I've heard in various places that tuning it to be a little extra rich right where the turbo accelerates the most makes it spool up faster than if you tuned for ideal AFR. Why is that?

I've heard in various places that tuning it to be a little extra rich right where the turbo accelerates the most makes it spool up faster than if you tuned for ideal AFR. Why is that?


Through my own testing, logging and analysis with datalogging, it appears that you want right in the 12.2-12.7 range for AFR at lower boost pressures(just entering boost). Leaner and you lose torque and it doesn't really help spool all that much, you actually will end up with some lag.
My belief is it all comes down to cylinder pressure vs time/crank angle. Again my own testing...slightly reducing Ign. timing results in an increase in boost pressure(entering into boost), however it comes @ the cost of cylinder pressure.The less cyl. pressure over less time is less torque and less HP.

So you're saying that by reducing timing as boost comes on it induces more boost more quickly, but kills power for that part of the powerband?

So you're saying that by reducing timing as boost comes on it induces more boost more quickly, but kills power for that part of the powerband?

Not by much, We are talking a few hundred rpms here. It helps you achieve your target boost a few rpms sooner only w/ optimum A/F. Every set up is different, this is all personal experience.I have tried almost every theory out there and this one seems to work well for me...

So what is causing the turbo to spool quicker if there should be reduced cylinder pressure due to reduced timing? It seems like running optimal timing and fuel to create the highest cylinder pressures would spool the turbo quickest.

By retarding Ign. timing from MBT (Maximum Brake Torque) The less timing provides higher EGTs(as will to much advance,not safe) since smaller portions of total energy are converted to usable work to the piston.

Ok, makes perfect sense now. Thanks.

Hmm, very interesting I like this new forum,does it have to do with more of the fact that the combustion process is happening later and there is more heat energy left over since it happened later in the process and there wasn't enought time to convert all of the energy into the piston, or that the less timing just produces a higher egt. Also what is MBT.

Michael

MBT = Maximum Brake Torque. I'm assuming that just means that it's tuned as good as it gets so it's making the most power possible.

The way I'm interpretting this is that instead of using the expansion of explosion to force the piston down, it's doing more expansion in the exhaust manifold so it's spinging the turbine rather than pushing on the piston.

correct Beau.

By retarding the ignition timing, the hot gasses from combustion are still expanding when the exhaust valve opens, thus helping the turbine spool up faster.