Dumb Question: Exhaust Size

[Andrew.Y]

Hi might forum,

I have a really dumb question but just can't get straight on my own:

Does 3"x2 - 3.5" mean

1, two 70mm diameter pipes joints to one 80mm diameter pipe, or

2, two 76.2mm diameter pipes joints to one 88.9mm diameter pipe

I see exhaust diameters are often quoted in rounded millimeters, which is quite confusing to me.

Any commends welcome!

[r32-25t]

The 2nd one

[GTSBoy]

Exhaust pipe is sized according the rules for "tube" not pipe. As such, tubes are always the outer diameter. 3.5" is 3.5" OD, and the ID is whatever that number is take 2 lots of wall thickness (which can vary but in exhaust pipe is usually not much more than 1mm).

So your ID on your 3.5" pipe is actually more like 86mm.

[Andrew.Y]

Thanks guys.

[scotty nm35]

All Jap systems use metric tube. It can be almost impossible to find over here, we use imperial for some reason...

[cobrAA]

From my calculation, the right size should be 2x70mm (most sized jdm downpipe) intro 1x 100mm ( 4 inch) for ZERO RESTRICTION.

However, most of them are 2x70mm into 1x80mm and some are 90mm.. which cause a restriction.

Then you could have a 90mm exit but if your exhaust or cat or resonator is only 3inch ( 76 to 80mm) it doesnt matter if its a 90mm (3.5 inch) because you still have a bottleneck at 80mm.

Once you have sorted this out... you should know that almost ALL dump pipe are 70mm or less ( hks are 60mm and R34 are about 56-58 i believe and then the r32s are about 50mm)...

As much as I would like to upgrade my exhaust, I havent found any informations on the internet about this problematic and don't want to be a '' piggy tester'' as I don't have any friend who work at exhaust shop who can fab me a 3.5 or 4inch exhaust for dirt cheap and I dont think my tuner will allow me to test-run his dyno for free for the sake of argument.

So I dont think your question is dump at all.

[discopotato03]

Yep 3" (76.2mm) here and 80mm in Japan . Saddedst thing is we used to have 2 3/4" (70mm) here as well but almost non existent now .Occasionally some Jap exhaust bits are in this size because they know in some instances a little higher gas speed works better than with 80mm .

A .