Myth?

[mechanixfetch]

I've heard that backpressure in your exhaust increases torque. This seems to me that it cannot possibly be true and just hamper the power of your engine by not allowing exhaust gasses to flow freely after combution. Is there any truth to this Backpressure = Torque equation?

[mechanixfetch]

Ah Crud! wrong fourm. Can a moderator please move this to the right place? Thanks

[megotmea7]

to answer your question you first have to understand the dynamics of exhaust velocity and flow. lets take your average 2 liter n/a 4 cylinder with a redline of say 8000rpm (warning this may take a while...) the stock exhaust is halfway decent its no huge in diameter but it flows good enough to make say 200hp at 6500rpm and ~140lb-ft at ~3-4000rpm. now if you take out the restrictions in the exhaust (cats/muffler) you'd get a slight bump in torque and horsepower because you have more overall flow and a freely breathing engine makes more power/torque. the backpressure=torque thing is based off the misinterpretation of backpressure and flow. say you swap on a 3" diameter exhaust all the way back on that bad boy, youll get good top end power but low end torque will suffer, why? because you took out the backpressure? NO, its because the pipe is too big for the engine to breath with effectively. pressure=velocity take a high volume of air moving at a low rate of speed and compress it and the velocity will increase (this technique is used in wind tunnels to achieve high speed simualtions with low fan speed)
the same principal applys with your exhaust, your bigger pipe has a lower exhaust gas(EG) velocity so your engine cant make the exhaust flow till its really spinning therefore making power up in the higher ranges. put say a 2" pipe on there and your top end power might suffer but overall yourpowerband would be wider. its all about sizing the exhaust for the aplication. if you want to know more in detail just ask. also its slightly different on a turbocharged car as the EG velocitys are higher than on a n/a car and different things can be accomplished with different sized exhuast. perfect example is rally cars...

[crisis]

megotmea7 may be able to expand on this but I believe that the resonation of the gases in the exhaust system help to draw the gases out which also reduces back pressure. Tuned length extractors are designed to achieve this.

[bum-man]

i think this applies better to 2-stroke engines.

[mechanixfetch]

Thanks I think I understand now. The enlightenment is much appreciated.

[-datsun 1600-]

So the back pressure is kind of a current and its pushing the gases out and if its too big theres no force to help push it out?

[megotmea7]

no, back pressure is generaly the restrictions in the exhaust, with a large pipe(for the application) flow is reduced because there is more almost stagnant air(a high volume of slow moving air which decreases overall flow) that cant be pushed out till the engine is wound up thus top end power increases but lower down the torque may suffer

[-datsun 1600-]

ohhhh i see cheers that straightend things out

[raz]

Any Brands that you recommend on a 2.0L Alfa 147?