exhaust length - how does it effect performance

[16bit]

I was wondering if I was to just have a straight pipe that exits out the back on the passanger side (ie no right angle bend then muffler then another right angle bend) how it would effect the engine? Would it reduce torque? i can't see a performance gain but would I lose anything? this would be a track only thing.

[wun911]

less weight?

[rossint]

A guy was doing this at the last paractice day I went to on his Renault Clio. Unbolt the standard muffler and bolt on a straight pipe. It really free'd up the standard system removing the restictive muffler and sounded great but I'm not sure how it would effect an aftermarket system, would you loose to much back pressure?

[greenMachine]

Don't forget the dreaded noisemeter....

[16bit]

i c - well it would still have the hi flow metal cat but no muffler. so the cat would create some back pressure. but would it be enough? i suspect not. thanks for the clear up.

[Fabio]

4 stroke engines are most efficient when there is no back pressure in the exhaust, and having the air/fuel mixture entering in to the exhaust will help ensure that the cylinder is empty of exhaust gases and full of the air/fuel mixture. Also see here for more info on exhausts.http://mx5cartalk.com/phpBB-3.0.2/viewtopic.php?f=76&t=22540

[Okibi]

... Ditch the cat...

Go a high flow cat, i'm pretty sure there's a massive fine if your caught running your car without a cat.

[greenMachine]

... zero back pressure and that is not a good thing as without some back pressure & a straight pipe on a small displacement engine you will loose all torque. ...

Any source for this info? Smells like a crock of the proverbial to me, but I don't build high hp engines for a living either....

What I do know is that my race engine will have the free-est flowing extractors, no cat and straight pipe to a flow-through muffler.

And don't get me started on the relationship between torque and power

[kazjim]

This old chestnut keeps coming up ... "You have to have back-pressure" ....

no, you dont.

It all started when grandad too the exhaust off his FJ, the thing ran worse and ended up burning valves....

Yep, that will happen every time (See Graemebc's post for symtoms) - the REASON is because the engine hasnt been tuned to reflect the change in volumetric efficiency...

an open exhaust has different characteristics to a muffled 'street' exhaust, and as such has different tuning requirements ....

its all to do with the cam profile, if you are running a race car, your torque peak will be much higher up the rev range, and you will have a lot more overlap - yes, you WILL loose fuel out the exhaust ports, this is going to happen, live with it ...

I could go on for hours (Airflow was one of my specialties during a past life as a mech engineer .....) but I think I'll just let this COW sit here for a while ....

J


PS - Case in point, how many top-fuel dragsters / V8 supercars / off-shore racers have mufflers / backpressure ? none, they are tuned to suit.

[16bit]

ok so I already have extractors - hi flow metal cat and a straight throw muffler. i will just be replacing the muffler with a straight pipe - still cat. might give it a go and see what happens.

[hks_kansei]

I say do it, head to a dyno day and have two runs, one with the stock pipes, the other with the straight.