back pressure myth

[Bushbasher]

It's my understanding , and I'm no expert, but ; efficient scavenging is achieved with tuned length headers where each pipe from the exhaust port is the same length so that the pulse of hot gas that is expelled from the cylinders reach the collector in sequence one after the other and then progress through the rest of the exhaust system with the minimal impedence possable. Cheap pipes and cast manifolds of unequal length cause the pulses to run into each other which slows down the flow and creates too much back pressure for maximum power BUT will give good economy when tuned properly. This is where gas velocity comes in. The less restriction means the faster those sequential pulses can get out and the more air passes through the motor at any given moment and as we all know - "more in = more out = more power". If you're after maximum power at all costs then go as large as possable with your exhaust BUT, (and this is where the backpressure argument should be focused IMO), if you want a street car that will return half decent fuel economy and efficiency then you do indeed NEED some back pressure. This is because of the valve timing overlap on most modern engines where the inlet is opening before the exhaust is fully closed. Without back pressure to regulate the flow of escaping gasses then a free flowing and well tuned open exhaust will suck or "scavenge" all the exhaust gasses out then take some of the incoming charge as well before the exhaust valve is fully shut. This in turn wastes fuel and increases emissions as well as heating your exhaust excessively when that unburnt fuel ignites. The most extreme example is any alcohol or top fuel drag car with huge flames blasting out the exhaust at WOT, you get the idea. Any exhaust is a compromise between power and economy/efficiency. The less flow means less power but generally, especially with stock engines, better economy and efficiency. The thing with factory fitted exhausts is they are even more of a compromise because they're looking to mass produce for the smallest cost possable and still meet minimum requirements, that's why when you fit a decent set of headers and exhaust system you will always get more power and usually better economy.

As I said, I don't claim to be an expert and I stand to be shot down in flames but that has always been my understanding.


Bushbasher