how important is back pressure?

[pinske]

i know this is an age old argument but i want to know how important back pressure is, recently put a full straight through on my N/A and my dad keeps going on about getting blowback through the valves and that their only really any good on turbo engines.

that said ive heard that a straight through on an N/A is fine, ive also felt a decent increase in power through the rev range and picks up A LOT quicker.

can someone settle this for me. straight through on an N/A, yes or no?

Sam.

[Foggy]

Depends from car to car. I know altezzas make more power with the cat in. I cant see much of a difference in a 1300.

[morgey]

Depends on the bore of pipe, if its still quite small 2" or under you will still have a good amount of back pressure anyway. If you were to stick a 2.5" straight through on then expect to loose loads of low down and midrange.

Are you running an after market exhaust manifold? These are also restrictive so cause back pressure as well.

I have a 2.5" on my 1.6 and people tell me its too big yet i made above stock power and torque.

Fitted a cat but put open filter on my throttles and i lost some midrange (expected with the open trumpets) but in theory the cat helped minimise the midrange loss due to the back pressure created. But i made more power and torque again lol

On a 1.3 that only revs to 6250rpm i doubt your'll affect it much to worry about.

[pinske]

Depends on the bore of pipe, if its still quite small 2" or under you will still have a good amount of back pressure anyway. If you were to stick a 2.5" straight through on then expect to loose loads of low down and midrange.

Are you running an after market exhaust manifold? These are also restrictive so cause back pressure as well.

I have a 2.5" on my 1.6 and people tell me its too big yet i made above stock power and torque.

Fitted a cat but put open filter on my throttles and i lost some midrange (expected with the open trumpets) but in theory the cat helped minimise the midrange loss due to the back pressure created. But i made more power and torque again lol

On a 1.3 that only revs to 6250rpm i doubt your'll affect it much to worry about.

a right, so nothing to worry about then running 2 inch from mani back, stock manifold, 3.5 inch jap cannon on the end.

[_shaun_]

It's a common kid conception you want back pressure in actual fact it's exhaust gas velocity you want back pressure is just a by product if this. I don't realy interest myself in the n/a aspect of tuning but for example on a turbo car you want to try maximise the exhaust gas velocity by running smaller diameter exhaust manifold runners to spool up the turbo as fast ad possible and then run the least restrictive exhaust you can. Althought I could be wrong but that's my understanding

[morgey]

^^ that makes sense on a turbo tbf.

N/a is a lot more complicated. Diameter of manifold runners and length are important. Then the size and length of the collector should be matched then the rest of the exhaust. Plus do you go 4-2-1 or 4-1 then depending on the exhaust setup you have, it changes the pulse waves the engine produces which are what can make throttle bodies really effective. Match these pulses to the length of the trumpets and you can make some crazy torque patterns.

Way too confusing, stick to your guns mate if u like it leave it. At the end of the day the best way to get performance from the n/a's is strip the bitch. Or find a 5efhe and dump that in or go crazy and stick a 4age or 2zzge in it ;)

[rmsnoel]

Keep the standard manifold, downpipe and cat then go 2inch from there back. Unless your fitting a 4branch but u still need a box where the cat is. Its important where the first box in the system is, its a tuned length because thats where the car thinks the exhaust stops in terms of pulse and velocity.

A 4efe with a peak power at 4500-6500rpm is best fitted withthe 5efhe manifold and downpipe and the cat in roughly standard postion.

The cat can be gutted out..

Search four stroke exhaust length calculator.

[Flynn]

I've just fitted a straight through system onto a 4-2-1 aftermarket manifold and I'm defo down on power....gonna stick the cat back in tomorrow and see how that effects things? Sounds lovely on idle but the expression "all show and no go" couldn't be truer!