Fuel Cut Rev-limiter - Safe?

[TrumpetRhapsody]

Alright so I spent the better part of last night searching the internet looking for personal accounts of the legitimacy of using fuel cut only as a rev limiter. Obviously I bring this up for us EDIS guys that don't have the option of spark cut. If a dedicated topic about this already exists I apologize, but I spent over an hour searching hybrid alone, so if there are topics here then the search function is useless.

 

The obvious concern everyone spouts is going lean at high revs, detonating, and blowing up your motor. The main argument against this is the fact that once fuel is removed completely, there is not enough combustive power (even if it pre-ignites) to cause a significant detonation event. The counter argument being that yes, there is enough residual fuel to det and cause damage.

 

Does anyone have any first hand knowledge on this, stories of someone hitting a fuel cut and blowing up their motor, or the opposite case where someone has used fuel cut in a drift car bouncing off rev limiter for months with no ill effects? Just trying to get some facts and weed out the baseless opinions.

Edited February 24, 2011 by TrumpetRhapsody

[cygnusx1]

I have had the same question in my mind for about a year now. Glad you asked and hope someone with more experience will chime in!

[BLOZ UP]

All that I've worked with that relies on coil output for tachometer use, uses only fuel cut.

 

If your injectors aren't leaking, you won't blow your motor up from a lean condition. No fuel isn't a lean condition, it's a no combustion condition.

 

If you are so worried, turn on Spark and Fuel cut (might be an MS-Extra setting only, though).

 

I prefer spark cut because it makes cool noises.

[TrumpetRhapsody]

 

Obviously I bring this up for us EDIS guys that don't have the option of spark cut.

Just trying to get some facts and weed out the baseless opinions.

 

Also if you'd read my sig, you'd see I am in fact running the extra code lol. Sorry to be harsh dude but that's exactly the kind of posts I spent the better part of my evening sorting through last night haha.

 

 

 

 

One thing I forgot to put in the first post, is that I read about the fact that a lot of older stock fuel systems exclusively use fuel-cut as a limiter. Many NA-T people claim no ill effects, and some say that aftermarket overboost protectors also use fuel cut.

 

Another consideration for us EDIS folk is whether to only cut fuel (if this thread proves there is no danger to do so), or cut fuel and spark to let EDIS default back to 10BTDC (if that is preferable to whatever timing you're at when you hit the limiter).

Edited February 24, 2011 by TrumpetRhapsody

[mobythevan]

Oh, now it hits me, you are talking about people using the EDIS module. At first I was having a hard time understanding why EDIS people cannot use spark cut but I forgot that some people use the EDIS module and the only thing that megasquirt tells the module is what timing angle. I have two wasted spark engines but I use the megasquirt wheel decoder and no external modules. Sorry for the sidetrack, I just didn't understand why you couldn't use spark cut at first.

[BLOZ UP]

 

 

 

 

Also if you'd read my sig, you'd see I am in fact running the extra code lol. Sorry to be harsh dude but that's exactly the kind of posts I spent the better part of my evening sorting through last night haha.

 

Since I'm on my phone there are no sigs by default. Let me put it this way: I've bounced 5 cars with MS and fuel cut only off the rev limiter, not all day long but enough. They didn't explode. Bouncing off the rev limiter isn't the greatest thing to do in the first place. Which is probably why you don't see lots of "facts" about it, especially since it depends wildly on the tune and engine and car.

 

 

 

One thing I forgot to put in the first post, is that I read about the fact that a lot of older stock fuel systems exclusively use fuel-cut as a limiter. Many NA-T people claim no ill effects, and some say that aftermarket overboost protectors also use fuel cut.

 

So set it and try it. You're never going to know for sure until you try it on your car.

[mario_82_ZXT]

Pretty much every FACTORY car with a rev-limiter uses fuel only, maybe fuel and spark.

 

Spark cut means death to cats and chambered mufflers, which is why OEMs don't use it.

[cygnusx1]

BOOM!!

 

I am not convinced that this is not the cause of my car detonating on the race track a few weeks ago. Hear me out. My tune maintains AFR 12's all over the upper ends of the map. It is not dyno tuned, but tuned with datalogging and correcting. Tuned without a rev limiter. Little by little I have developed a pretty stable map for my car. When I switched to EDIS, I shut off spark cut limiter, and went to fuel cut limiter. I set my fuel cut start point to 6500rpm's and end point to 7000rpm's. That means that the fuel cut gradually happens from 6500 to 7000. So in theory, for example, at 6750, half of my fuel is now gone! To me that is BAD!! Full throttle, full boost, and half fuel. All of a sudden the AFR's are up in the 18's or so. Then a few visits to the 6700 rpm range on the track and BAAAM three rear pistons lose their skirts, and the head gasket went to lunch.

 

 

 

Tell me I am wrong?

Edited April 27, 2011 by cygnusx1

[TrumpetRhapsody]

Hmm I certainly wouldn't think a gradual fuel cut would be a good idea, like you say it would just lean out and detonate. I use spark retard, then a hard fuel cut. So far I've felt the retard first and avoided the fuel cut.

Edited April 27, 2011 by TrumpetRhapsody

[Nigel]

SDS uses fuel cut for overboost and rev limiting. It's a 100% cut and nothing bad has happened from the few times I've experienced it. It sure gets my attention though, because it's a very abrupt loss of power. It's definitely meant as a safety feature, and not a control for holding a boost or rev level. I agree with TrumpetRhapsody, a gradual fuel cut sounds like a bad idea.

 

Nigel

'73 240ZT

[BLOZ UP]

BOOM!!

 

I am not convinced that this is not the cause of my car detonating on the race track a few weeks ago. Hear me out. My tune maintains AFR 12's all over the upper ends of the map. It is not dyno tuned, but tuned with datalogging and correcting. Tuned without a rev limiter. Little by little I have developed a pretty stable map for my car. When I switched to EDIS, I shut off spark cut limiter, and went to fuel cut limiter. I set my fuel cut start point to 6500rpm's and end point to 7000rpm's. That means that the fuel cut gradually happens from 6500 to 7000. So in theory, for example, at 6750, half of my fuel is now gone! To me that is BAD!! Full throttle, full boost, and half fuel. All of a sudden the AFR's are up in the 18's or so. Then a few visits to the 6700 rpm range on the track and BAAAM three rear pistons lose their skirts, and the head gasket went to lunch.

 

 

 

Tell me I am wrong?

 

I think you are wrong. What you set sound like activate and deactivate points for the rev limit. In other words, some hysteresis so you don't bounce like crazy off of a strict 7000 RPM limiter. You rev to 7000, it cuts ignition/fuel/whatever till RPMs drop down to 6500.

 

On Megasquirt, there's a similar algorithm. It drops X cylinders at the first soft point, then the hard limit drops all cylinders.

 

The AFRs will be completely inaccurate when you hit a rev limiter.

[cygnusx1]

I am going to dive into the MS manual again a dig out the section about rev limit / fuel cut. If that doesn't clarify it, then I'll ask Matt at DIY. BLOZ UP, I tend to agree with you because a gradual fuel cut, as conceded above, is a BAD idea. I don't have my MegaStim handy, or I would test it out.

Edited April 27, 2011 by cygnusx1

[cygnusx1]

You are correct sir!

 

Quote from the Megamanual- setup section

 

"For example, if you set the upper limit to 6000 and the lower limit to 5800, then in:

 

 

Spark Retard mode:

Spark would be applied normally (from the advance table, etc.) until you reached 5800 rpm.

It would then be cut linearly as the revs increased to 6000 rpm.

Note that the spark is retarded with rising revs between 5800 and 6000, as well as when the revs fall.

 

Fuel Cut mode:

 

Fuel would be applied normally (from the advance table, etc.) until you reached 6000 rpm.

Fuel would then be cut completely, and restored ONLY when the revs dropped to 5800 rpm.

Note that the fuel is NOT cut with rising revs between 5800 and 6000, only as the revs fall."

Edited April 28, 2011 by cygnusx1