Exhaust diameters.

**Steve Webb** · 02-03-12, 20:50

The subject of exhausts has come up in my mailbox again.
So a quick question to you all, what would you say the recommended exhaust diameters for various power outputs?

I saw 270bhp using a 2.5"/63mm exhaust (removed Cats and single rear box)

What have other people found?

**bussone** · 02-03-12, 21:51

Very curious to see replies

**mpampis_** · 02-03-12, 23:36

I saw engines up to 400whp using 70 mm exhaust without cats. Everything depend on what power do you want. I think 70mm is a middle diameter for not being very loud, you have less lag and you will be ok up to 300 hp. Mine is 76mm decat and my wife is angry all time because of loudness.

**jimnielsen** · 03-03-12, 08:02

For a racecar Q4 - it has to be 3" - that's 76mm - end to end - with a single straight through exhaust at the very end - like Magnaflow. It makes a fair noise at idle - but is much quieter than non-turbo race cars of similar power. Easily gets under the 75dba noise limit here. It may be easier to use the dual 65mm tubes around the section under the front cross member to get a few more mm in ground clearance...

**Steve Webb** · 03-03-12, 08:29

Originally Posted by jimnielsen

It may be easier to use the dual 65mm tubes around the section under the front cross member to get a few more mm in ground clearance...

That raises an interesting point, would there be any downsides to running a dual system? Fitting large diameter tube under the car is not easy, so 2 smaller tubes may be easier to site.
Given the turbo I guess the usual calculations regarding runner length are no longer valid, so does it make any difference at all as to where is best to split the pipe?

**jimnielsen** · 03-03-12, 09:00

some interesting numbers - 2 x 60mm has more flow than 1 x 3" - but it is heavier has more joins and is more trouble..

using area of a circle = Pi x (radius x radius)
where Pi = 3.14 rounded

60mm = 2.3622 inch diameter
Pi x 1.1811 x 1.1811 = 4.38029 sq inches
100% area to flow through

single 3 inch
Pi x 1.5 x 1.5 = 7.065
1.6129 times more area to flow through than stock

dual 2.25 inch
Pi x 1.125 x 1.125 = 3.9740625 x 2 pipes = 7.948125
equivalent flow to a single 3.18198 inch exhaust
1.8145 times more area to flow through than stock

dual 2.5 inch
Pi x 1.25 x 1.25 = 4.90625 x 2 pipes = 9.8125
equivalent flow to a single 3.53553 inch exhaust
2.2401 times more area to flow through than stock

dual 2.75 inch
Pi x 1.375 x 1.375 = 5.9365625 x 2 pipes = 11.873125
equivalent flow to a single 3.889 inch exhaust
2.7105 times more area to flow through than stock

dual 3 inch
Pi x 1.5 x 1.5 = 7.065 x 2 pipes = 14.13
equivalent flow a single 4.2426 inch exhaust
3.2258 times more area to flow through than stock

**Evodelta** · 03-03-12, 10:21

I would prefer a two pipe system over single large any day as they are quieter. I've got a 3" SS with two straight through silencers and a race cat on my 16vt and it's still far too loud.
I hate the looks and boy racer comments I get from non-petrolheads (I'm getting too old).

Does sound good though!
The cams make a big difference to the tone and volume, a quite high lift race cam will make more and a 'hard edged' sound.

**Steve Webb** · 03-03-12, 11:36

Given those flow areas, and assuming that the bhp an exhaust can handle is directly related to the flow area, how accurate is it to assume the following BHP capabilities for each size.
I'll assume 200bhp for standard power with a standard exhaust.

60mm = 2.3622 inch diameter
Pi x 1.1811 x 1.1811 = 4.38029 sq inches
100% area to flow through
200bhp

single 3 inch
Pi x 1.5 x 1.5 = 7.065
1.6129 times more area to flow through than stock
322bhp

dual 2.25 inch
Pi x 1.125 x 1.125 = 3.9740625 x 2 pipes = 7.948125
equivalent flow to a single 3.18198 inch exhaust
1.8145 times more area to flow through than stock
362bhp

dual 2.5 inch
Pi x 1.25 x 1.25 = 4.90625 x 2 pipes = 9.8125
equivalent flow to a single 3.53553 inch exhaust
2.2401 times more area to flow through than stock
448bhp

dual 2.75 inch
Pi x 1.375 x 1.375 = 5.9365625 x 2 pipes = 11.873125
equivalent flow to a single 3.889 inch exhaust
2.7105 times more area to flow through than stock
542bhp

dual 3 inch
Pi x 1.5 x 1.5 = 7.065 x 2 pipes = 14.13
equivalent flow a single 4.2426 inch exhaust
3.2258 times more area to flow through than stock
644bhp

**Evodelta** · 03-03-12, 17:47

I think something is wrong with the calcs Steve, a 3" system should be good for more than 322bhp before being restrictive...

**JBSOUND** · 03-03-12, 18:15

Well I asked Mr. Google and he said this:

http://www.magnaflow.com/07techtips/faq/question10.asp

http://victorylibrary.com/mopar/header-tech-c.htm

http://www.bgsoflex.com/bestheader.html

**JBSOUND** · 03-03-12, 18:25

An other link...

http://www.exhaustvideos.com/faq/how...pipe-diameter/

**mpampis_** · 03-03-12, 19:06

This is interesting, does anyone tried to put dual exhaust? How does fit it? I mean single downpipe? What diameter of down pipe? I guess the pipe split in 2 parts at the area of the stock cats! What is going on to the fuel tank area?

**Steve Webb** · 03-03-12, 19:27

All the links seem to err very much on the conservative side of things, specifying a large diameter pipe for the bhp produced.
Is there any difference between the requirements for a turbo and normally aspirated engine?

I'm not sure I've ever seen a Q4 setup with a dual exhaust run, well not one I can remember.

**mpampis_** · 03-03-12, 20:45

I think it is possible if you have a race car. With gas tank removed and place an aluminum one to the trunk.

**jimnielsen** · 03-03-12, 22:42

And the fuel tank in the Q4 is much bigger than is needed for a club-day racecar - it would be 'simple' to put in a smaller tank of (say) 25 liters in the center rear under the car and leave plenty of room for the 2nd exhaust pipe.

**Steve Webb** · 06-03-12, 20:19

For some reason I've got a hankering for a side exit exhaust. Might take a look at how feasible this would be when I finally get my hands on a GTV.

Also, is there anything special in the construction of exhaust tubing?

**jimnielsen** · 07-03-12, 01:19

The best thing about a side exhaust with a GTV is that you can avoid getting the exhaust involved with the dedion tube - Its a real pain having to remove parts of the exhaust to remove the gearbox..

The bad part is that there is not much run length between the exit point of the extractors and the exhaust tip and you really need two mufflers on that pipe. The main muffler on a normal GTV is at the rear where there is plenty of room for it. If you use a side exit - you have to use mufflers that are not too thick as it ruins the ground clearance.

On my giulietta race-car (same layout as a GTV) I made floor cutouts so that the mufflers did not interfere with the ground - but its a lot of mucking about..

**Steve Webb** · 07-03-12, 08:59

I was only really planning on using one silencer due to it being a turbo engine. You think it would still need 2?

**jimnielsen** · 07-03-12, 19:31

No, I forgot your gtv is going to use a turbo!!! If it has a turbo, it will be a lot quieter - especially if you stay at < 2.5". I am sure just a single muffler will do in that case!

**Steve Webb** · 07-03-12, 20:53

I'm not planning on trying to get the motor much above 200bhp, at the start, so 2.5" is going to be plenty.

I was thinking something like this, but more square, for the side exit.

**1NRO** · 14-03-12, 06:42

No exhaust is best for a turbo engine! Not practical I know but that's the truth, every psi of back pressure in the downpipe is felt by the turbine and in turn seen on the manifold/engine side of the turbo, that essentially is a pressure working against the turbine from both sides, number one enemy.

**Steve Webb** · 14-03-12, 08:03

Just another wacky thought, but does siting the exhaust tip /exit in a low pressure area of the car help at all with reducing back pressure. If its in a low pressure area, then surely at speed there will be a slight suction effect, is this so small as to not have an effect or is it something worth thinking about?

**1NRO** · 14-03-12, 18:14

%'s of improvement, they all help :-)

If you car is really fast an upturned side exit (on both side) helps a little with stability/traction, like zoomies on a dragster or as RedBull did in F1 last year so well.