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Originally Posted by taxonomy  I think if you put a 1 HP shop vac in blower mode to the intake and of an exhaust it would blow through pretty easily, even a "tight" stock exhaust. It's not like you're using up gobs of HP forcing air through teh DOT pipe that you suddely win back when you take a sawsall to the muffler. |
You can't visualize the system like that for a few reasons. First, a shop vac provides a steady state flow as opposed to pulses. Second, the shop vac doesn't flow enough air to mimic an engine (especially larger ones). Third, a shop vac doesn't produce enough pressure to mimic an engine.
The muffler isn't the only portion of the exhaust that restricts flow. Messy welds, improper bends, catalytic converters, etc., are all part of the equation. You also use more HP than you realize forcing exhaust out of the engine. Modern 600's are lucky to make 50lbs-ft of torque which is substantially less than what an average person can muster with a wrench. Even though their torque is low, they make large HP numbers because they rev extremely high. At high RPMs, a small change in torque can translate into a significant change in HP. If a 600 is producing 45lbs-ft at 13K it is making about 111HP. A small increase in torque to 47lbs-ft at 13K translates to 116HP which is a rather significant gain.