Confusion of pipe and tubing makes this a frequent question. Despite common usage there is no exhaust pipe on cars. It's tubing. Pipe is measured by the nominal inside diameter (ID) and tubing by the outside (OD). And, for more confusion a cat-delete is really a "test tube" not a test pipe.

-- Chuck

 

Tubing is used for structural engineering and pipes is used to convey fluids, gases or similar.
Just stupid to define them from how you would measure the diameter.

 

Opened up a can of worms here !!
Or is it.a tin of worms

 

Another question that I saw that I could not find an answer for it, was is it beneficial to space the cat closer or farther from the header to reduce the impact of the dip in vtec? My goal is smooth consistent power 4K and up, not total HP gains.

I will be installing a replacement cat since mine is dying, and tuning with AEM. Stock exhaust and header, but I can get the weld in cat and space as needed if it's beneficial.

 

Farther away from the header is usually better, but it might struggle keeping hot enough to keep the emissions down.

 

So a possible improvement might be too fiberglass wrap the stock header and exhaust before the cat, and move it back as much as convenient?

 

Sounds like a good idea.

 

If you just want to lower vtec, just get a 63.5mm htc, bolt it on, done. This will kake you very happy.

If you want to go crazy, moving the cat farther away from header could help some. But then its all custom. If cat dies at some point, its a hassle to replace. If exhaust gets damaged, can't just find another used stock one and bolt it on. Then there is the cost, and dealing with crappy exhaust shop welds, and rust due to nom stainless filler wire...

If you really want to go down this road, its already been well paved by a guy named Gernby. Search for his thread on how he did extensive testing on this very thing, and came up with a cat of his own design that he sold for a while. It moved the cat farther away, and used a megaphone shaped pipe to transition from pipe diameter to cat diameter. Eventually he made a whole header back exhaust that he sold for a while. You could just replicate what he did.