Question about Testpipe/HFC and Bottlenecking
 

I've always wondered about the following questions:
1) Wouldn't a 63.5mm testpipe/hfc creates bottleneck since the header is 60mm and the cat back is also 60mm? I see people that replaces their cat for a 63.5mm without problem though..
2) I currently have a j's racing header (60mm as well) and plan on getting an exhaust which has a 60mm piping diameter as well, which companies produce an exact 60mm hfc? I'm trying to avoid a test pipe but I also don't want the much restricted oem cat
3) Is the OEM cat truly 60mm? I see numbers all over the place after a search ranging from 60mm to 65mm...

Well, 63.5 is bigger then 60, so no, a 63.5 mm testpipe is not a bottleneck. There are no exactly 60 mm cats. 60 vs 63.5 is within 6% of the size, very small difference.

You want to try and match cat, or tp, to exhaust size. Otherwise you create disturbances which rob power. For example don't use a 70mm cat with 60mm exhaust.

The stock cat actually flows exceptionally well. You aren't going to make more power with just a hi flow cat or tp. Only if you use flashpro or similar to lower vtec rpm will either of these help.

A tp or hi flow cat will allow lowering vtec rpm even lower, so more power sooner. Its not a flow thing, its a resonance thing. No real difference in top end power.

Yes, I understand a 63.5 would not cause a bottleneck after the stock header, but wouldn't it cost a bottleneck if it connects to the stock exhaust? since you're going 60mm (header) -> 63.5mm (tp/hfc) -> 60mm (stock exhaust)
Now I don't actually have a stock exhaust, but I've always wondered about this..
I do have flashpro for a MY06 so I see some power gain from a hfc and exhaust after a tune

Anyways, I'm going to go from a 3" exhaust to a 60mm exhaust (that age where I need to tone down on the noise and flamboyant character)
Just need to get the cat/hfc/tp part figured out.. I remember a company did sell 60mm hfc before many many years ago.. but forgot which one...

:iagree:
These lines pretty much summarize all the info I got from searching last month.

Bottleneck is when there is a restriction, wide, narrow, wide. The narrow bit is the bottleneck.

If you have 60mm header, 63.5mm cat, then 60mm exhaust, that would be wide, wider, wide. No bottleneck.

Isn't bottleneck simply just means going from big to small? So the wider -> wide will be a bottleneck no?
I mean, we wouldn't see people putting a 70mm test pipe, pairing with a 60mm cat back because that would be considered bottlenecking?

Bottleneck is a narrowing, not widening. Pressure builds up. fluid moves faster. Exhaust, however, reacts to moving to a wider pipe: fluid velocity slows. Anyone who has ever played with a garden hose knows this. All sorts of harmonics in an exhaust system I don't pretend to understand so I kept my system very close to stock.

Some pipe makers list the outside diameter (OD) rather than the inside (ID).

-- Chuck

You're ignoring what's going on before the cat.

No. That is not a bottleneck. If something goes from wide, wider, wide, those last two steps, from wider to wide, are NOT a bottleneck. The reason is you aren't imposing a restriction.

Thought experiment. Imagine its jellybeans through a pipe. Imagine at some set Jellybean pressure you can stuff 10 JB/sec through the wide pipe, and 15 JB/sec could pass through the wider pipe. But when the JBs get to the wider pipe, there are only 10 of them/sec, as that is how many got to the wider pipe, since they got there through the less wide pipe. So when pipe steps back down, there is not any restriction to JB flow.

Well two things...

One, there is no practical transition difference between a 63mm aftermarket test pipe and the OEM 60mm exhaust.

Two, when speaking of bottle necks, you nee to look at the system as a whole. Think of it as a choke point. There is no choke point going from 60 to 63 back to 60. Again the transition difference diameter is almost non-existent. The shape and smoothness of the transition from pipe to pipe is more important than a diameter difference that small.