Could some one explain the importance of back pressure in the exhaust system If at maximum revs you had a pressure of 1 psi (just a made up number) then at half revs it would be none existant so why bother..

THe performance books i have read say it is a myth you need back pressure.

Think about the combustion chamber, if there is pressure in the exhaust system on the exhaust stroke it takes more energy to force the spent mixture out.
What you want is a high flow system which helps assist in this exhaust extraction.
That is where the black magik of tuned exhaust systems come into play, i.e. lets only consider pipe diameter; to small it restricts flow, to large you will have very low exhaust velocity.
Then we get into a situation where it can control the peak power and torque curves of the engine, thats when my head explodes...

Harley

Kevin,

Back pressure has been discussed a few times on here in the past - here's a thread that starts off asking for tips on how to separate rusty pipes & ends up as a debate on back pressure! It may be of some use?

viewtopic.php?f=7&t=727&hilit=exhaust+back+pressure

Neil.

Ta Neil ... have just ordered a silencer off ebay for a ford dexta tractor max diameter around 100 mm looks very similar to a 2cv one but bigger bore £26 delivered.

I certainly wouldn't lose any sleep over what effects back pressure - or lack thereof - would have on a 2CV.

But, as a general rule, the less the better. Running with no exhaust - or a through-flow type - will in theory give you a teeny bit more power as it 'breathes' more freely, and might also make the engine run a teeny tiny bit leaner, tho' not to the point where it should cause overheating. So, as far as I understand, you should get a marginal increase in power coupled with a fractional increase in economy with a freer-flowing exhaust.

Back pressure - or 'tuning' - is important with performance engines, tho', as it determines how each cylinder affects t'others and also any slight change in fuel mix would have greater consequences. Take the above with requisite pinch of salt as it's just a basic overview, and reflects my 'understanding' of it gleaned over years of fragments of info.

The myth of back pressure seems to be based on a misconception.

People stuff huge exhausts on cars and then notice that they've lost power. Surely that can't be right as bigger must flow better, and better flow must equate to more power? At that point the back pressure thing rears ugly head.

Actually what they've done by oversizing the exhaust is lost velocity. You need exhaust gas velocity so the gasses are drawn away from the combustion chamber. Ergo what is required is a pipe diameter that optimises the velocity of the exhaust gasses exiting the engine whilst minimising the back pressure.

Well put.

Harley

On my motorbike, they use to restrict the engine to 30mph by restricting the amount of exhast gas that can escape. Ie make it smaller, often done by a washer. In mine they put a little tab to restrict it, so what dd ido? Ground the thing of, welded the hole up, and welded the thing on for good measure so it looked legit

Now when i took my exhast of a few times, ( i put it on a bonfire once)
i put a hoover on the exhaust output of the engine, the revs rose on idle, and it idled a bit wierd. I didmt try revving, but forced exhaust gas at idle made it run with more revs

is it important?

not on this forum it would seem.

its complicated

pressure can be negative as well as positive

I dont know the feed in and out diameters of the silencer yet and dont forget it has a 800cc BMW engine.. also I have bought a lambda sensor and gauge off ebay for £66 I can not recommend it highly enough as it identified problems with carburration as they cropped up as I have tried 2 bing carbs , one bing carb and even a 2cv one