Originally Posted by
BenFenner 
I think a spark and fuel cut at the same time (or spark cut right before fuel) would be the best compromise. However a soft limiter using spark retard would be a sweet option as well.
Like a lot of people I've never liked a fuel cut on aggressive turbo engines and I think the science backs me up there. From the video they found as much as a 1/3 of an injection worth of fuel just chillin' in a puddle in the intake runner. This puddle is kept liquid by the boost pressure and sheer amount of fuel. When the boost pressure drops (like when you lift the throttle) all of that fuel instantly evaporates in the low pressure air and is ingested by the engine giving natural deceleration enrichment (which is nice) but during a WOT rev limit hit where the fuel is cut I believe the fuel is also able to be evaporated into the dry air (which now is not saturated with fuel) and also ingested by the engine causing potentially lethal lean ignition events.
As for cutting things cylinder-by-cylinder instead of all at once. I think if you can do that easily and cut each cylinder in a safe way I don't see why not to do that.
The puddle I referred to is called the "Tau"? puddle, and is referenced in a lot of tuning books I've read. It has been very difficult to actually piece all of this information together for me. It seems like every other person I talk to has conflicting statements about whether or not it would cause detonation. Some said there is not enough fuel to actually combust. I've talked to several tuners about this puddle, and a lot of them looked at me funny, scratching their heads.
I even brought it up to JWT and their tech said they have never heard of this. The factory Nissan computer cuts injectors one at a time for the rev limit.
I had my ignition cut set at 7400rpm with the JWT limiter at 7500rpm and I would still "bounce" the limiter. I set the MSD to 7300rpm and it stops right there! Also, the torque curve suggests that the MSD pulls timing by a few degrees on the top just before the actual cut. It makes it very very smooth. Look at my 3rd gear burnout video and you can see how smooth it is.
I would also be interested in seeing how the engine responds to throttle opening and this puddle. If in fact the puddle exists in an area of high pressure, the change of throttle may effect the properties of evaporation of this puddle as well. Perhaps the "letting up" on the throttle just before fuel cut created a lean condition?
Be the first to like this post.
Back to top
