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There is a point where larger flow areas are counter productive as the flow needs to be speed up again just before the turbine. Slowing the flow then speeding it back up again creates dynamic losses. And it is considered good to have a bit of a step at the head gasket, which counters flow reversion. So port matching is not an objective at the exhaust like it is at the intake.
When on boost at 15 PSI, you might have 30 PSI or more of pressure in the exhaust maniflold. So the densities are higher and the velocities are lower as a result. With the lower velocities, the resistance drops in proportion to V**2. Flow power losses are more proportional to V**3.
Smoothing a cast manifold is a great idea. Thats not easy. A tube header does produce smooth passages!
So did you have to drop the turbocharger to get the manifolds swapped? Any other details that you can point out?
Whats the thinking on how tight the manifold stud nuts should be? If they are too tight, then stesses from thermal expansion cannot be releived, something has to move. Then studs will break from shear loads. Often the flange of an exhaust header is saw cut between ports to some degree to reduce stresses from the expansion of a continous flange. If this is not done, the welds of a tube header can be over stressed. With an aluminum head, there is some expansion of the head that will reduce the stresses from the flange expanding. But I still see too many reports of busted exhaust studs.
The need to create equal length tubes in a header is not a critical objective for a turbo header. As the header has a major objective of been quite short and compact, the contortions and tight radii create flow restrictions themselves. So I would prefer to see equal length tubes a secondary objective and smoother less contorted flow paths as the major objective. When I see well done tube header, I am mostly impressed that someone was able to get at all of the man circular welds in the middle of it!
posted by 65.68.10...
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