First of all, I have read the numerous threads discussing piping size transitions and how they affect performance in N/A and FI applications, including the testing done by Gernby.
My question is not about performance, rather it is about fitment.
I bought a modified AP1 which is supercharged and the exhaust setup is a PLM header (2.5in collector) going into a 70mm test pipe going into a 70mm Invidia N1.
I want to change the TP and cat back to a 63.5mm Berk HFC and a stock exhaust. I will get the car retuned after.
I have trouble finding information on the exact piping ID/OD for the PLM and the Berk HFC.
From the PLM website: <- is this the collector ID and the size of the bit that sticks out past the flange?
I assume the 63.5mm listed for the Berk is OD.

Will the 63.5mm Berk HFC physically fit and bolt onto the PLM headers?

 

Header to cat, and cat to cat back, and cat itself, all have flanges. They bolt together, not slip fit together. So you don't really care about OD (however pipes are historically sized by OD, and you have to assume the ID).

Further, virtually all S2000 specific aftermarket headers, cat or tp, and catback, all use the same flanges as stock, so they can be bolted in place.

That means you can use virtually any mix of stock and aftermarket exhaust components made specifically for S2000. They will all just bolt together at their flanges.

That also means you could bolt together pieces that aren't smartly spec'ed. Like a 70 mm tp with an otherwise stock system. That would be little, big, little, and cause power robbing turbulence and annoying rasp.

So all you need to worry about is choosing components wisely, which means similar ID, and know they will all just bolt up.

 

I thought that this bit may have not fit in the HFC. My understanding was that this bit is 63.5mm ID and the HFC would be 63.5mm OD but it is just me being unfamiliar piping sizes. Thanks for the help.

 

There is a donut gasket that goes around that collector outlet and the HFC part is conical so it should probably fit.
But that 63.5mm measurement on the header is most likely OD.
Just for comparison, the stock header only measures 54.6mm inside diameter.

 

In short: Tube is measured by outside diameter, pipe is measured by inside diameter.

The exhaust is tubing and the ID is smaller than the stated OD measurement. I measured the inside diameter of my OEM header and it was just over 57mm. Neglected to measure the ID of my 63.5mm OD Berk cat but I'll bet it's within a couple mm.

-- Chuck