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!

[greenamex2]

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.

[Chatters]

I'm sure this would do the trick.

[Al Weyman]

Years ago I built a tuned 1750cc Ford Escort Mk1 van, anyhow it was a road vehicle so nothing too drastic on the cam but the only exhaust manifold I had lying around was a big old meaty full race 1700cc HotRod one so I stuck that on and I can say it spoilt the whole thing, the van was a dog to drive around London in until it got to about 5000 rpm and gobbed petrol, stick with the smaller pipes all the rest if just for Chavs. It lools like that particular sect of people has spread to the coloneys Chatters. :-)

Also i was having this conversation a while back with a certain Rover SDi racer of some repute and he downsized his system from twin 3" to a siamesed 3.5" because he recons it improved the torque out the bends and ended with a faster lap time. Oh and he runs a 5.2 litre i believe.

[Notso Swift]

Originally Posted by Al Weyman

stick with the smaller pipes all the rest if just for Chavs. It lools like that particular sect of people has spread to the coloneys Chatters. :-)

yeah but no but yeah!

[Chucky]

[Al Weyman]

You win Chucky, you have the biggest one for sure, do you have big feet as well as a matter of interest:-)

[JohnMiller]

Good, but shame exhaust is spelt wrong.

[JamesH]

Originally Posted by 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.

If you could 7hp just by changing the exhaust, surely Toyota would already have done it????? Filter yes, a few HP maybe, and maybe a few on exhaust but with lots more noise (not road legal anyway), but 7 - I wouldn't have thought so...

I'm with the others - size isn't important, it's what you do with it. Er, no, that's not what I meant.

James

[Al Weyman]

I don't get the filter thing either, if that was worth power and more power means more efficiency which if used correctly equates to better fuel consumption something of increasing importance, again why would Toyota not do it in the first place. I can understand chipping i.e. sacrificing power for economy/reliability/more emmisions, head porting and narrow valve seats(expence and loss of longlivity), cams (a compromise at best), balancing and blue printing, bigger intercollers/turbos etc. but simply sticking a different filter on even if by K & N I don't really buy.

[greenamex2]

Exhaust = Cost, noise and emmissions
Filter = Cost and noise

And I am sure the fact that they make money out of replacing these disposable parts has NOTHING to do with it!

[Al Weyman]

Surely a company as huge as Toyota could get a good price on an effiecient filter and if by fitting it made say 10bhp or 2 or 3 mpg extra then they would do it as that would sell more cars than the couple of quid it would cost extra on the sale price.