Turbo exhaust size- spool vs creep
 

I have a 3.5" turbo back w/ one muffler (JIC Spec 90- OK, really 90mm turbo back).

I have constantly been trying to rectify boost creep- it is down to starting ~6,500rpm. Believe me- I have tried all the other methods of decreasing creep that I thought would not kill the boost response.

I will be moving out of the sticks in 2 months and realize I will need a "civilized" exhaust system. Actually, several friendly officers have pulled me over just to kindly tell me this even up here in the sticks.

I thought, perhaps adding mufflers may add enough back pressure at high rpm due to turbulence to stop the boost creep.

Luckily, I saw RB has greatly increased the quality of their 80mm turbo back and it is now almost entirely stainless construction! So, though it is a step down in size from my 90mm it will not be such a step down in quality (yes, I do own many RB products and am familiar w/ their quality levels).

I will buy the RB turbo back now. But, if my spool is not as good from the smaller piping I was thinking of adding my 3.5" downpipe and a presilencer- or even a 4" downpipe that is lying around here from a diesel.

Will this bring back the boost creep? I was thinking possibly not if there is an abbrupt transition from the 90mm or 4" downpipe to the 80mm RB exhaust just before it splits into its 2 60mm tubes.

It would provide good spool due to large DP diameter/volume, but at high rpm/flow the exhaust would hit the neckdown and cause reversion and restriction adding that backpressure I need to prevent boost creep.

I usually use a tip silencer that totally nullifies the boost creep- though the top end suffers.

I have put on a 2.5" downpipe/presilencer which nullifies the boost creep and offers suprisingly good top end- though the low end suffers alot...

Does anyone have experience or observations about this large downpipe to smaller rear section exhaust idea?

Ian