Single turbo downpipe... Does size matter?

Stevo585

Lurker
Dec 16, 2022
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Just thinking out loud here -

I have been building my own single turbo kit and will be using a 67/66, 0.85 a/r with a 3" turbine outlet.

My thoughts would be using a 3" to 3.5" transition directly off the turbine and making a full 3.5" downpipe as I have a full 3.5" exhaust as well.

Would any gains be made here or would it still be bottle necked at the 3" turbine outlet regardless of the downpipe size.

Has anyone ran a 3.5 inch downpipe or have any input on this?
 

1and1

Specialist
Dec 28, 2016
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Tally, FL
I'm no pro, but I've always understood that the exhaust is defined by the smallest restriction.....

You have several feet of 3.5" pipe, but it's being fed through a 3" restriction, so it can't move air any faster than that 3" restriction can provide...
 

Torgus

Brigadier General
Nov 6, 2016
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Boston
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ACF 6466 E92 + METH
Yes. A larger downpipe is ideal. The N54 VM/ACF kits all came with a 3.5'' downpipe standard. I never understood why every other n54 turbo kit seemed to have a 3'' DP other than for convince sake I guess. Less expensive, easier to fab up(or let China fab it up for you). It will fit fine. Backpressure is your enemy on a turbocharged car, the less backpressure, the faster your turbo will spool, thus creating power earlier in the rev range. This is why people do hood exits: short huge pipe = barely any backpressure.

Having your waste gates vent open to atmosphere w/ screamer pipes instead of into the exhaust also can help out.

The faster you can transition the downpipe from 3'' off the turbine V band to 3.5'' the better.

With the stock N54 turbos everyone replaces with 3'' downpipes. No one ever said ZOMG that downpipe is way too big for the turbo that makes 200whp(200x2=400whp which is where stockers usually end up around). You now have a 800-900whp turbo using a single 3'' downpipe. Turbo has increased output by of 4x but still same sized downpipe.


Basically the bigger that better:
michael-scott.jpg
 

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Stevo585

Lurker
Dec 16, 2022
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Both view points are exactly what I been debating myself.

I guess going with a 3.5" is not going to hurt anything, benefit or not.
My wastegates will VTA.

At some point I would love to do a dyno comparison with a 1.0 AR and a 3.5" turbine outlet.
 

wheela

Lieutenant
Jun 4, 2021
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Twin Cities, MN
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Totally anecdotal, but this guy claimed noticeably better spool and response going from a 3.5" to 4" downpipe, even with 3.5" exhaust the rest of the way. It was on an s55, with a big BW EFR turbo:


Talks about it about half way down on page 5 if viewing on your mobilešŸ‘
 
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