Questions about MS3 soft rev limiter - Miata Turbo Forum - Boost cars, acquire cats.

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Old 11-15-2014, 05:48 PM   #1
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Default Questions about MS3 soft rev limiter

Because I have a Rotrex SC the old style of rev limiter is very hard on the SC. I see new options of soft limiters on the MS3. Do those allow me to keep my foot in it without the typical "bamp bamp bamp" rev limters? It would be great to find a way to softly hold it at redline.
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Old 11-15-2014, 06:56 PM   #2
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It softens it by pulling timing before it hits the fuel cut. Maybe some further options if you run spark cut, but I don't because of cat. With timing and then fuel, it is softer, but no, I cannot ride the limiter without on /off /on /off (which I do not do). There is adjustable deadband.

In for other answers that discuss the use of the spark cut and how it may differ.
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Old 11-15-2014, 07:05 PM   #3
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I think you pretty much summed it up. It progressively retards timing, but still won't "stop" the engine like the actual rev limiter will, just softens it. Be careful running too much, I can see some serious damage happening to the hot parts.
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Old 11-15-2014, 08:35 PM   #4
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There is also a progressive fuel cut option that is really smooth. I don't know much about it but it was really smooth when I played around with it. Can't find any documentation on msextra though.
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Old 11-15-2014, 10:58 PM   #5
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Quote:
Originally Posted by 18psi View Post
I think you pretty much summed it up. It progressively retards timing, but still won't "stop" the engine like the actual rev limiter will, just softens it. Be careful running too much, I can see some serious damage happening to the hot parts.
Good point. As I havn't yet tracked the car, I don't hold it in high RPMs. I'm only running through the gears. Will have to re-think it then. Sounds like OP is tracking or autocrossing though.

aidandj, I haven't seen that. Only for fuel cut and return on decel.
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Old 11-15-2014, 11:00 PM   #6
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Quote:
Originally Posted by DNMakinson View Post
Good point. As I havn't yet tracked the car, I don't hold it in high RPMs. I'm only running through the gears. Will have to re-think it then. Sounds like OP is tracking or autocrossing though.

aidandj, I haven't seen that. Only for fuel cut and return on decel.
Here's a screenshot of my revlimiter settings.

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Old 11-16-2014, 10:02 AM   #7
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I have my limiter setup like this on the AEM. In the last couple hundred rpms before the limiter is dumps boost back to wastegate pressure, and starts retarding timing. Then it starts pulling fuel and spark in scramble pattern (I hope, I'm not 100% on the pattern). Its nice for auto-x since it makes the limiter not abrupt, you kind of come up to it really fast and then the car just stops making power and if you sit on it for about a second then it starts pulsing but its not all that noticeable to the driver, you hear it though. I cut fuel and spark because I dont want it to pop on the limiter because sound regulations.
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Old 11-17-2014, 11:53 AM   #8
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Great info thanks guys!

My car is a dedicated track car. I was warned to stay off the "rev limiter" due to potential damage to the SC. Sounds like the soft limiter will go a long way to helping solve the abrupt style rev limiter.

It has not really been an issue on track but now I want to do some skid pad work and practice car control. That means drifting and potentially bouncing off the rev limiter. The tips from above will help.
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Old 11-17-2014, 12:18 PM   #9
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The soft limiter will make your exhaust really hot when it retards the timing. Just know that.
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Old 11-17-2014, 12:25 PM   #10
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Quote:
Originally Posted by sixshooter View Post
The soft limiter will make your exhaust really hot when it retards the timing. Just know that.
Got it. It's a E85 car so EGT's are normally pretty low. It can handle a bit more temp. I do need to keep that in mind though. Thanks!
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Old 11-17-2014, 05:02 PM   #11
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E85 will make it a non-issue.
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Old 11-17-2014, 07:00 PM   #12
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The progressive fuel cut Rev Limit is a 1.3.3 FW feature. I loaded that firmware and tried it today. I removed the progressive retard.

The fuel cut works so well at trimming power, that I lifted and shifted before the hard cut occurred. Perhaps tomorrow I will hold the pedal until hard cut does occur just to see how it feels. The soft cut seems pretty good. There is a loud farting out the exhaust, though. I suppose it it due to the break in pulses. I don't know how good this is on the Turbo and exhaust system, but it does give an auditory feedback along with the feeling of power drop.

It is rough enough to bring in my knock control, so in the end, there is some retard occurring. Well, I actually was seeing some knock retard just before the 7K initiation of progressive fuel cut.

I think I'm going to like this system. My hard cut is at 7200, with 200 hysteresis, that also sets the onset of the soft cut at 7000.
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Old 11-17-2014, 09:59 PM   #13
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What about detonation. Isn't slowly cutting fuel just asking for super lean conditions?
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Old 11-17-2014, 10:07 PM   #14
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Why dont you just use a hysterisis on 5rpm or something. That way it just stops dead on the limiter and holdws within 5rpm.

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Old 11-17-2014, 10:11 PM   #15
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Quote:
Originally Posted by aidandj View Post
What about detonation. Isn't slowly cutting fuel just asking for super lean conditions?
Though I have not seen documentation on this feature, it is fuel cut, meaning that on a given cycle / cylinder, either normal level of fuel is injected, or no fuel is injected.

Thus, it on a per cylinder basis, it is either running normally, or simply not firing.

This ignores cross-talk, and pooling on walls, but realistically, this is only occurring at high RPM, so that should be no issue. I do understand that is not rotating, so it is cutting one, two, three cylinders (hard would be the forth?), but is not going from cylinder to cylinder... again, if I understand.

I think I will put this question to the MS forum and see what the authors say.

This should only be used with sequential injection and is not to be used with throttle bodies or staged injection, for the reasons you just mentioned.

However, I did say I saw some knock indicted by the sensor. Not sure if it is really knock or just the un-even firing.

Perhaps I will run both retard and the progressive fuel cut and see if I get the same knock indication.

You do make a good point. Just because the FW writers put a feature in, does not necessarily mean it is a good idea to use it.

TLDR: Perhaps, but I don't think so.
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Old 11-21-2014, 02:53 PM   #16
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Quote:
Originally Posted by DNMakinson View Post

The fuel cut works so well at trimming power, that I lifted and shifted before the hard cut occurred. Perhaps tomorrow I will hold the pedal until hard cut does occur just to see how it feels. The soft cut seems pretty good.
this has been my experience with it so far. its much softer than i thought it would be, sometimes the only way i noticed is the rev limit lamp i rigged up comes on. i think it works really well as long as you dont just sit on it.
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Old 11-21-2014, 05:22 PM   #17
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To answer the question about individual cylinders going lean or not, we should get Reverant to run this. He has O2 Sensors on each tube of his headers, so he can see individual exhausts.
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Old 11-23-2014, 01:03 PM   #18
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MS2 -- the soft cut for fuel works like this:

It seems to drop 1 cylinder at a time in the amount of range you give it.

Let's say the hysteresis is 1000 rpm and hard is 7300.

So at 6300RPM it drops cly 1, then at 6500 it drops cly 3, then at 7000 it drops cly 4, then at 7300 (hard) it drops the last: cly 2.

this was with AND without progressive fuel cut. I feel like that's only how it's supossed to work with it ON.
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Old 11-23-2014, 01:58 PM   #19
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On the MS3, there is a total difference between ON and OFF, so they must have the code correct this time. The progression is like you said, starts at the bottom of the hysteresis and progresses up, but only when turned ON. I don't know the order, the effect is as you describe.

*EDIT* Or are you saying that the soft cut effect is the same whether by spark or fuel?

Last edited by DNMakinson; 11-23-2014 at 02:14 PM.
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Old 05-12-2017, 02:33 AM   #20
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This thread came up in a search I was doing re: fuel and spark cut for limiter.

I don't want to use spark cut because I have a cat. I'm currently running the progressive timing retard function as well as the fuel cut with progressive fuel cut enabled. I was just wondering if this would cause lean conditions under boost. From what I've gathered so far, it appears that it just cuts out fuel to one cylinder at a time over the course of the limiter so it doesn't cause a lean condition and just causes a 'no combustion' condition.

Is this accurate?
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