General Flat-shifting (NoLiftToShiftin') potentially causing detonation and fuel-air mixture getting ignited prematurely?

[Kryndon]

Had the pleasure of speaking to possibly the best/biggest tuner in my country this weekend at our national JapFest. We touched upon a few subjects including flat shifting.

Now, I wasn't certain if we were talking in general or for specific applications, but his general opinion was that flatshifting was extremely bad for the engine itself, disregarding all the benefits for transmission health and switching gears. He said that as you have flatshift enabled and keeping WOT between shifts, the boost doesn't drop as much so let's say you retain 14 psi between 2nd and 3rd gear. At the same time, your MAP (if equipped) is reading those 14 psi worth of air and is then demanding the necessary fuel to be injected. So you end up having a substantial amount of fuel-air mixture going into the chambers but at the same time some of the exhaust valves are open which from scavenging and possible valve float or weak springs start to let the hot exhaust air back into the cylinders. This suddenly ignites the fuel-air mixture prematurely, way before the piston reaches adequate travel for ignition. Essentially, he said this "forces" the piston back down before it wants to do so, and this leads to extreme pressures on the piston crown, pin and rod itself, not to mention bearing surfaces. This is the loud popping and banging we can hear if running NLTS.

On my Talon with ECMlink, I have it enabled without additional antilag, and I can definitely hear some loud banging as I shift gears, but I always assumed it wasn't putting heavy load on the motor during the actual shift as our NLTS works simply by cutting spark.

For us to retain the RPMs high between shifts and thus the boost pressure, some sort of combustion needs to happen, so the ECU needs to inject fuel to keep this process going. Yet, this apparently produces immense stress within the cylinders. So now I'm rethinking about disabling NLTS and just shifting like a granny. I'm trying not to take all he said for given but the guy's tuned more engines than years I've been alive so maybe it's wise to listen to him?

Curious about you guys' opinions and you tuners out there. Do you use flatshifting on any car at all and have you ever had mechanical damage or knock issues from it?

 

[steve]

I haven't dug around in ECMTuning's code in a long while. Are you sure that they are using spark cut for NTLS. When we did it back in the day, we just changed the rev limiter which uses fuel cut.

 

[Kryndon]

I just checked some of my logs where I was shifting thru gears with NLTS and yes we do use fuel cut to maintain the target RPMs between shifts, however it cuts fuel then re-adds to keep the motor revved so the effect is the same. Even if you cut spark and running wasted spark as is our system, you could still prematurely ignite the mixture if the above conditions are met, hence the popping and banging. I can't recall if the guy specifically said which type of NLTS he was referring to but his overall stance was that it's not worth it. Personally I love it because it makes me hate the manual trans a bit less and it's actually healthier for the transmission if you powershift it this way. But the topic is engine health.

 

[steve]

It's not the same. Cutting spark and letting the engine load up on fuel is a problem. Cutting the fuel turns the engine into an air pump purging the system and when the fuel is turn back on it's burnt in cycle it's sprayed. Yes, both are hard on the engine (if you have hit the rev limiter or fuel cut you know) but guess which of the two has the issue you describe above and worse.

 

[DrewmHolland]

as long as you can keep WOT then you should be able to stay in boost so a fuel cut limiter is the way to go, spark cut limiters can cause backfiring and detonation because of wonky afrs, found that out playing with the msd boxes on my friends rx7