Converting exhaust sytem to single pipe

[Bristol_Jer]

Just toying with the idea of converting my exhaust system to a single pipe instead of twin, mainly for ease of assembly and should be a bit lighter (going to use Jetex parts to make the system).

The m30 system should be twin 2" pipes, so what single pipe size should this equate to, does anyone know?

[jmc330i]

My maths is probably way out, but I think a single pipe of 2.75" will be a little larger than the twin 2".

[Bristol_Jer]

Ok thanks, is it just a case of working out the total circumference of the pipes, or is it more complicated than that?

[German-Whips]

Ok thanks, is it just a case of working out the total circumference of the pipes, or is it more complicated than that?

It would be in terms of cross sectional area. As essentially the exhaust pipe is a "nozzle" from which gasses escape. In order to maintain flow characteristics the cross sectional area of the nozzle must remain the same.

The path which they take can be different, you can have 4 tailpipes with the combined area of 2 without detrimental effects.

[German-Whips]

In that case your single exit pipe should be exactly 2.828" in diameter.

[jmc330i]

I just worked out the area of a circle.

2", 3.141x1x1 = 3.141. For the twin pipe that's 6.282" squared (my phone won't do the squared symbol)
2.75" is 3.141x1.375x1.375 = 5.93
3" is 3.141x1.5x1.5 = 7.06

So a 3" pipe would be slightly too big, the 2.75" slightly too small - sorry got that wrong in my first post. As German-Whips said, 2.82 would be perfect, but good luck finding that size at Jetex
I think they do 2.75" now, so I'd probably go for that.


As a side note, I was once told that an exhaust system should get slightly smaller as it gets towards the end. Something to do with as the gas cools down it gets less dense, so if the pipe is the same size all the way through, the gas starts to tumble and slow down. By starting to restrict the pipe as the gas cools, it will hold its velocity throughout the pipe. Fairly complicated to work out where along the system and by how much the pipe needs to get smaller though.

[HairyScreech]

yep, works out to 70.7mm diameter, so may be available as a metric size just probably not from jetex.
Area difference to a 3" tube (75mm) is 3926mm^2 vs 4417mm^2. A 12% increase in CSA.

[Bristol_Jer]

Thanks for the replies, much appreciated

Would going slightly larger (3") be such a bad thing? The plan is not to leave the engine standard, so with a better exhaust manifold, better induction, and a better flowing head would that be able to make the most of the extra pipe diameter, or is it still best to keep standard diameter?

[e301988325i]

My BTB goes 6-2-1 running a single 2.5" pipe from the back of the gearbox on my M20 b25 (planned b28). It's much lighter, although the bigger pipe is tight around the rear suspension area & of course a 3" pipe much more so.

[redcar]

Doesn't twin pipes have a far greater surface area than a single pipe also?

I'd imagine 2.5" single would be more than enough. After all, that's what 300bhp turbo cars run.

[e301988325i]

Doesn't twin pipes have a far greater surface area than a single pipe also?

I'd imagine 2.5" single would be more than enough. After all, that's what 300bhp turbo cars run.

Too much back pressure = Fuel air mixture not getting in hence the term forced induction

[zaust]

Mine goes 6 into 2 into 1 and straight through. I use 2.75 pipe and it is perfect.

[Green]

I was considering getting a single pipe exhaust for my M52 when the time's right, was wondering there any benefits worth keeping the twin pipe system?

[redcar]

I was considering getting a single pipe exhaust for my M52 when the time's right, was wondering there any benefits worth keeping the twin pipe system?

with the twin pipe with an x-pipe it's easier to run the car a lot lower (only if well fit!)
Twin pipes tend to be quieter as well.

Then again, you won't have any problems with ground clearance with a 2.5'' single on an e30. Can't comment much for larger sizes but if you want to keep your heat shields then I can't see fitting greater than a 2.5'' will be very easy. Especially as the 2.5'' is very very close! I can't imagine you would need anymore than a 2.5'' on an m52.

[steve_k]

What would the effect off fitting an X pipe be on an M30 with a ported/flowed head, better induction & a decent 6 branch manifold be?

[redcar]

Supposedly better exhaust gas scavenging which may equate to higher peak power.