The biggest restriction in the exhaust of this car is the cat. Unfortunately I think you need to keep a cat for STR. HFC will help but nothing is as good as completely getting rid of it.

I don't have dyno numbers to back it up but on my first s2000 I had started by running a 70mm single with stock cat. Car lost about 40lbs but it felt the same as stock just barely louder. Shortly later I ditched the cat and went to a test pipe and all of a sudden I had the sound and response I was expecting to see. I ultimately sold the exhaust to pay for nicer wheels but decided to just leave the test pipe in place. To my surprise the car still felt the same. I later sold it and put the stock cat back on and I immediately noticed the difference.

 

I presume you mean the stock Cat back since you are replacing the header and cat. You may be able to find a dyno plot some where on s2ki that shows the difference. My guess is that it is slightly restricting because of all the turns in the pipe, especially in the mufflers where it looks like the tubing wraps around for three passes through (in, back toward the front, out, turn around, out the back). According to the Honda S2000 book by Carney, Honda spent a lot of time developing the exhaust to maximize power and meet government sound regulations. I've also read in some old s2ki threads where people lost power (5 HP) when putting on after market exhausts. I certainly agree with //steve\\ that the cat itself is the biggest restriction. This was evident in the sound difference after installing a HFC on my car. For the same cat back (stock cut-off) the sound level went from about 91 dB to about 97 dB. Both readings were by SCCA at tour events.

The primary reason to remove the stock cat back is to reduce weight. It weighs 55 pounds which is significant. When using the stock cat back cut off at the Y there is a 45 pound savings.

 

Good info, thanks. Seems like I'd be alright buying the 63mm Berk HFC and sticking with the stock exhaust. My driving isn't at the level where I'm concerned about weight, I'm more interested in a bit of a bump in power to make the car more fun on both the auto-x course and on regular roads.

 

Car: 2006 S2000 AP2

Header: PLM (about 11 lbs)
Cat: OEM
Cat Back: First Racing F1 cat-back, 60mm, dual exhaust
SCCA Sound Rating: ?? dB measured at 2010 Nationals on West course


I missed the Black Friday sale with Berk to get their 63mm HFC. Is Berk our only option for 63mm HFCs?

 

It's the only one that I have found that is assembled and ready to go.

If you want to save money (about $100) then buy a test pipe, Magnaflow HFC, and bung. But, you'll have to find someone that can weld it all together for you. See photo below which is the same as in first post of this thread. It's been working great with no CELs. YMMV

 

Thanks, if I had the welding tools I would have fabricated one myself but just decided to purchase the Berk HFC today since I made some money selling Miata parts.

Another question popped into my head though. I recently found out the piping on my catback is 60mm and not 63mm. Is it critical that the piping be the same diameter for the best flow? Currently the header is 63mm, and HFC is 63mm, but the catback is 60mm. Should I find a 63mm catback to keep the piping diameter equal?

 

From my limited understanding it is generally better to go larger the further down the exhaust stream you go. that helps produce a scavenging effect that basically pulls the exhaust gasses out. although when pushed through a smaller pipe the gasses move at a faster speed due to the pressure increase from pushing the same volume through a smaller pipe. However, I am no engineer and I cannot say what actually makes more power or is better. I have just always heard it is better to go from smaller to larger rather than the opposite.

 

If you had your choice, which size race cat would be the most desirable: 60mm, 2.5", 70mm, or 3" i/o dia.?

I'm trying to determine the most applicable size for a weld-in 100cell cat that has been dominant in STU, and is now legal for STR.

Thanks,

Rick

 

A second OEM resonator will get you down under 93. in cold weather a turn down added to that.
cheers.

 

I was originally thinking I would go with the lite spec, but given the combination of my budget shrinking and them being backordered and I'm not sure I'll get it in time, I think I'm going to do the custom route. I'm thinking I'll use the OEM exhaust up to the split, then put a v-band flange there, then have a vband flanged turndown at the axle for national events. For street driving most of the time, local events, and/or when I have problems with meeting sound I'll have a second axle back section that has a small muffler/resonator and a turndown at the bumper. So my questions:

1) For those of you that have done the v-band on oem exhaust approach, where should I weld the flange? at the split, or upstream from it?Early on there was some talk that the turndown at the axle had to be welded on at a certain point so that the axle assembly doesn't take out your v-band clamp.

2) I'm trying to shop for a small muffler/resonator thing for the second axle back section, and it's really tough to shop for mufflers. Anyone know a vendor that lists weights? I tried jegs and summit racing, neither list weights. Amazon lists weights, but they're very approximate at best. There's burns stainless obviously, but they are $$$$$. Not to mention there's pretty much no way to know which do a good job of reducing sound.

3) Anyone have any ideas for what to use as that small muffler/resonator? Ideally I want something that just barely gets me to 93 dB worst case with that second axle back section. If anything I want quieter sound at the expense of a couple more pounds. I haven't measured the space I have to work with yet, has anyone already done that? I'm guessing one of the 6" round 12" long jobs should be adequate.

Thanks for any input on any of those questions...