4" Downpipe

[DA BIG ONE]

4" Downpipe, been knocking this project idea around for sometime now, using 4" round to 4" oval transistion to get past frame rail restriction. However, I need input as to how long the downpipe should be, or?

I know, in a high rpm gas turbo motor a larger downpipe into a smaller exhaust spools faster, and helps make power, but we are dealing with a low rpm diesel, and on turbo motors scavenging becomes less an issue.

I see many using 3" downpipe into larger exhaust system on 6.5td's w/good results, so say 4" downpipe into the same size, or smaller say 3" exhaust should be a major improvment, or?

Any thoughts?

[john8662]

3" is a good size, mainly because the outlet on the turbo is just 3", so it won't really make any difference other than maybe a little less restriction.

With a mandrel bent downpipe there is minimal restriction in the airflow through the pipe, because of the smooth transitions.

Someone on here is running 3.5" but I think they're running a different turbo.

[Kennedy]

Put a bigger turbo on first...

[DA BIG ONE]

Originally posted by kennedy:
Put a bigger turbo on first... JK, how about a suggestion for a trubo, I currently have the GM-8 adjusted to 14+psi after the IC, w/occasional spikes of 18+psi, "surely at, or close to stall and super heating aircharge right now."

[Hubert]

You might gain a little bit but IMHO its a diminishing return for increased effort after 3" DP and 4" exhaust.

I read somewhere the exhaust manifold ID is about 2-3/4 on ID on the crossover flange it looked about that when I had the crossover pipe off. I did not see the turbo exit though. The only high performance exhaust manifold I have seen is on the Peninsular sp? diesel with there hot motor. They also have a bigger turbo. I want to say its a Garret turbo ??? on size or model?

There is a ceramic coating that will improve the flow of the OEM manifold but I don't know details.

[Kennedy]

I haven't seen anything that I really liked for the 6.5. I had a Switzer from Peninsular, but it was very lazy for day to day driving...

[DA BIG ONE]

Originally posted by kennedy:
I haven't seen anything that I really liked for the 6.5. I had a Switzer from Peninsular, but it was very lazy for day to day driving... Surely spoolup issues.

I'm knocking around porting the GM-8 turbine outlet some, then somehow going a little larger on the compressor side, but do not know if a larger compressor can be had for this turbo?

[HANK1948]

I made a 3.5 i downpipe I used stainless flex dyno pipe w/ a marmon clamp on the end connecting to 4 in striaght pipe all the back and oh man does it sound bad ass!!! the turbo whistles away at idle and I cant stop puttin' my foot down to the floor! :D