Getting ready to put together my hot side on my 521 CI big block with a single 94mm Thumper turbo but I'm unsure to the size pipe I should use. My headers have a 3.5 inch collector so would it be better to keep everything the same size and stay with 3.5 inch piping or should I reduce it down to something smaller.

 

I have always thought about this. Does hot side matter? if you look in your turbo all of your exhaust must go through that small area?Even on a big turbo like yours.I do not think turbo hot side plumbing matters so much.I have seen beautiful turbo headers but do they make a difference over a log header not really only a couple hp.It will only flow as much as the smallest part of the header or turbo.

 

If your looking for constant pressure turbine drive, use big pipes, if you want to utilize pulse energy and not loose energy in your hotside by slowing down and speeding it up, keep the pipes small, usually a lot smaller than you think

 

A smaller header collector size with your set-up, say 3 inch... used with a 3 inch crossover into the turbine housing will def. create faster exhaust gas velocity/spool up. Almost every pro outlaw, sso or drag radial turbo car i've seen use a very small collector size (2.5 inch) with a 2.5 inch crossover into a large frame (t6) turbo. Even using a 2.5 inch collector & crossover will support upwards of 2000hp.

 

As a real world example, I had a single 3" crossover and went to a 2.5 and the power (spool) comes on much sooner. This is on a 331 t-6 setup. On your set up I would reduce to 2.5" after the collectors and run the dual 2.5" to the turbine flange, or a single 3".