
ryan wrote: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.
