Quote:
Originally Posted by jessrayo
Some backpressure helps the engine maintain boost pressure and keeps the power higher.
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This statement is inaccurate. Usable boost pressure is not positively affected by positive exhaust manifold pressure.
Ideally, the exhaust valve should be fully closed when the intake valve opens. So exhaust manifold pressure would be irrelevant during the intake stroke. If there is overlap in the timing between the closing of exhaust valve and opening of the intake valve, then only negative exhaust manifold pressure (vacuum) is benefitial. The optimal amount of vacuum needed in the exhaust manifold varies with engine speed.
Properly tuned headers are so valuable because they reduce the backpressure and heat that the exhaust manifold normally retains.