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I switched to a Haltech Elite 2500 mainly for traction control. My Supra and Z are both making enough power now that hooking is a big concern for me. With the Elite they have the choice of cutting either ignition or fuel. Which is the better way for a 600-800 hp E85 powered turbo car? Advantages / Disadvantages?
I've tried both fuel and ignition in our 86 and I tend to prefer fuel cut. There's understandably pros and cons with either method. The downside of using an ignition cut in a powerful turbo car is that you're likely to be putting a lot of unburnt air/fuel through the exhaust when traction control is active which can result in a lot of popping and banging. Fuel cut tends to be a little more subtle however there is the potential for fuel cut to be a little less responsive due to the fuel film on the port walls being consumed and then needing to be re-established when the cut engages and is subsequently removed. This also can result in a very slight lean condition momentarily, although by the nature of the fuel cut, you won't be able to measure this on a wideband.
I'd suggest trying both options for yourself and see which you prefer now that you know what to expect.
From my experience i would use ignion cut very cautios. Especially on engines with hydraulic lash adjusters. The big pop and bangs can push the exhaust valve open. I've seen some traces on the piston, because the valves has touched on a few engines. Most likely due to the ignition cut or missfires. Another point is that rev limiting for a long period of time can lead to boost windup.
Of course fuel cut can lead to detonation in theory, but i've never seen a damage due to it until today. Good ECU's allows some ignition timing reduction during limiting. That can lead to boost wind up on fuel cut, too.
Adrian - Can you elaborate on "until today"? Unless I've skimmed over things too briefly I haven't seen any reference to hurting a motor using fuel cut, did you experience an issue today? At this stage I tend to bias fuel cut for most things where I can get away with it on the assumption that if there isn't an appreciable amount of fuel around you can't really do much damage
I think there's a common misconception that when using a percentage fuel cut that the ECU is removing a percentage of fuel from each injection event which would understandably result in a lean mixture. What actually happens is that the ECU cuts a percentage of injection events completely, meaning those cylinders don't see any combustion and hence the mixture is unaffected.
That is the basis for fuel cut however as I mentioned previously you will still have some residual fuel on the port walls that makes its way into the cylinder when the injection is cut. This is minimal and I'd make an educated guess that the resulting mixture would be so lean it wouldn't ignite. When the injection is reestablished there will also be a momentary affect on the air fuel ratio as the fuel film on the port wall builds back up - This is similar to what you'd expect to see with transient lean conditions if you had no accel enrichment. In my own experience this isn't anything to worry about though and I've never seen damage as a result of a fuel cut.
Yeah that was my thoughts/understanding on it as well - which is why I my curiosity was piqued by the "until today" statement... did I miss something, or does that need to be elaborated on? It makes it seem like he hurt a motor, or knows of someone hurting something by using a fuel cut.
No i've had no problem at all with fuel cut. But as Andrea raised, there are rumors that fuel cut can do some sort of damage.
From my experience I've had more problems with ignition cut. I've seen pumped up hydraulic lash adjuster and rocker arms that fallen off due to ignition cut.
Andre whats your experience with ignition cut?
Ignition cut needs to be treated with care, particularly on turbocharged engines. The inevitable popping in the exhaust from the unburnt fuel/air charge igniting can pop the exhaust valves back off their seats. This is more likely with a soft factory valve spring. Other than the possibility of bouncing an exhaust valve into a piston crown or possibly an intake valve, this will instantly result in a hydraulic lifter pumping up as Adrian mentioned. You can also have issues with rockers popping off and other associated nasties.
The only time I'm a strong fan of ignition cut limiting is for two step launch control where it is a necessary evil. What you can get away with is very dependent on the basic engine design. For example on the serious 4G63 drag engines I built, I would convert to solid lifters and use quite a high seat pressure on the exhaust valve springs to combat these issues. A stock hydraulic lifter in a 4G63 severely limited how aggressive you could be with the ignition retard on the launch control and this wouldn't work if you had a very large turbo.
Basically if you don't need to use ignition cut though, you're probably better to avoid the potential pitfalls and use fuel cut.
Thanks everyone for the great responses! I have one more question with this. When doing an ignition retard instead of a cylinder kill for traction control what are some good guidelines? Is it ok to take out 5 degrees? 10?
It will depend on the specific engine, as well as how much timing you're running under normal conditions. If you retard the timing too far you're going to end up with the engine running poorly and this in itself may introduce a misfire. If you're running significant retard and the traction control is being kept pretty busy controlling wheel spin, you're also going to end up with excessive EGT which can bring its own down sides. Lastly if it's a turbocharged engine, large amounts of retard will cause boost control problems.
The flip side is that you're probably going to need to use at least 20 degrees of retard to have a significant on engine torque and hence make the traction control actually effective. In my own experience retard alone is seldom enough to be effective on its own in a moderately powerful engine, and the higher the power level, the less likely you can eliminate wheel spin with just retard. It's common place to use retard followed by a cut if the wheel slip continues to climb.
This doesn't specifically answer your question, however the reality is you're going to need to try it out and see how it works. Testing on a dyno with various ignition timing will give you a good guide as to how much power reduction you'll see with retard.
as simple as it sounds, i have never tried testing the amount in real power loss when using retard in the dyno... good move there Andre... going to test that in my next session.