Why is a test pipe or HFC needed to lower VTEC rpm
 

I recommend searching for some of Gernby's threads on this subject through Google. There's a ton of information in them :thumbup:

The high flow cat is needed "just 'cuz it's needed." ;) I know no reason why but my Gernby FlashPro tune required it to lower VTEC to 3600 rpm. Greg isn't selling these parts, just the tune, so I don't suspect anything other than it's needed. :) I'm running the 63.5mm Berk high flow cat on my '06 with his tune. Astoundingly more midrange power and acceleration. I was bouncing off the rev limiter in lower gears so I installed a red line alarm horn on the car.

There's a myriad of fuel settings and other arcane adjustments that need to be made so it's not as simple as going into FlashPro and typing 3600 in the little box marked VTEC.

I don't have money to burn on dyno tests but the "after" number shows 221 horsepower at the rear wheels. A buddy's fast but non-Gernby tuned S2000 was closer to 190 within minutes of my test on the same Mustang dyno. He has some exotic headers and his car was always faster than mine. Past tense. In his words my car is now a "beast." (Certainly a relative term. :) ) Both cars were running FIPK intakes and Tanabe Medalion Touring exhausts we installed together. Some claim a Mustang dyno reads at least 10% lower than others. Proof ain't in the numbers but in the mid range power this car now puts out.

This tune is the best money I've sent on the car. Strongly recommended. Just too simple to do.

-- Chuck

You can lower vtec to right about 5300rpm before the stock cat causes issues, and you will actually lose power if you go bellow that engagement, tested and true every time. I cant tell you exactly why, but I suspect it has something to do with the available flow with cat in place along with the resonant frequency in that rpm range, coupled with the increased valve lift. So you need either a TP or good HFC to open up the flow and get around this and once you do you will realize gains as low as 3500-3600rpm. Peak gains in hp/trq happen right around 5.5k rpm - 15whp/8trq or so. If you look at a stock dyno, there is a big bump at 5900rpm, lowering vtec at the natural resonate frequency bump of 3500rpm makes a linear power curve/line from that point all the way to 5900rpm, filling in all that area, so obviously there wont be a 15whp/10trq increase at 3800rpm for example.

https://cimg0.ibsrv.net/gimg/www.s2k...b034beef0b.jpg
Here is an example. This is obviously with a full tune, so there is gains above 6krpm as well, but you get an idea of what happens in the mid range as I described from lowering vtec and power improvement expected.

With the lowered VTEC it's not necessary to run to 8000 rpm before shifting to avoid falling off the high power cam during the shift which is engaged at 6000 on a stock tune. And with the extra torque available it's often not necessary to downshift in many situations.

-- Chuck

it works the same on my integra, stock vtec is set at a very low 4400 rpms in factory form. With stock cat vtec was not noticeable at all.. Once I put on a test pipe the vtec transition became much more pronounced , you could definitely feel the extra kick and certainly the sound changed. I really don't want to run a test pipe for a number of reasons but it feels so good. I hope to try a HFC in the near future to see if some of the benefits will carry on with it. The reason it works is due to changes in air flow, resonance, and air-fuel mixtures , all better than the stock setup.

Just so you know, VTEC Yo! is eliminated with a 3600rpm VTEC engagement. High power is there right away, not at 3/4 of the way up the rpm range. Exhaust note deepens and the power rolls on smoothly.

-- Chuck

I run a Karcepts AP1 Emanage setup on mine. VTEC lowered to 4000 rpm, and as some mentioned, you do not feel the engagement, but rather it is a nice, linear power delivery. The intent of the tune is to make more power lower in the curve (To help minimize the power delivery gap between the AP1 and AP2). I can say, with the stock CAT it had a huge dip in power around 4000 RPM. With a Berk it is perfect. Granted, the tune was created with the high flow cat and PLM header, so it is expected to be worse without the cat swap, but I do not believe they could get as good as delivery with the lower VTEC and stock CAT due to flow reasons. Brian Karwin would be a good person to ask that question of since he spent a lot of time tuning and autocrossing the S2k. I have to run a CAT for SCCA STR ruleset, so have never run with a test pipe.