Exhaust Size

[AngryRB]

I was measuring a catback exhaust ( supposed to be 3 1/2inch catback) with a vernier caliper and noticed that the pipes outside diameter is 3.2inch and the inside diameter of the flange is 2.7inch, i can see the pipe before the flange is also smaller, probably 2.7inch as well .

so what id like to know is , if an exhaust is said to be a 3inch or 3 1/2inch system, is it normal to have the inside of the flanges at a measured 2.7inch, surely the inside of the flange should be the same as the pipe itself so that it is the same size all the way thru.

can anyone shed some light on this? as i need a 3inch or 3 1/2 inch system to get 300kw and wondering why these flanges are smaller than the pipe.

[R31Nismoid]

It is not normal.

Is it Jap made? Jap off-shelf exhausts often are say - 3.5" piping. However they have tapers/restrictors that go as small as 2.5" to keep the noise down, and at the same time cause restrictions

If it was made locally - Then no idea why they would use smaller flanges...

[AngryRB]

It is not normal.

Is it Jap made? Jap off-shelf exhausts often are say - 3.5" piping. However they have tapers/restrictors that go as small as 2.5" to keep the noise down, and at the same time cause restrictions

If it was made locally - Then no idea why they would use smaller flanges...

Im not sure, i have a different one on my car which is Jap and has the 2.5 restricter in the muffler, im getting a dump pipe made ( 3 1/2inch to go on the end of my gtx3076) so when i get it back ill have to figure out what the go is with the flanges.

If i were to buy a 4inch cat would it have these small flanges? or larger but the bolts still be in the same position.

[R31Nismoid]

Well most just come as a CAT body, and no flanges.

So if your exhaust is 3.5" - then an exhaust shop will just make a taper and weld some 3.5" flanges on.

Very simple and common

[discopotato03]

From what I've seen the Japanese don't use tube made to imperial dimensions so often if you have a Jap "3 inch" exhaust its more like 80mm , 3" converts to 76.2mm . I'm just trying to remember if its pipe or tube thats measured in OD because the other is ID . I think the Japanese 2.25 inch comes out at 60mm rather than the imperial 57.15mm .

3 1/2 inces converts to 88.9mm so I imagine in metric its 90mm .

[AngryRB]

one last question, what can i do about the restrictor? i dont want a loud droning cop attracting noisey car, what options are there here?

[boostn0199]

You either cut the restrictor out have loud exhuast, or change the mufflers and get some decent ones that flow well and wont be loud.

[KiwiRS4T]

That's what you pay for with the better Jap brand exhausts - decent sound without impeding flow.

After a (too) quick look at my Blitz rear muffler I decided it was too restrictive and got a Chinese 3in straight through muffler which was much louder and sounded cr*p.

Looking at the Blitz that I had cut off I realised that it probably wasn't too restrictive and it certaily sounded a lot better.

Luckily my TradeMe muffler came with a silencer which reduces the sound to an almost acceptable level and I will leave it in except when tuning or at the track.

[GTSBoy]

Note for that that want to know. Tube is specified by OD. Pipe is specified by a nominal ID. I say nominal, because pipe is only ever approximately the nominal ID. For example. You can get pipe in various wall thicknesses, from schedule 5 up through 10, 20, 40 (which is generally the standard wall thickness) to 80 an higher in some cases. The only thing that remains constant with all those wall thicknesses for any given nominal bore size, is that the OD remains the same. So various different schedules of 1" pipe will have different IDs, even though they're all 1" pipe. 1" pipe has an OD of 33.4 mm, well over 1". The ID of sch40 1" pipe is more than 26mm (can't remember exactly what it is just now, and my pipe chart is in the office).

[AngryRB]

The last thing i want is a loud crappy exhaust note, the jap system i have which uses a fujitsu muffler with restricter is great for the stealth factor and sounds good, im hoping that with the turbo upgrade and larger pipes comming off the turbo that it will sound a bit louder though,

will be interesting to see how much the restrictor affects power at 300kw, the muffler might be a dilema if its restricting too much.

Edited January 11, 2012 by SliverS2

[TiTAN]

i hacked up my apexi N1 system to get it to a legal noise level and remove the 3" section from just after the cat so my exhaust starts at 4" off the turbo and is a true 3.5" all the way from the end of the dump back to where it exits the car. to boot its only marginally louder than a stock system and good for an easy 300+kw, so nice for cruising around town too as there is no intrusive exhaust noise the stereo has to overpower all the time.

loud exhausts only bring more attention to you and your car from all the wrong people and then you complain that they pick on you for everything they can (not saying the OP, just in general) or maybe im just old and cant be bothered dealing with all that crap for no good reason.