regularman said:
More backpressure will generally limit your power a bit when absolutely wide open throttle but give you a little help on low end torque.
A wide open pipe allows some relatively cool air to go back up the pipe and get pulled into the cylinders.
Both of these are old wives' tales.
Gas velocity helps low end torque, not back pressure. A smaller dia longer pipe will give more low end than a larger dia shorter one. This is due to the gas' inertia creating a lower pressure behind each pulse as it travels down the tube helping evacuate the cylinder behind it.
Torque increase with volumetric effiency, not back pressure. Back pressure does not increase torque as it hurts, not helps cylinder filling.
People falsly associate back pressure with this as the smaller pipe creates more of a restriction on top end.
Yes, an eight inch zoomie may introduce some cold air back up the exhaust on a top fuel dragster at idle due to enormous valve overlap. Maybe, and I do say maybe; a short zoomie on a bike MIGHT too. A six foot pipe on a street four cylinder engine won't.
Jim, his center baffle is a plug on the softail type mufflers.
I, as well as everyone else who hears my car at idle, is shocked to learn it's a straight through muffler. It's just as quite as stock.
The 1500 may idle quiter than a 1275. My friend's 1275 w/ sport muffler is louder than mine. Both systems are straight through. If it's too loud for you now, try to bump your timing up just a bit.
I've said this over and over again. The stock Harley center baffles MUST BE PUNCHED OUT to allow proper flow! There were desinged to flow only a little more than HALF of the volume you are now putting through it at any given RPM.
This is way too much back pressure for these engines. Mark the stick before you push it out. That way you can reinstall it if you think it's too loud.
If back pressure induces low end torque, you should be snapping axles at idle.
Kim, the reason you saw fuel consumption increase w/ straight pipes is due to the pulses evacuating the fuel charge through the combustion chamber out the exhaust purging spent gases. The cylinder still gets it's fill, just a little bit is wasted. "Thy cup overfloweth" kinda thing.
Back pressure will reduce/eliminate this, but it doesn't increase power.