Ah the stumbles.....
I first noticed this when I drove SpicyMSP's car. Under partial and full throttle acceleration in every gear the car would feel like it was mis-firing or bucking under boost. It wasn't huge enough to toss you around in the seat but enough to feel the car was not at its peak potential.
First few thoughts that ran through my head......
1: Knock Sensor was very sensitive and retarded the timing too much.
2: Poor ECU Tuning from the factory leading to stumbles.
3: Bypass valve leaking boost and causing a surge.
After we had Injen do the development with the intake the 3500-4500 surge went away but all of the little stumbles seemed more pronounced. So if you thought the intake was restricting the motor and causing it to stumble, it wasn't quite the problem.
Now we move on to the knock sensor. I don't have a way of checking this while the car is running to tell if this is the main cause right now.
On the other hand, my buddy Brett from Vishnu Performance up north who dyno'd the 138whp MSP said that the A/F ratios from their wideband sensor were reading in the 10:1 range. A full 2 points richer than it should be under this amount of boost. This could lead to a rich stumble if the mixture is not getting fully burned. This is my main suspect for the stumble right now.
The final reason may lie in the bypass valve, it seems to be improperly shimmed for the car. A normal valve will release all of the air back into the intake and this seems to only release a small amount. Thats why you'll hear a slight cough like a turkey when it goes off. I will be trying to find an H25 TurboXS valve to adjust it manually and see if that helps.
If anyone else finds out any info let me know, we'll be testing things out on her car until my MSP comes in.
