The nasp engine and back pressure.

[VVC-Geeza]

Does a tuned nasp engine require exhaust back pressure to perform at it's peak?

I don't necessarily mean the standard 120 bhp 1.8K, although it's all relative.I'm thinking more of a tuned version of around 190bhp as in my VVC coupe.

Would a decat pipe increase or decrease output?

Some say the more freeflowing the better others say too freeflowng and it slows down the exiting gasses and looses power.Both sound reasonable arguements.

Has anyone done back to back RR runs with/without cat?

[Ravinder]

This thread may help

http://forums.mg-rover.org/mgf-mgtf-...ml#post1885537

[rrobson]

I did quite a bit of research into this some time ago.

What I gathered basically was you need some back pressure in a nasp, especially something like your vvc because what back pressure does is restrict the gasses path out of the engine. This increases heat in the system which increases energy thus speeds up the velocity of the gas. With higher velocity comes lower pressure (Venturi effect) and actually pulls more gas out of the engine. This obviously helps with flow through it and therefore power as the quicker you can get rid off the burnt gas the quicker and more fresh air and fuel into the cylinders.

A sports cat would theoretically lower back pressure as it is less restrictive but I can't advise whether or not it would be detrimental to performance. If the rest of the exhaust is standard I'd imagine it would still be fairly balanced as your shoving more gas through.

There is however a fine balance between too much or too little. A good exhaust place should be able to advice you but you'd be able to work out yourself the size of the bore. I don't know where to start but if you know the displacement, temperature and revs it should be a relatively simple equation.

[minimutly]

Increasing back pressure increases gas flow, well you'll have a job to calculate the theory on that one!
Lower backpressure reduces pumping losses, reduces combustion, or more accurately post combustion temperatures, and will, provided the camshafts are optimised for it, increase mid and top end power.
Note that i mention optimised cam timing. What can happen with a drastically reduced pressure is that the engine scavenges the exhaust gas to the extent of sucking out some incoming mixture. This will cost you power.There is another potential issue at lower rpms where the inertia in the exhaust system helps scavenging, a larger pipe will reduce the velocity of the gas, thus potentially reducing the scavenge effect.
So, although i disagree with the reasons given, its highly possible that at certain points in the rpm range, a larger, and also lower backpressure exhaust could cause a reduction in power. I would say that in all my years tuning road and competition engines, i would allways look at improving the exhaust as a cheap step in increasing performance, lower backpressure good, but too big is allways bad.
If your car is std, leave the exhaust allone.

[VVC-Geeza]

Originally Posted by minimutly

Increasing back pressure increases gas flow, well you'll have a job to calculate the theory on that one!
Lower backpressure reduces pumping losses, reduces combustion, or more accurately post combustion temperatures, and will, provided the camshafts are optimised for it, increase mid and top end power.
Note that i mention optimised cam timing. What can happen with a drastically reduced pressure is that the engine scavenges the exhaust gas to the extent of sucking out some incoming mixture. This will cost you power.There is another potential issue at lower rpms where the inertia in the exhaust system helps scavenging, a larger pipe will reduce the velocity of the gas, thus potentially reducing the scavenge effect.
So, although i disagree with the reasons given, its highly possible that at certain points in the rpm range, a larger, and also lower backpressure exhaust could cause a reduction in power. I would say that in all my years tuning road and competition engines, i would allways look at improving the exhaust as a cheap step in increasing performance, lower backpressure good, but too big is allways bad.
If your car is std, leave the exhaust allone.

Thank you for that.

The VVC optimises cam timing throughout the rev range.My coupe is heavily engine modified including a 4-2-1 tuned length SS exhaust manifold and freeflowing SS exhaust.

I guess the only way to proove if a cat helps or not is to do back to backs on the RR.Perhaps a sports cat would be the best option.