Hello fellow S2k owners.

I need to swap out my test pipe for a high flow cat. Will there be a significant loss in power throughout the powerband once the HFC is installed? Also will my exhaust be a lot quieter? I'm going to be installing a 70mm Berk HFC.

My car is still NA with full bolt-ons (Exhaust/Header/Intake) with drop in cams.

 

Is your car tuned? If not, there won't be a significant power change even if you put the stock cat back on. Our stock cats actually flow quite well.

Unless... Is your catback also 70mm?

You'll actually lose power due to turbulence caused by mismatch of inner pupe diameters. A tp of the wrong diameter will actually lose power compared to stock cat that matches catback diameter.

The only way any cat or tp makes any real power on this car is if its tuned, and really the only gain is midrange, as the only way you gain is by lowering vtec rpm (potentially as low as 3800 rpm).

Lowering vtec any significant amount with stock cat actually loses power. But not because of flow, but because of resonant harmonics.

If you haven't lowered vtec via tuning, then no cat or tp osq gonna do anything other than make noise, and more importantly, harmful greenhouse gases.

Regarding noise, yes, a tp is a little louder than hfc, and hfc is louder than stock cat. This effect is amplified by how loud the catback is. On a stock exhaust, hfc or tp makes a barely perceptible difference. But even just a uk mod stock catback it'll make a significant difference.

 

I'm tuned and have a test pipe. How do you lower Vtec to 3800 rpm's?

 

Many of the tuning solutions allow lowering vtec.

The hfc or tp makes very little difference even tuned EXCEPT THAT IT ALLOWS LOWERING VTEC. So if you're tuned, but vtec isn't lowered, then 1, your tune left all the real performance gains on the table, and 2, tp vs hfc is not going to have much performance effect, but 3, you really should get it retuned after making an exhaust change, as it could affect afr.

 

Thank you for your feedback. My header outlet is 63.5mm and I have a 70mm exhaust and test pipe. I have now purchased a 70mm Berk HFC. My car is not tuned at the moment, just waiting for the HFC to arrive before I hook up my Flashpro and get it tuned.

The HFC may cause me to lose a few hp cause of the restriction, but after tuning I should gain a lot more power than I have lost.

 

I'm convinced that abrupt exhaust diameter changes disrupt gas flow by inducing harmonics. 63.5mm high flow cat seems to better match the 57mm OE exhaust heater and matches a 63.5mm header even better. With a 70mm exhaust I guess it depends where you want the exhaust to slow down when it jumps to the larger diameter. Not sure if it really matters or if this is PII.

I bought a FlashPro for my 2006 car thinking lowering VTEC was as simple as typing 3600 into a box in the software. Yeah it is -- but it ain't gonna work without a dozen other adjustments to fuel-air and other settings. There are guys here who will remote tune your car. You capture data in several test runs and send it to them and they make calibration changes and send the data back to load into your ECU. Mine took 3 weekends. Gernby tuned but he's out of touch for the past year or more.

-- Chuck

 

Yes. I recently read a paper that mentioned ideal pipe diameter change to not disrupt flow (by inducing turbulance) involves a 7 degree slope. So a long, slow cone shaped transition from one diameter to the other.

If you checkout the Gernby catback (still sold by Urge), that looks like what it does.