3.5"downpipes on stock turbos?

[IanH755]

I think there's been some mis-understanding going on, certainly on my part where I got DP back pressure and turbine A/R back pressure mixed up (my statement above about cylinder gas velocity is correct is correct for turbine A/R).

[FUBAR]

I have experience going from 2,75" to 3" on the same car with identical setup before and after.
My car made 977ps/1300Nm on 2,75" DPs with Supersprint resonated exhaust (no cats). Due to gearbox failure I had to take engine out again. As I was so close to 1k ps I figured it could not hurt to put on 3" DPs to see if I could get a few ps more out of it. So I put on MRC 3" decat DPs on and sent it to MRC for a MAP tweak. As MRC is not willing to send me the dyno sheet after the MAP tweak I guess it made less than it did before, I don´t know. They said the car is fine, but the rest of my exhaust is the problem.

In my case it is the complete system not only the DPs that is setting the limits.

It made 749ps/1100Nm on stock turbos with 2,75" DPs on stage 2 map.

[CHEQUERED]

3.5 " DP are too big even for hybrids IMO because you will struggle with flow velocity. I've used 3" catless DP for my 9xxps and didn't have a problem with flow. Imagine blowing through a large pipe compared to a straw... the flow velocity through the straw will be much quicker... just my opinion of course.

You can never go too big with a turbo exhaust, I've dyno'd 1000s of cars and tried all combos, bigger the better regardless on a turbo car.

[FUBAR]

3.5 " DP are too big even for hybrids IMO because you will struggle with flow velocity. I've used 3" catless DP for my 9xxps and didn't have a problem with flow. Imagine blowing through a large pipe compared to a straw... the flow velocity through the straw will be much quicker... just my opinion of course.

You can never go too big with a turbo exhaust, I've dyno'd 1000s of cars and tried all combos, bigger the better regardless on a turbo car.

Maybe for peak dyno numbers. If you use it for street driving I agree with CLASSBA. Going bigger did zero for me.

[CHEQUERED]

3.5 " DP are too big even for hybrids IMO because you will struggle with flow velocity. I've used 3" catless DP for my 9xxps and didn't have a problem with flow. Imagine blowing through a large pipe compared to a straw... the flow velocity through the straw will be much quicker... just my opinion of course.

You can never go too big with a turbo exhaust, I've dyno'd 1000s of cars and tried all combos, bigger the better regardless on a turbo car.

Maybe for peak dyno numbers. If you use it for street driving I agree with CLASSBA. Going bigger did zero for me.

it just means you have a bottle neck elsewhere, got nothing to do with peak numbers. bigger exhaust will free the whole rpm range everytime (except NA where it will only improve peak) it will also improve transients which wont show on a dyno (will show in logging on street and circuit though). TBH the hp is not really high enough to benefit peak but if you can control the noise bigger is always better (Turbo).