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I was thinking of it the other day ..

try to stay off the limiter and boost cut.

fuel cut only could end up with a lean condition at the set point.boosted or not.
 

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i use fuel cut for mine.

i never really thought about it being bad. ive hit the limiter plenty of times when the engine was making around 200hp. ive hit it a few times since the new turbo(300ish hp probably) the limiter is at 8500rpm.

so far no problems but i also dont sit on the limiter either.

i can see where timing would be safer and ill consider changing it.
 

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Meat Popsicle
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Discussion Starter #4
The way I'm thinking about it...

Fuel cut - There's no fuel to ignite so there's no change of pre-ignition of detonation.

Ignition cut - There's a lot of extra fuel in the cylinders and in the exhaust. Ignition takes place outside of the cylinders.


It also seems like fuel cut is used on most, if not all, production vehicles (FI and NA).
 

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Meat Popsicle
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Discussion Starter #5 (Edited)
I guess I should be more specific. In terms of launch control, why does ignition cut seem to be the preferred method?

Is this specific to turbo engines where you want to use ignition cut for anti-lag or does it provide other benefits?
 

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I guess I should be more specific. In terms of launch control, why does ignition cut seem to be the preferred method?

Is this specific to turbo engines where you want to use the two step for anti-lag or does it provide other benefits?
fuel cut is used on factory cars because spark cut would blow up the cat.


disregard anything I say ,just living with crome ptsd.

rev limiter I like to fuel and spark cut..when give the option..
 

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Meat Popsicle
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Discussion Starter #7
Yeah I’m stuck with just fuel cut since I’m using turboedit. A friend of a friend was saying “never use fuel cut on any boosted engine” so I started reading up on it.

A lot of guys say there’s no problem with fuel cut, others say that it’s dangerous to use for launch control.

Given that every high HP drag car seems to use ignition cut these days, I’m interested to understand why that is. I’ve searched but it just seems to be an ongoing discussion with no clear answer as to why one is better than the other.
 

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oh you mean launch control.

well i also use that though its more like at 3000rpm. i use fuel cut with it as well.

i wouldnt worry much about detonation as there isnt that much load on the engine and no boost (not using an anti lag)

but i could see what your saying about using ignition retard and allowing the combustion to happen later. this means hotter exhaust to spool the turbo and you may be able to launch with boost as that is basically what anti lag does.

so i could see where that may be better but i wouldnt think its any safer as i would have concerns for your exhaust manifold and turbo over time.
 

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why does ignition cut seem to be the preferred method?
Because #flamelife bro.....

lol

That is a great question, but you and robgoof are correct. Fuel cut is used in almost all OEM applications due to always having to put care of the catalyst up high.

It all honestly comes down to particular build styles and the characteristics valued out of the engine, as both methods offer pros and cons. I think this is all it really comes down to, most engine management solutions offer either cut option, or both at the same time and it is up to the user which one(s) are more beneficial to use in terms of it's characteristics.

We all know what happens with ignition cut and a bit of retarded timing, #flamelife, boost buildup during launch control, but the compromise is always more heat soak/increased wear and tear on in-cylinder engine and rotating assembly components, exhaust valves, turbos, exhaust sealing devices and gaskets/etc.

Fuel cut is usually always recommended when trying to safely control RPM where you are not looking for an after-effect like #flamelife or continued turbocharger operation. During fuel cut, the injector is turned OFF. It's not like it's half way or 1/4 of the way open, during fuel cut ALL pulse is turned off until RPM is brought under the target cutoff RPM by a certain amount, then the injector is turned back on and fuel is allowed to flow at the same rate as normal as determined by the operating fuel table cell.

It is safer because you are closing off all possibility of combustion, turning off the taps if that analogy works here. Sure, there may be an injector shot worth of fuel that was pooled up at the back of an intake valve waiting to be drawn in to a random cylinder when fuel cut occurs, but if you are near or at set RPM cutoff, that single shot of fuel is drawn in and used in combustion so quickly that the next time the timing is right for fuel to be brought back in, there is nothing there anymore.

In-cylinder lean spots if any would occur so fast that at those speeds, even at average launch control RPM speeds, the lean condition (if any) is almost negligible. When the injector gets turned back on at speed, you have a correct amount of fuel ready to be burned anyways. Our engines use port injection, meaning the fuel injected by an injector is destined for a particular cylinder ONLY. Thus, if the intake valve is closed, nothing gets pulled in until it is time.

With Throttle Body Injection (TBI) systems, fuel control is less precise and the chances of running lean in-cylinder are higher during a fuel cut event because the fuel is injected before all intake valves, and the delay time of fuel cut off to being drawn in to a cylinder is longer. Fuel is accepted by any cylinder ready to take it in. This is why my S10 backfires on throttle lift off a couple of times, even though it runs a fuel cut strategy. #flamelife lol. Port injection removes that negative characteristic.

Of course with fuel cut on any injection system, your A/F sensor would register as way lean due to the extra oxygen being pumped through the exhaust due to no combustion happening at the time, but that's NOT the same as in-cylinder combustion being lean. You are cutting fuel, so you should EXPECT a lean reading. If you were injecting fuel, but still getting a lean reading, THAT's a problem!

Throwing a button controlled wet nitrous setup in the mix coupled with engine injector fuel cut can be dangerous if it was tuned to run at close to stoich A/F during nitrous injection. At high RPM fuel cut if you are still on the go-baby-go button when engine injector cut occurs, you could DEFINITELY run lean in-cylinder here. A dry nitrous setup would not have this occur, as nitrous from the bottle by itself (just nitrogen and oxygen) won't combust by itself without an accompanying hydrocarbon source.


The way I see it: if you have full precise control of all combustible fuel sources through the primary fuel cut device (usually a computer), and the computer knows to turn them ALL OFF at fuel cut, it's pretty safe to use only fuel cut in almost every situation.

Ignition cut only if you desire some sort of effect where excess fuel in the exhaust is a good thing like during launch RPM control where you want higher boost pressures than could be normally attained by regular in-cylinder combustion at the lower RPM limit, anti-lag functionality when you are off-throttle, OR if you have an additional combustible fuel source being injected that is NOT controlled by the primary fuel cut device and you are likely to hit fuel cut at some point (like button-controlled nitrous in a drag racing environment).


OR, just use both if available through your engine management when in doubt if you really don't want to experience the #flamelife bro....
 

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Meat Popsicle
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Discussion Starter #11
Thanks; that was a very well thought out post. Those were the same lines I was thinking along.

Since at least one of my OLD DSM injectors got stuck open on the dyno after three pulls that bounced off the rev limiter, I do wonder if the fuel cut contributed since the injectors went from rapid pulsing to complete shutoff to rapid pulsing again. However I also know that I’m not the only one that has experienced this with DSM injectors and, even though I had them cleaned and flowed, I know that the age of the injector likely contributed.
 

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All limiters are pretty abusive and should be used as little as possible. Fuel cut is the safer of the two options. Your thought that there is not enough fuel to even fire is right on point. It also makes for a more aggressive torque cut in my experience.

Ignition cut has its uses too. As you pointed out, it can have an anti-lag effect and is also great for things like shift cuts, where duration is short and you want to reduce torque enough to let the dogs come apart and shift into the next gear without shutting the engine off momentarily. The down side of ignition cut is that you really do not want to be in it for an extended period of time, your exhaust temps will increase quickly and you risk your exhaust components, valves and seats, etc.
 

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Thanks; that was a very well thought out post. Those were the same lines I was thinking along.

Since at least one of my OLD DSM injectors got stuck open on the dyno after three pulls that bounced off the rev limiter, I do wonder if the fuel cut contributed since the injectors went from rapid pulsing to complete shutoff to rapid pulsing again. However I also know that I’m not the only one that has experienced this with DSM injectors and, even though I had them cleaned and flowed, I know that the age of the injector likely contributed.
Damn! I've never used DSM 450's, but I've also heard as you have of others using DSM 450's and them sticking open mechanically. I've only ever experienced gasoline engine low pressure injectors staying open due to electrical issues, maybe I've just been lucky!

Honestly in all my years as a technician, I've never seen a gasoline engine fuel injector (mounted within the air intake tract) mechanically stick open, aside from the stupid spider injector poppet valve system in GM 4.3L and 5.7L engines.

I've also replaced many diesel engine mechanical poppet style injectors and electromechanical hydraulic spool valve style injectors from internally breaking and resulting in a stream of liquid fuel into the cylinder, as well as piezoelectric diesel and GDI engine injectors for the same thing, but never a low pressure gasoline injector!

Cross fingers and knock on wood!
 

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Ignition cut is also great for things like shift cuts, where duration is short and you want to reduce torque enough to let the dogs come apart and shift into the next gear without shutting the engine off momentarily.
Never even thought about this :) but it really is how almost all sequentially shifted or h-pattern dog boxes coupled to high powered engines are tuned for, wide open throttle shifting where the strain gauge mounted in the shifter is the trigger for the cut delay or through the paddles. Almost always results in a :rl: out the tailpipe under WOT shifting!

Here's a good video of this occurring:


I guess my train of thought definitely hangs out with the cheaper crowd, who can't afford sequential setups or who think dog boxes are the things you put animals in to take to the vet with lol
 

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Meat Popsicle
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Discussion Starter #15 (Edited)
Damn! I've never used DSM 450's, but I've also heard as you have of others using DSM 450's and them sticking open mechanically. I've only ever experienced gasoline engine low pressure injectors staying open due to electrical issues, maybe I've just been lucky!
I definitely wasn’t expecting it. After the engine stumbled on the 4th pull, we smelled fuel so I asked the dyno operator to prime the pump so I could look under the hood and check for leaks. I meant prime it once...

Little did I know, the guy was repeatedly turning the key and priming it which resulted in fuel filling the cylinders, supercharger, s-tube, and finally leaking out of the throttle body. Ended up having to tow the car home, remove the supercharger to drain the fuel, and change the oil. Not a good day!

Fortunately there was no engine damage and, most importantly, there was no fire
 

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I guess my train of thought definitely hangs out with the cheaper crowd, who can't afford sequential setups or who think dog boxes are the things you put animals in to take to the vet with lol
Fair enough. I never really thought about it either until I found myself spending large chunks of my day tuning shifting in those Xtrac boxes. FYI the one in that BMW is probably way fancier than the ones I work with in IndyCar.
 

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Fair enough. I never really thought about it either until I found myself spending large chunks of my day tuning shifting in those Xtrac boxes. FYI the one in that BMW is probably way fancier than the ones I work with in IndyCar.
Dude, you work in IndyCar? Driver?/Tester?Mechanic?/Floor sweeper?/Anyone close to those cars? That's badass man! What kind of work do you do?

Do you need an extra set of hands lol
 

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Trackside Engine Calibrator. Get an engineering degree and c'mon down.
Working on my electrical engineering degree, while working full time as a test engineer and supporting family/kids..... yay fun... :)

Sounds like a blast. If for some reason I still remember this post in roughly two years, I'll PM you and see what you're doing haha.
 
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