The age old backpressure question! Please help

[ryan]

Please help me clear this up with a mate of mine. He insists i'm not letting my engine 'breathe propelry due to my big fat exhaust pipes on my Dep 2 exhaust.
He said that after 15 mins switching the ignition off, there is still smoke coming out of the exhausts. He says that I have too little back pressure and this will be bad for my engine etc. So is he right?
One thing he did point out to me was that Rally cars, Touring cars etc do have 'peas shooter' exhausts compared to ours, so why is this?
Surely these cars are designed to run to gain as much torque/bhp as possible.
I did argue with him for a while over it, but he is a mechanical engineer, and I work for the NHS , so i'm a bit out of my depth.
I cant really imagine i'd get any gains from putting the std system back on
I know that some torque is lost when an n/a loses back pressure (by de catting?), but how about a turbo?
Anyone explain this in laymans terms and tell me if my mate is right/wrong
Thanks guys/girls

[Jim_Mr2]

I was under the impression that with a turbo, you don't want back pressure. The more free flowing the better.

[^Trickster^]

Turbo cars want no backpressure, they get plenty of that from the turbine side of the turbo, you want the gases away as quick as possible. Your mate is talking rubbish

Graeme

[The Morris]

Dont understand that, but you need some backpressure to avoid boost creep

[^Trickster^]

Back pressure to avoid creep is a bodge way around it that people use by fitting cats etc, a well setup turbo and wastegate system will not creep.

Graeme

[BLOODUK]

I havent had boost creep never seen it happen, and my car isnt exactly in top form atm..

[The Morris]

remove the cat fit a downpipe to your ct26 and see what happens, mine went from 8psi to 19 psi above 4000rpm

[^Trickster^]

Thats becuase the ct26 wastegate is tiny, i have a free flowing mongoose exhaust with external 38mm wastegate and i get 0 boost creep.

Turbos spool easier when theres less restriction, my upgraded ct26 crept with a 3inch aussie downpipe.

Graeme

[The Morris]

Thats becuase the ct26 wastegate is tiny, i have a free flowing mongoose exhaust with external 38mm wastegate and i get 0 boost creep.

Turbos spool easier when theres less restriction, my upgraded ct26 crept with a 3inch aussie downpipe.

Graeme

I had to change to the single port TT turbo to stop mine, expensive way round

[^Trickster^]

Yeh, the ct26a? they have a larger single wastegate and the housing is alot less restrictive apparently.

Graeme

[The Morris]

Yeh, the ct26a? they have a larger single wastegate and the housing is alot less restrictive apparently.

Graeme

According to TT, they're exhaust wastegate creates more backpressure, thus prevevting boost creep. I ported my twin wastegate ct26 exhaust housing but it made no difference

[^Trickster^]

not too sure on that one, but i doubt it, a bigger wastegate causes less backpressure, its bigger so the gas vents through it instead of the turbine causing more boost, the bigger the wastegate, the better the control.

Graeme

[DeusMR2]

the best exhaust for a turbo is no exhaust at all!

[BLOODUK]

the best exhaust for a turbo is no exhaust at all!

I heard that before on a skyline, wow still sounds the dogs!

[ryan]

Turbo cars want no backpressure, they get plenty of that from the turbine side of the turbo, you want the gases away as quick as possible. Your mate is talking rubbish

Graeme

Thanks Graeme, i'll let him know

[jonno]

Please help me clear this up with a mate of mine. He insists i'm not letting my engine 'breathe propelry due to my big fat exhaust pipes on my Dep 2 exhaust.
He said that after 15 mins switching the ignition off, there is still smoke coming out of the exhausts. He says that I have too little back pressure and this will be bad for my engine etc. So is he right?
One thing he did point out to me was that Rally cars, Touring cars etc do have 'peas shooter' exhausts compared to ours, so why is this?
Surely these cars are designed to run to gain as much torque/bhp as possible.
I did argue with him for a while over it, but he is a mechanical engineer, and I work for the NHS , so i'm a bit out of my depth.
I cant really imagine i'd get any gains from putting the std system back on
I know that some torque is lost when an n/a loses back pressure (by de catting?), but how about a turbo?
Anyone explain this in laymans terms and tell me if my mate is right/wrong
Thanks guys/girls

Mechanical engineers often struggle with auto engineering, its a different discipline.

Strictly speaking "backpressure" has little to do with it on an NA engine, the issue with exhaust gasses is not letting them back inside the cylinder when the exhaust valve is open, exhaust gasses mixed with a fresh charge is bad for power and efficiency. The exhaust manifold is crucial in this regard but also so is the first length of exhaust, since the "wave" tends to bounce back a shockwave from the first thing it hits, if this wave returns to the cylinder when the exhaust valve is open it can cause the exhaust gases to go back inside the cylinder. So, the real problem is designing an exhaust that is able to get rid of all of your exhaust gases quickly without causing any standing waves that cause the exhaust gases to get back inside the cylinder.

Rally cars dont generate any real power, its all low revs torque so therefore they dont actually generate huge amounts of exhaust gases. If you look at something that does generate lots of power like an F1 car, you will also notice huge and very short exhausts that are aerodynamically designed to actually suck the exhaust gasses out of the pipes. Dont get confused by the tailpipe diameter, the important bit is the actual exhaust pipework leading upto it.

On another note, forced induction cars absolutely hate any form of exhaust restriction, the turbo gets all of its power from the speed of the exhaust gasses, once they leave the cylinder they have to go through the exhaust wheel which drives the compressor - if your exhaust is too small it will cause even more resistance to the exhaust gasses and a pressure will build up, this pressurised exhaust gas is highly likely to make its way back into the cylinder on the next stroke - this causes high cylinder temps and also increases det.

Golden rule? Dont buy a cheap exhaust.

[ryan]

Thanks Jonno! Thats just the sort of info I was looking for.

[crazylegs]

let me throw one in for the non turbo boys.you can acheive better power on normally aspirated car if the exhaust is not a drain pipe. on some set ups because the induction is not forced the gasses escaping fron the exhaust valves need to be travelling quickly. the speed of the gas moving down the pipe creats a sort of vacuum and draws the following gas out of the cylinder.i`m sure there is a good name for this but cant remember.if you have a larce pipe the gas slows down and effectively chokes the engine. so on na motors the 6" monstrosity is even more laughable than it looks.anything over about 2" is a complete waste with the sort of power the na`s acheive.

[Fizzy]

Ive read some daft answers on here but some of these take the biscuit!! If you were daft enough to run your ct26 with no (zero!) backpressure two things would happen, firstly your turbo would leak oil into the intake, and secondly it would soon thereafter fail!.