working just fine.
12-15-10, 01:46 PM
#6
2900 RPM - 10 psi
4500 RPM - 8 psi
4550 RPM -10 psi
*but* in a 100% stock car (and I mean 100% no DP or anything)
6500 RPM - 8 psi until redline
Now I'm working off memory from what I read and from my own setup (10 years ago) so don't hold me to the exact RPM.
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12-15-10, 03:43 PM
#10
Well the idea came from people adding a boost gauge in their perfectly working stock set up. With zero modifications the patter was/is basically:
2900 RPM - 10 psi
4500 RPM - 8 psi
4550 RPM -10 psi
*but* in a 100% stock car (and I mean 100% no DP or anything)
6500 RPM - 8 psi until redline
Now I'm working off memory from what I read and from my own setup (10 years ago) so don't hold me to the exact RPM.
2900 RPM - 10 psi
4500 RPM - 8 psi
4550 RPM -10 psi
*but* in a 100% stock car (and I mean 100% no DP or anything)
6500 RPM - 8 psi until redline
Now I'm working off memory from what I read and from my own setup (10 years ago) so don't hold me to the exact RPM.
12-15-10, 03:56 PM
#11
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Don't miss sequentials, and I drove the car bone stock for at least two years. It was pretty gutless below 3k rpm, even sequential. The rotary lives and breathes in 4k+ rpm land, and I'm ok with that. If you need low rpm power, buy a vette - and for heaven sake don't buy an S2000, they're gutless below 6k 
12-15-10, 04:20 PM
#12
and if that's how your car behaves, there probably isn't anything wrong with the way it's boosting. But if it hits 12psi and then tapers down it may be running fine too. For all we know the 10-8-10 pattern is how it was enginreered to run under certain conditions and the 12 psi tapering pattern is how it runs under other conditions.
ps my car hasn't had that boost pattern since like 2000
12-15-10, 08:47 PM
#13
I am one of those people who is always worried about my boost hitting 12. Most people were telling me not to worry about it, starting with arghx. Nice to know there is a bit of a safety net. I did have a perfect 10-8-10 before I put on the HKS dp. In my opinion the sequential system is perfect for the car and probably the only good option since the 3 rotor wasn't able to happen from the factory.
04-30-17, 10:34 AM
#14
Just an anecdotal update to this. My 50k mile stock 1995 (except K&N drop in) boosts to 12psi and tapers down to 8 at redline, with a dip in between when the secondary comes online. But it depends on the exact way you drive it. Peak boost and the secondary turbo staging varies by gear. This is with stock precat and catback too.
The stock ECU is more sophisticated than aftermarket controls.
The stock ECU is more sophisticated than aftermarket controls.
Last edited by arghx; 04-30-17 at 10:40 AM.
04-30-17, 12:49 PM
#15
Just an anecdotal update to this. My 50k mile stock 1995 (except K&N drop in) boosts to 12psi and tapers down to 8 at redline, with a dip in between when the secondary comes online. But it depends on the exact way you drive it. Peak boost and the secondary turbo staging varies by gear. This is with stock precat and catback too.
The stock ECU is more sophisticated than aftermarket controls.
The stock ECU is more sophisticated than aftermarket controls.
A PFC maybe but I'd be hard pressed to be convinced that the 8 bit ECUs can hold a candle to late 90s MoTeC x00 series ECUs et cetera. Given that there isn't a gear sensor I'd be more inclined to say that the plumbing decisions, orifice sizes and other physically engineered pieces are more sophisticated than we give them credit for.
04-30-17, 09:51 PM
#16
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A PFC maybe but I'd be hard pressed to be convinced that the 8 bit ECUs can hold a candle to late 90s MoTeC x00 series ECUs et cetera. Given that there isn't a gear sensor I'd be more inclined to say that the plumbing decisions, orifice sizes and other physically engineered pieces are more sophisticated than we give them credit for.
so the ECU knows if you're in gear, and if its 1st, 2nd or 5th. i guess if you're in gear, and its not any of those, it assumes 3rd or 4th.
05-01-17, 11:49 AM
#18
You can calculate gear position based on rpm and vehicle speed, both of which are inputs to the stock ECU. High rpm, low vehicle speed-->low gear . High rpm, high vehicle speed --> high gear. That's how aftermarket ECUs do it. In fact, if you have an Apex'i AVC-R external electronic boost controller you set up boost-by-gear by configuring the rpm & vehicle speed relationship to detect gear position.
Neutral switch is important for emissions. The Federal emissions testing procedure starts out in neutral, and then puts the vehicle in Drive or 1st gear after a certain amount of time elapses. That's why the accelerated warmup system only works when you are in neutral. It's not something regulators would let you get away with today.
Neutral switch is important for emissions. The Federal emissions testing procedure starts out in neutral, and then puts the vehicle in Drive or 1st gear after a certain amount of time elapses. That's why the accelerated warmup system only works when you are in neutral. It's not something regulators would let you get away with today.
Last edited by arghx; 05-01-17 at 11:53 AM.
05-01-17, 12:56 PM
#19
You can calculate gear position based on rpm and vehicle speed, both of which are inputs to the stock ECU. High rpm, low vehicle speed-->low gear . High rpm, high vehicle speed --> high gear. That's how aftermarket ECUs do it. In fact, if you have an Apex'i AVC-R external electronic boost controller you set up boost-by-gear by configuring the rpm & vehicle speed relationship to detect gear position.
Neutral switch is important for emissions. The Federal emissions testing procedure starts out in neutral, and then puts the vehicle in Drive or 1st gear after a certain amount of time elapses. That's why the accelerated warmup system only works when you are in neutral. It's not something regulators would let you get away with today.
Neutral switch is important for emissions. The Federal emissions testing procedure starts out in neutral, and then puts the vehicle in Drive or 1st gear after a certain amount of time elapses. That's why the accelerated warmup system only works when you are in neutral. It's not something regulators would let you get away with today.
I understand calculating gear based on vehicle speed v RPM.
So at this point you're just speculating what the ECU is capable/is in control of versus what is designed into the (boost control in this case) system. Remember this ECU was released at the tail end of 1992 and most likely in development 2-3 years prior to that. This is very old tech, and I don't believe they would have been able to reduce production costs to warrant that kind of sophistication. Manufacturers just weren't doing digitally controlled gear based boost control strategies back then.
Also please remember that I'm not hating on what Mazda did to reach their design goals. In point of fact I praise them for their ingenuity. I do however take a bit of issue when someone says that a factory ECU from realistically 1989/1990 is as sophisticated as (and this is important) high quality ECUs from the late 90's to today.
So I don't sound like a total ***, I will admit that if anybody can point me to hard data regarding this topic I'll gladly eat crow as many 'facts' offered up here are pretty much anecdotal, and I most certainly lump my own in that category as well.
05-01-17, 01:42 PM
#20
No worries. In the second post of this thread, I quoted page 8 of SAE paper #941030 , "Sequential Twin Turbocharged Rotary Engine of the Latest Rx-7." It is remotely possible that this paper doesn't accurately represent a mass production Rx-7, so we can't be 100% sure.
Here's the full quote from the paper, emphasis mine:
You can shoot me a PM about the paper if you'd like.
Well, we know the ECU itself in terms of processor and RAM etc are clearly way behind modern times. But it is possible that the logic (timers, maps, and other calculations) in the ECU had some unique features as a result of the R&D Mazda put into their flagship sports car at the time.
Here's the full quote from the paper, emphasis mine:
Turbocharger Operation Control System: Timing of
change-over from single to twin turbocharger operation is
controlled by electronic computer unit(ECU). The ECU
monitors gear position, throttle valve opening and engine
speed, and makes change-over just when the engine
output is the same between single and twin turbocharger
operation conditions. For example, during the drive at
first gear, engine speed rises rapidly and therefore turbo
control valve would fail to open at an optimal timing. To
resolve this problem, ECU is made to send a signal to
open the valve earlier during first gear drive than the
other gear drives.
change-over from single to twin turbocharger operation is
controlled by electronic computer unit(ECU). The ECU
monitors gear position, throttle valve opening and engine
speed, and makes change-over just when the engine
output is the same between single and twin turbocharger
operation conditions. For example, during the drive at
first gear, engine speed rises rapidly and therefore turbo
control valve would fail to open at an optimal timing. To
resolve this problem, ECU is made to send a signal to
open the valve earlier during first gear drive than the
other gear drives.
Also please remember that I'm not hating on what Mazda did to reach their design goals. In point of fact I praise them for their ingenuity. I do however take a bit of issue when someone says that a factory ECU from realistically 1989/1990 is as sophisticated as (and this is important) high quality ECUs from the late 90's to today.
Last edited by arghx; 05-01-17 at 01:52 PM.
05-01-17, 03:14 PM
#21
No worries. In the second post of this thread, I quoted page 8 of SAE paper #941030 , "Sequential Twin Turbocharged Rotary Engine of the Latest Rx-7." It is remotely possible that this paper doesn't accurately represent a mass production Rx-7, so we can't be 100% sure.
Here's the full quote from the paper, emphasis mine:
You can shoot me a PM about the paper if you'd like.
Well, we know the ECU itself in terms of processor and RAM etc are clearly way behind modern times. But it is possible that the logic (timers, maps, and other calculations) in the ECU had some unique features as a result of the R&D Mazda put into their flagship sports car at the time.
Here's the full quote from the paper, emphasis mine:
You can shoot me a PM about the paper if you'd like.
Well, we know the ECU itself in terms of processor and RAM etc are clearly way behind modern times. But it is possible that the logic (timers, maps, and other calculations) in the ECU had some unique features as a result of the R&D Mazda put into their flagship sports car at the time.
Fair enough. I stand corrected.
I think I focused on the sophistication comment and attributed the act of sequential transitions to both sequential control as well as a comprehensive boost control strategy.
I'll also PM you RE that paper.
05-01-17, 07:05 PM
#22
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05-01-17, 07:11 PM
#23
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Well, we know the ECU itself in terms of processor and RAM etc are clearly way behind modern times. But it is possible that the logic (timers, maps, and other calculations) in the ECU had some unique features as a result of the R&D Mazda put into their flagship sports car at the time.
some of the FC ecu logic is more sophisticated than the aftermarket, there are a few things, like the air control valve which have an rpm OR throttle strategy, and the aftermarket can only do one of those.
in the scheme of things its not a huge deal, and the faster processor speed really really helps.
05-01-17, 07:36 PM
#24
Sorry if I come off as a bit rabid but I think that MUCH of the FDs bad press can directly be contributed to a somewhat limited stock ECU and completely outdated aftermarket solutions like PFS boxes in the US and ECU reflashes in Japan as well as (I await the hate) the PFC/Datalogit in combination with other band-aid fixes.
05-01-17, 07:45 PM
#25
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Sorry if I come off as a bit rabid but I think that MUCH of the FDs bad press can directly be contributed to a somewhat limited stock ECU and completely outdated aftermarket solutions like PFS boxes in the US and ECU reflashes in Japan as well as (I await the hate) the PFC/Datalogit in combination with other band-aid fixes.