question about revlimiting

[Yves]

On MS2 I believe there are 2 ways of doing a revlimit : one is with ignition retard and the second is with a fuel cut.

I assume the fuel cut is the 'hardest' where it would be most noticable.

What exactly is the result of using a ignition retard as revlimiter, and how much is it noticable.

[grom_e30]

You can use ignition retard, to set the timing to a fixed angle or to progressively retard the timing to slow the engine when it reaches the soft limit, then use spark cut at the hard limit. or you can set the hard cut limit to stop the fuel, the spark cut is not advised for cars with cat's in the exhaust and can be fairly noisy, iv heard that the fuel cut can feel softer and is what most oem's use.

[nathanhardy]

What about turbo oem I'm hesitant to cut any fuel in boost? Am I wrong?

[grom_e30]

lots of ppl have brought that up before, lots of oem use fuel cut on turbo cars thats why you get a fuel cut defender for over boosting a stock car. if the fuel is cut you cant be running lean as there is nout to burn.

[dontz125]

I didn't want to chime in too soon with something that turned out to be wrong, but grom has a good point - the danger comes when you reduce the fuel to 70-90% of what is needed. Cut the fuel entirely, and there's nothing to detonate / burn the engine.

That said, how much fuel is in 'the puddle', and how many cycles does it take to drop below useful consideration?

[racingmini_mtl]

The puddle has always been something I wonder about with the fuel cut limiter. Not when you cut fuel because I think there isn't enough contribution from it to sustain combustion. However, when you go back to fueling, the puddle will be dry so it will need to be replenished and that will mean lean running for a while. So I wonder what happens if you're keeping the engine stuck on the fuel cutting limiter.

Also, is that something that EAE should be compensating for.

Jean

[nathanhardy]

What do you guy mean by puddle?

[grom_e30]

thats the fuel thats stuck to the walls of the intake tract.

[Rick Finsta]

Pistons can be consumed as fuel if there is enough primary fuel to initiate a combustion event. I think there is a low chance of it happening or otherwise the OEMs wouldn't do it. My Honda cuts fuel off at 8400rpm from the factory and I've never noticed frosty aluminum oxide bits on the spark plugs...

I do like the drop X out of Y scheme for softer limiting but I'm not sure it really makes much of a difference in the end. If the sparks are in empty chambers the only issue I see is poor ring seal causing issues due to the lack of combustion events in a forced induction car.

[Yves]

Looks like a fuel cut is the only way to do it.

[BigLou240sx]

Looks like a fuel cut is the only way to do it.

Spark cut is different than spark retard... if you choose spark cut, the spark is removed (not all of it, rotating ignition events that happens in the code) at the hard rev limit (similar to fuel cut)

spark retard affects the soft limit zone.

----
Speaking of rev limiters, in the manual (http://www.msextra.com/doc/pdf/Megasqui ... ce-3.3.pdf TS reference section 8.3.1):
"Soft Limit Zone / Hysteresis (RPM)
Sets an RPM deadband, soft limiters start this many RPM below the hard limit. If fuel cut is enabled, then the RPMs must fall by this much before fuel is re-enabled. A number of hundred RPM is suggested."
This is true for spark cut too right? IE: "If spark cut is enabled, then RPMs must fall by this much before full spark is re-enabled"
Would it make sense to add a similar statement for spark cut for clarity?

[Yves]

A question on the side of this : I want to be able to limit the amount of power the engine makes in the lower rpms during tech inspection. Car has a rather powerful engine, which might raise suspicion. I was thinking of using an rpm limiter at the rpms where these engines are normally limited (ca. 4500-5000 rpm) but that won't reduce power output at the lower rpm range.

Would retarding timing during the time of tech inspection help ? Normally the engine has about 19° at idle and a total of 36° which is all in at ca. 2500 rpms. If I retard the off idle timing to about 14° and only have 36° at 4000 or so rpm, how much would that reduce power output approximatly ?

Thanks

[suberimakuri]

Some people hold cars in Rev limiter when drifting or burnouts but you have to ask yourself why ? Same with hitting Rev limiter before shifting gear.

Yves , think you'll need to pull more than 5 degrees.
Why not use table switching and have a second spark table that has less timing or is additive with negative numbers. I think you'll need to pull down to life 20 degrees to make a serious difference but this will affect your afrs.

[Yves]

Some people hold cars in Rev limiter when drifting or burnouts but you have to ask yourself why ? Same with hitting Rev limiter before shifting gear.

Yves , think you'll need to pull more than 5 degrees.
Why not use table switching and have a second spark table that has less timing or is additive with negative numbers. I think you'll need to pull down to life 20 degrees to make a serious difference but this will affect your afrs.

Hm, 20° ? That's a lot yes, my curve starts with 19° at idle.

Idle however is the only time they test emissions, so above that the CO is not tested. Even then, mixing in some Ethanol seems to do wonders.

I do think that exhaust temps would go way up.

Dual tables ? Haven't thought about that.