Hey tuners, I recently switched from a Garrett gt4094r w/106a/r turbine housing to a BW S366 with .91a/r housing. I have since adjusted my 4094r fuel map(perfectly tuned to 28psi) to the needs of the S366. Generally, I have had to add fuel to the boost map to maintain the same afr's as the 4094r, but the tunes are very close. This could be because I also did a refresh on the motor with new seals just before I add the new turbo. I am now beginning to tune high rpm above 24psi and the motor is running very rich as I reach 7500 and up. It still pulls without breakup, but not as hard, and gets very rich(below 10.1afr) above 7500.

Is this caused by the restriction of the .91a/r housing? Is this the expected result form choking turbine at high rpm?

 

When you say it's running very rich, what happens when you reduce fuel injection pulsewidth at that high rpm and boost? A more restrictive turbine A/R means more residual gases and backpressure at very high speeds. This reduces the amount of fresh air that gets into the combustion chamber. However, the low speed turbine power increases. That means that you should be able to get more boost when you floor it in 5th gear at say 2000rpm.

 

I have not tried leaning it. All in all, I just need to to know is the result in respect to afr's when switching to a smaller turbine housing with everything else remaining the same including the tune.

Will the choking of the exhaust show richer condition because volumetric efficiency is reduced?

 

^ if there's less fresh air trapped in the combustion chamber because of residual gases due to higher backpressure--if that's in fact what's happening--yes.

 

Thanks...not going to try leaning
It yet. Going to check/change the plugs first since they have been in there for a while. Also going to do some more pulls to see if it petsists