Exhaust tail pipe size

[woodyracing]

Hi.
I've got a Toyota MR2 mk2 Rev3 which I am going to be racing, currently it has a standard exhaust but I am fitting a cat replacement pipe and also removing the existing system (from the cat back) which has 2x 2.5 inch tail pipes.
My new system will have a single tail pipe, but what diameter should the tail pipe be and how will it affect performance and sound ? I was thinking 3, 3.5 or 4 inch.

thanks
Andy.

[Al Weyman]

4 inch! What have you got in that thing a V8?

[woodyracing]

Originally Posted by Al Weyman

4 inch! What have you got in that thing a V8?

no just a 4 cyclinder, 2 litre, non turbo

the pipes are about 2.5 inch diameter, so I'm just wondering about tail pipe size, popular twin exhausts can be 3.5 inch for the normally aspirated car.

[GreenSaxo]

i would say at least 6"

or something like this :P

[Notso Swift]

Just because it has 2 x 2.5" at the moment doesn't mean that it needs them

Search the net and you will find some information that will give pipe flows. If you motor is reletively standard seriously doubt you would need more than one 2.5" pipe (providing there were no other restritions) If it is a full race 2litre (275hp) I doubt you will need more than 3".
You will need to know how much air you are pumping and how many revs. My guess, Car is 2 litre, Twin cam 16V so max VE will be about 85% and revs will be 7200. Remember VE will be lower at high revs, and you you don't need to look at ultimate revs, but max "normal".
If you go too big you will (maybe) gain 0.5% in power but will lose 5% in torque through the range, so you will be slower.

Or you could go the max power route.

[woodyracing]

hi thanks for the reply, ive no idea how to find out the air flow but you're right it does rev to 7200. The engine is completely standard apart from decent spark plugs and ht leads. this is for racing not show so I will go with whatever is best.

that saxo is terrible btw

thanks.

[Al Weyman]

No the Saxo is cool, they are rocket boosters not exhausts! I still cannont see you need to be bigger than 2.5 inches personally. I doubt it will make a jot of difference going any bigger!

[Denis Bassom]

There is absolutely no advantage to be gained by the tail pipe being a large diameter than the system. All it is going to do is weigh more!

[Chris Y]

I would even say a 2.25 inch pipe may do you if it's just a standard power plant. With a smaller diameter pipe you may get some benefit from exhaust pressure waves

[ChironWSC]

I think about 2.25inch would be about right certainly no more than 2.5inch. We fit 2.25inch on most of our 2ltr engines in our race cars they are between 230 and 280bhp.

[woodyracing]

ok thanks for the replies, much appreciated. The output of my engine is about 176bhp.

cheers
Andy.

[Al Weyman]

Only 176! Stick a 1" mini system on it then! :-)

[woodyracing]

well its 176 plus whatever the exhaust and K&N give me.

[JohnMiller]

176.1 then!

Actually, the K&N will be likley worth a few.

[woodyracing]

any improvements would be good but this is production standard spec (apart from the few bits i've mentioned). Some exhaust manufacturers claim 7 bhp on these cars.