Wow. Guys, it's like this: It's never the back PRESSURE that helps. When smaller exhausts are delivering more power, it's because air is a spring and has weight and velocity and momentum and pipes have resonance...
So when you have loads of low end torque on a smaller pipe, it's not because the pipe is PRODUCING MORE PRESSURE, it's because the pipe shows resonant behavior and is sucking (scavenging) the exhaust out of the cylinders. The whole system is a huge resonant behavior, not a simple 'flow resistance'. The actual 'flow resistance' is never good, but you cannot totally destroy the resonant behavior of the system in attempts to have less flow resistance...
I know this not because of being a mechanic but because I'm a sound engineer with years of experience designing speaker enclosures... the phenomenon is not unlike trying to get the most loudness out of a bass reflex enclosure, you don't simply use larger and larger pipes or all your LOW END will be destroyed and the peak reinforcement will move upwards in frequency- just as the power peak moves upwards in RPM as you make the frequencies of the resonant system higher.
And smaller pipes have LOWER frequency resonances at the same length than larger ones- but way more flow resistance at the same time. You try to get away from the flow resistance (w. larger pipes you quickly do that) but as the pipes get larger and larger, the resonant behavior goes up very quickly. With a pipe big enough it's like there isn't any resonant behavior at all- like it's an open header into a large box.
You never want BACKPRESSURE, but you can't have this torque-giving resonant behavior without also accepting some backpressure. You could have a big vacuum cleaner on the tailpipe and suck ALL the backpressure away, and if the exhaust resonance was able to give you serious torque it would STILL be giving you even more low-end torque with the vacuum cleaner there. No backpressure, but still all the resonance.
Or if you had a crazily powerful vacuum cleaner on the exhaust port you'd be golden- always maximum scavenging and suction out of the exhaust ports. No such thing, though- but if there was, that would be the open-headers full torque fix.
The thing is, when you're setting up a resonance in a tuned pipe, the high pressure waves are matched by LOW pressure waves. You send a pulse of pressurized exhaust down the header pipe and exhaust, it goes shooting off and the valve closes and the momentum of the air continues but now the valve is shut. Depending on the size of the pipe, it will take some time for the air to reverse itself and come to equilibrium. If the pipe isn't large, there's still negative pressure at the valve when it opens again, aiding the start of the next pulse. If the pipe is huge, the air reverses itself really fast and there's equilibrium when the valve opens, not suction.
Sorry... it drives me crazy when I see this argument come up and people say things that are confusions- it's not about the back pressure at all, but you CANNOT just have open headers and low end torque too. You're stuck with some back pressure when you do it right, that's all.
Maybe there should be a big vacuum cleaner mod- so you can have no back pressure but still have smaller pipes for maximal low end torque
*insert sarcasm emoticon here*
