Why would you run a bigger downpipe than the outlet to the exhaust housing on a turbo?

IE: t3 .48\.60 exhaust turbine is ~2" so why would someone run a 2.5" downpipe?

wouldn't it be benifitual to run the same size tubing throughout the whole exhaust as the exhaust turbine outlet?

 

You would think that. Same with the outlet and inlet on the comp., but no for a turbo car the least exhaust resistance the better. Remember not all stock turbo cars can have it ALL now.


JP

 

A 2" pipe all the way back is going to be restrictive and kill spool time. You might not notice any huge difference if just the downpipe was 2" and the rest was 2.5" though. Turbos love open downpipes.
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W0rd. I go 2 1/4 to 2 1/2 all in the DP then cat back is 3. Gansta.


JP

 

The 2" extended pipe would end up being restrictive on a turbo vehicle I believe. A vacuum is desired for exhaust, which ends up pulling on the piston as the exhaust valve is open, and with a long stretch of piping that is not large enough, that vacuum would be unachievable. Short pipe yet same width, fine, long pipe larger width would be fine as well though.

 

Ok, so what is better. Having a large downpipe leading to a smaller exhaust, like lets say 2.75" dp --> 2.25" exhaust OR a 2.25" dp --> 2.75" exhaust?

 

2.75 to 2.25 but thats pretty silly to do.
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With a 2.25" DP, you will still run into that restriction problem that you would with a 2.25" exhaust.

 

so should i just start out with a large downpipe to begin with on my t3? Maybe 2.5, then up to 3? I eventually want to get a 3" exhaust but the outlet on the hot side is 2.25 and i want to use the stock wastegate until i can upgrade.

 

The decrease the pressure differential on the turbine wheel. Less pressure at the exducer, the faster the spool time will be. If you can hook up a vaccum to the downpipe it would be beneficial