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Our 1.75" Hedman headers merge into a single 3" exhaust system with a 3" catalytic converter, and it showed a load of back-pressure, registering nearly 6 psi when we gauged it. Pushing against that kind of resistance will cost plenty of power. To find out how much, we uncorked it and suddenly had 326 hp at the tire (+15 hp). So much for the muffler-man's wive's tale that an engine needs back pressure to make torque. We were up everywhere.