Quote:
Originally Posted by Domman56
Um not sure you can count this accrate statement either When TURBO cars have less restriction they often produce more boost therefore producing more HP and Torque Many turbodiesels pick up gains in this way so it May be more efficient to reduce it in turbo cars
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Usually there is large piping after a turbo to increase the pressure differential across the exhaust turbine. This lets it spool up faster and helps it work more efficiently. This is also why turbo mufflers usually provide much less restriction. The fact that the turbo is in the stream somewhat makes up for these mufflers' lesser sound absorption.
Back pressure doesn't make torque, it is flow restriction. This can help control flow in cars with cams with larger overlap by restricting exhaust speed out of the cylinder at low rpm, which improves vacuum on the intake stroke. Most headers are tuned for high rpm scavenging because most people who buy them are looking for more HP, but if you tune for low rpm scavenging (which would mean very, very long header primaries) and have a cam that supports low rpm power (less overlap) you will have good vacuum and little back pressure. In fact, if your headers are tuned correctly you will have negative pressure at that rpm you chose, and that's why headers make power in the first place. It will just be at your cruising rpm, and instead of the goal of making power it will be to lessen the work your engine has to do (exhaust helps the piston pull itself out and pull fresh air in).