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Old 03-15-2018, 11:07 AM   #10
Homemade WRX
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Member#: 33782
Join Date: Mar 2003
Chapter/Region: MAIC
Location: Mooresville, NC
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I would have the initial pipe diameter be as close to the port size as you can get but try to avoid being smaller than the port. That's just bad.

As for an egg head, I'm one of them. If you knew more about it, there is no ideal exhaust. It comes down to a lot of variables specific to each engine and what you're doing with it. If making peak power is your goal and transients don't matter, then the obvious goal to to make it as efficient as possible. If you want the best spool, scavenging be damned and make the tubing as short as you can (see KillerBee).

Turbine size, location, cam size, exhaust port flow data, etc, etc, etc....there isn't a simple answer to this. Even in racing (NASCAR and Ford GT) we did 1D calculations, backed into CAD, and validated on the engine dyno. Obviously the NASCAR stuff this was more important. In the GT, everything is BOP, so we made a good set up and left it alone. No point in spending a ton of time optimizing when IMSA and the FIA would just penalize you for it. Heck, that engine practically ran zero boost at redline by the time we were done with homologation. BOP killed the race engineer's fun.

I will say the turbo cars are far less 'header' driven performance than an NA car. They do however still benefit from the same principles.

As for your spool issues, that can come from a myriad of other things too...intake manifold size, intercooler volume, intake plumbing volume/length, etc. All of this is also neglecting the tune and shear size of the turbo you've chosen.
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