Thread: Rev Limiter Discussion

The subject of rev limiter operation came up in this thread by Coheed:
I blew IT! 56k don't do it!

The following are the relevant posts from Coheed's thread:





















I have some questions to hopefully continue the discussion:

1) How does the MSD 6AL "soft touch" work? Does it cut spark to one cylinder at a time? Does it alternate which cylinders are being cut?

2) Would a timing retard be better than a spark cut?

3) For the turbo folks, should anti-lag be incorporated into the launch control, and if so, how?

If we can understand exactly how a good rev limiter and launch control system should work, there's a very good chance it can be incorporated into TunerCode.

Thanks,
Dave

I held off saying this earlier but for whatever reason the way I have the AEM rev limiter set up (fuel cut at 7,500 and ignition cut at 7,550 if I remember correctly) at least feels much smoother than the OEM rev limiter. Calum let us know how the OEM rev limiter works, and you'd think by the description that it would be pretty "soft" but what ever AEM is doing feels miles better. I have no idea if it's "safer".

It's so subtle and smooth that I've actually spent plenty of time at the limiter WOT where as before I never would have done that.


Edit: I think the rpm hysteresis is why AEM's is so smooth. I believe it is set at (maybe I set it?) at 50 prm which means fuel is cut at 7,500 but then re-instated at 7,450 rpm. Rinse and repeat. Instead of OEM which must have a larger value for hysteresis. That would explain why it feels smooth. However, it wouldn't necessarily be safer regarding the combustion events. It might shock the value train less making value float due to chain slap less likely. *shrug*

you can change in the AEM software how hard the limiter is. at factory settings for the sr its set soft. turn it to 10 on the hard and it will shake your fillings out. i tested the soft-hard setting with a rev limit of 4500 rpms for my 2 step and stumbled on the same soft-hard setting for the top rev limit. i have my 2 set 200 rpms diff since on the soft setting it trys to keep a 300 rpm limit (IE 300 rpm bounce) i cant remember off hand if it cuts fuel or spark first. i am N/A atm so i don't know if it matters.

on a side not with my old turbo setup with a MSD 6al for a rev limiter i did throw a rocker once by hitting it at 7500 rpms. i don't think it hits as hard as the stock rev limit but its not as nice as the AEM's on full soft.

looking at the AEM base map calibrations for several Nissans (some turbo others N/A) it looks like they have the spark always being cut 200 rpms before the fuel.

You're not talking about AEM's ability to implement a true soft rev limit are you? We are still talking about the hard limit right? (I think it's time I go look up what a true soft limit is.)

Edit: The best I can find is that it is ignition retard versus ignition or fuel cut. (Link)
Others think a per cylinder fuel cut is also technically a soft rev limiter but Calum says otherwise. I'm leaning towards agreeing with Calum as well based on the SRT-4 guy's description.

If that's how they had it, I've certainly changed that. Like I said, mine cuts fuel first for sure, then maybe as much as 200 rpm later cuts spark and it's like a pillow (again not saying it's safe).

There are actually 2 rev limiters in the stock ECU (3 counting the stationary rev limiter, which is converted to a launch control rev limiter in TunerCode). One is a hard fuel cut rev limiter that cuts all 4 injectors at the limit with a stock hysteresis of 500rpm. The other is a soft cut rev limiter that cuts 1 injector at 150rpm below the limit, 2 injectors at 100rpm below, 3 at 50rpm below, then all four at the limit. The resolution for the limits and hysteresis is 12.5rpm.

Due to misunderstanding of these 2 rev limiters, most people disable the soft cut limit by setting it way high, and use the hard cut limiter by setting it to their rev limit, with the stock 500rpm hysteresis. Another problem with the stock hard cut limiter is that it's on a 1 second timer. ie: the hard cut rev limit has to be exceeded for more than 1 second before the fuel cut occurs.

TunerCode implements both of these limiters as a choice for the user, but documents them fully so there is no misunderstanding as to how they work. The limits and hysteresis can be edited with TunerPro. Also, the timer on the hard cut limiter is removed.

Dave

Good to know, and holy crap 500 rpm no wonder it sucks so much to hit. Although I guess it makes sense to go back down below the "soft" limit before letting fuel back.

Calum explained this a while back as well. Then I asked him if it was a "soft" limit and he said no (post #13).
http://www.sr20-forum.com/tuning/1430-stock-rev-limit-what.html#post19535

Such good info to have. That 500 rpm and 1 second part is killer!

i will start off by saying i don't know much about this topic but i am very interested in how this stuff actually works so please don't take my info as if its law or fact i am just saying what i am seeing or have felt.
in the EMS software there is a wizard for the 2step and the main rev limiter that has a soft threw hard setting (1-10).looking at the wizards info it has a 300 rpm hysteresis at 1 and a 100% cut at 10. so by AEM's terms the bigger the rpm hysteresis the softer the cut?
as for the 200 rpm difference from fuel to spark i am just curious as to what is the "Safe" way of limiting the rpms.

Where they take affect depends on where they are set. If both are set the same, the hard limit will never be reached. If the hard limit is set within 150rpm below the soft limit, the soft limit will start at 150rpm below, but the hard limit will take over when it is reached. The hysteresis for the hard cut limit is subtracted from the hard limit to determine when to re-enable the injectors and doesn't come into play until the hard limit has been reached and the fuel is cut.



I think some of the rev limit over-run expressed in that thread is due to the 1 second hard rev limit timer. I'm calling the soft rev limiter "soft" because it cuts only one injector at a time, rather than all injectors at once.



The hard limit hysteresis is 200rpm in the stock S13 SR20DET bin. (500 in the B13 SR20DE)

Dave

Good info, thanks for sharing.