Differences? Which one is better to switch out my stock cat with?

 

Your stock exhaust is 60mm and from what I've read going from larger to smaller is bad.

 

It's best for the test pipe to match the diameter of the exhaust, so if you plan to stick with the stock exhaust, get a 60 mm test pipe.

 

Good to know!!! I currently have the stock cat with a 63.5 mm exhaust
Soo im guessing the 63.5 tp would be ok

 

Just to make it a debate. And somewhat directed at Gernby if he checks in.

Would going from a 63mm tp to a 60 mm exhaust matter much? Berk 76 does neck down from the header flange but it's a sloped transition. I see the negative of going up a size but such a small step I don't foresee losing much power over a matched TP and exhaust.

If you know what you are getting then a correctly sizes TP is the way to go as to not obstruct flow. My question is really what do you lose using a 63mm tp and a 60mm exhaust?

 

Any change in pipe diameter will cause a pulse reflection back to the engine. If the diameter change is positive, then the reflected pulse will be a scavenging pulse (good), but if it's a negative change, then it will be a plugging pulse (bad). Since these pulses travel at the speed of sound, the location of the diameter change determines the RPM ranges that the scavenging or plugging pulses will cause the most impact.

 

Gernby, how does one determine whether the diameter change is positive or negative? I have a 63MM HFC paired with a 60mm dual exhaust (Fuji Legalis R and would like to know in your opinion whether you think I would be losing performance compared to running the same 60mm exhaust but with the stock oem cat?

 

I should have been more clear. A positive change in diameter would be to go from 60mm to 63.5mm, and a negative change would be to go from 63.5 to 60. If I had a 60mm exhaust, I would not use a 63.5mm test pipe.

 

Gernby, In your experience do you think I will be losing power if I had a header with a collector size of 63.5mm paired with a 63.5mm HFC and 60MM Dual exhaust? Would this setup be worse than stock? I plan to tune with a vafc2 to optimize vtec point and a/f ratios.

 

Yes, I do believe you would make more power with a 63.5mm exhaust. However, I don't think you're going to get much benefit from a VAFC, since you'll need to add a bunch of fuel and ignition advance in order to lower VTEC much, and a VAFC can only be used to reduce fuel.