Quote:
Originally Posted by ar design
It has been proven a billion times over that no exhaust is the best exhaust on a turbo.
We just built 3" downpipes for the B5 Audi S4. Picked up over 30awhp with test pipes and over 20awhp with HFC secondaries over a set of gutted primary cat OEM downpipes. Going from HFCs to test pipes, we lost virtually no torque, and picked up around 10awhp. OEM downpipes are either 2.5 or 2.25in.
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An open dump is best yes as that maximizes flow and exhaust gas velocity is optimal as it is straight to atmosphere. But we are talking about street cars and not drag cars so that point is moot. You are forced to run a long tube exhaust so the question isn't about just flow, it's about thermal efficiency/exhaust gas velocity as well and there is a trade off with bigger pipes.
The question is not about HFC vs catless on the same diameter pipe, it's about the volume of air in the exhaust that has to be pushed as the turbo spools. With a 3" DP there is more air to be pushed which could lead to slight slower exhaust gas velocities.
All these things are just theory and it needs to be evaluated on an engine by engine basis. Since you asked VK for their test data I assumed you must have done the same which is why I was hoping you could share some of it. Again this it not mean to be any kind of attack, I'd love to see the VK test data as well! Just an interesting dicsussion.....