iTac

I just read this great article on back-pressure and the common misconceptions that usually tag along. Its a little on the long side, but well worth it.
http://www.mustangevolution.com/forum/f312/t296628/

jonbar87

Yep, read that a few years ago. It's so true. So many people don't understand that with EFI, the need for backpressure is no longer a must.

tamnalan

Very interesting! He hits the idea that a modern internal combustion engine is complex - spark, fuel, engine speed, temperature... a bunch of things need to work together for the engine to produce good power. Fuel injection, sensors and computer controls give the engine more self-regulating ability to maintain a good fuel-air ratio than the old carbureted engines had. On the other hand, changing only one thing "too much" may still push the engine beyond its ability to self-regulate air, temperature and spark.

The exhaust gas coming from a particular cylinder's firing "pulse" has mass. Enough velocity is needed to keep the exhaust gas pulse flowing reasonably smoothly before the next cylinder's firing and helps create some vacuum in the pipe behind it to assist the next pulse's flow. The momentum helps all the exhaust gas move along.

Not enough momentum? Pipe is too big for desired RPM range and the engine is inefficient. Too much momentum? Too much velocity means the pipe is too small and drag is increasing between the gas and the pipe walls... the engine is inefficient.