Amazing! That's the first time that I've EVER heard anyone online explain exhaust flow correctly. Usually its just, "Backpressure ownz joo, yo needs it fo torque."
Now if we could just get people to talk about HP and torque correctly this discussion could really be intelligent and informative.
If nothing else, let's just try to avoid using "torque" as this catch all term for low end power. When you slap on a huge exhaust, you don't lose torque. The engine probably makes more, it just makes it at a much higher RPM. Even more so than the misunderstaning of exhaust flow, its the misunderstanding of HP, torque, the power band, and how they relate that causes the exhaust size discussions to turn into mass stupidity contests.
As for STREET (that means full car length) exhaust. Just think of it like an intake manifold. The flow is optimized to make torque at a certain RPM. Smaller sizes make more down low and less up high. Larger sizes make less down low and more up high. The trick is to find the best balance of flow (biggerness lol) and velocity (smallerness). But it has NOTHING to do with backpressure making torque or the load on the engine. It has only to do with optimized scavanging and velocity and what RPM that occurs at.
Think of it this way.
A smaller exhaust will have a higher velocity (ideal gas law) but less flow (common sence). At low RPM, the small pipe is easily filled and the velocity is good. That decreases backpressure and makes good low RPM torque. But as RPMs build the engine is exhailing more than the exhaust pipe can smoothly flow. That makes backpressure and kills high RPM torque.
A large exhaust will work opposite. It will have lower velocity but more flow (at any given RPM). So at low RPM the huge pipe isn't filled enough for the gas to have good velocity (ideal gas law again) and that makes the gas tumble. That reduces flow and creates backpressure, killing low RPM torque. But at high RPM the large pipe is now filled correctly and the velocity is high. Now you have very little back pressure and that frees up high RPM torque.
So they both have their uses. Again, sizing is key. Just like sizing a turbo to fit the engine's flow (CFM), your driving style, and the operating RPM. Its the same for exhaust. But notice the VERY KEY part of all that explaination. In every case backpressure is what kills the power. This is why race exhaust (a large diameter dump pipe) usually makes gains everywhere. When the exhaust is extremely short the ideal gas law doesn't matter. The pipe is so short there's no time for the gas to tumble, back up, speed up, whatever. The only time you have to worry about tuning for velocity is when the exhaust is the full length of the car and you need to keep it flowing out a long pipe quickly.
In conclusion.
BACKPRESSURE DOES NOT MAKE TORQUE.
I hope that was all helpful and will prevent having this discussion every other week on this board (like all the other car forums).
