exhasut back pressure

[jimbo1]

ok rite, ive always been led to believe, that for an engine to perfrom well, it needs a certain amount of back pressure to operate well, give good fuel economy. and that by decreasing back pressure, the engine loses torque, etc.

having been reading up on it on the web, ive seen sites saying, that the less back pressure there is, the better a n/a engine perfroms. so if i removed the cat on my car, removed as many silencers and power sapping bends etc as possible. get it remapped, the engine wud produce more power n torque all over the board?

if thats the case, then why did renault sport use a small diamter tubular manifold for the williams as it gave better torque than the 16v cast manifold and downpipe? is to do with the length of the headers as well?

can u make a quiet low pressure system, or is that impossible?

jimbo

 

[16V_Matt]

the back pressure is a myth for more info go to

http://wizz.proboards.com/index.cgi?board=tech&action=display&num=1037036138http://wizz.proboards.com/index.cgi?board=tech&action=display&num=1037036138http://www.sunfinder.com/sweepstakes/entry.php

theres some usefull info for what u r asking on there!

matt

 

[coolspot007007]

Heres a pretty simple test to do Jimbo, take your back box off and have a drive around for a day or two. The car will make hell of a lot of racket (its great) but the low down power just aint there, once you get going say 4k revs+ the revs seem to shoot up a lot quicker than normal, it really pulls hard. Fuel economy goes out of the window but the noise is mental, gives you a headache after about 10-15 mins

 

[Jellyhead]

What youre describing is a mismatch between that the ECU is expecting to see from its sensors and what actually "is".....

Pull the airbox off and a similar thing will happen......

Messing with flow rates either side of the engine will require a re-tune to make the most.

 

[BenR]

backpressure is a relatively misunderstood quantity, and alot of pub talk goes on about how you need lots of it for torque etc. but back pressure doesnt neccesarily mean resistance to flow. as tunned lengths with the right radii bends will make more power than a pure straight pipe.

anyhow, it works like overlap, overlap being the time in which both the inlet and exhaust valves are open as the piston is nearing TDC. anyway, little overlap gives great torque as at low rpm (and gas velocity) threre is enough time for the cylinder to fill, but not for the fresh charge to excape out the exhaust, but at higher rpms (gas velocity) there is simply not enough time for all the exhaust gasses to excape, and the inlet charge can use the inertia ( or rather relative negative pressure area behind the exhaust valves) to pull in more fresh cahrge, increasing the VE and power. so lots of overlap leads to top end power roughly and basiaclly. but teh trent for less duration and higer lift is more popular in todays cams.

anyway, backpressure works roughly like that low BP = lots of overlap, high BP = little overlap

we can go into all sorts........hope we do......CAPTAIN!