What size exhaust piping do you racers use for NA's? 2.5'' or 3'' or something else?
I'm not exactly looking to race, maybe some lapping w/ a club, I think that would be cool. What I am really looking for is optimum performance out of a cat-back exhaust, and thought someone in here might be able to tell me if there a performance difference with 3'' piping compared to 2.5'' piping on an NA. thanks.

 

The RX8 Renesis race car is using 3" tubing with the rear sectioned Y'ed with two mufflers for some muffling. It is a loud and aggressive track-only sort of system.

 

Sub 200 WHP, 2.5 is plenty. Any larger and the exhaust gasses can cool and condense causing more backpressure. Seems like the best power is from long primary tubes closer to 2". wyed into 2.5" 'cat-back' section.

I've also seen most renesis motors with 3" from the header back...

And, yes, race exhausts are ridiculously loud.

 

okay, i have a 91 coupe so instead of compromising for something else like 3'' which is readily available on ebay I think im going to wait on corksport and get their 2.5'' cat back power series exhaust

 

If you are going with their full header-mid-single exhaust setup, I would call them and ask about what their measured dB levels are with that system on a stock engine *without* their silencer insert (which necks the exit down to about 1" diameter). With their cat back hooked up to a Racing Beat header/resonate-mid-pipe, it will not pass sound at any track.

Their mid-pipe/silencer may well take care of this issue, but check first.

Good luck,

-bill

 

I think 2.5" is the compromise, but probably the right one for you if your going to drive the car on the street at all.
You will likely end up wanting 2 exhaust systems- one with a pre-silencer and a large muffler so you can drive it around on the street, and another with just enough restriction to pass sound control at whatever track you choose to run, but as always YMMV.

 

The corksport without the instert, even with the pre-silencer is SUPER loud. Borderline for most tracks. But, we have not been told to leave one yet...

I am redoing mine with a RB road-race header, pre-silencer section, and wying into a 2.5" corksport style cat-back.

Once i got it i did not at all like how the corksport pre-silencer looked. Its just a 3" one adapted to a 2.5" exhaust. Big chamber for gasses to slow, cool, and become more dense. This all means backpressure and less potential power..

 

Racing Beat race header. 2" primaries into 3"





We have not had an issues at Infineon with sound (97db?)

 

yeah its surprisingly quiet

 

At the end of the exhaust pipe, where the exhaust energy is at its lowest, it doesn't matter too much.

My exhaust system is 2.5" from the collector back, with a pair of straight through mufflers. At the tip is a firearm style "silencer" with two sharp edged 1 7/8" rings. They are worth at least one muffler's worth of quiet, and on the half bridge engine there is zero difference in idle vacuum, measured acceleration, or engine tuning (MAP based) compared to having them in. Mind you the engine IS sensitive enough to exhaust that I can shut it off by putting my hand over the tailpipe, the slightest change is instantly noticeable at idle vacuum.

For my next trick, I plan on trying to acoustically decouple the header from the rest of the exhaust system, so I can use a baffled muffler, which is kinda needed if you want to be quiet. (Racing Beat mufflers are baffled... at least, the one on my friend's long primary "street port" system was)

Incidentally, I was able to make 170whp through the stock FB 1 7/8" midpipe, with a street ported engine.

 

i notice one of the honda challenge guys had something like that going on too, they actually bent the pipe to make that shape somehow though.

its kind of weird on a honda, and i didn't get to see if it worked, cause they found the wall

 

Without giving away any team secrets, do you have any pictures of what you are talking about? I can't quite visualize it.

Thanks,

-b

 

No team, no secrets.

Detailed in this thread: https://www.rx7club.com/showthread.p...light=silencer

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I took two 1 7/8" ID pipe adapters that fit inside each other and the 2.5"OD exhaust pipe, cut the small pipe off leaving just a ring, welded them together and to the end of the pipe. Cuts the drone fantastically.

I think the adapters were 1 7/8" ID to 2 1/4" ID (which will be 2 3/8" OD, which is the ID of a 2.5" pipe), and the other was 1 7/8" ID to 2.25" OD.

I used to hammerfit them, but sometimes they'd blow out, so I just welded them in place.

The exhaust at the very end is at its lowest velocity and coolest, so the added restriction doesn't hurt power any, but it totally kills the drone below 3500. One ring kills it below ~2800.

Again, the rest of my exhaust system is two straight-through 2 1/2" perf-core mufflers. The drone was awful beforehand.
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My main goal is street drivability, but they certainly do make the car a lot quieter. It is about the same as my friend's RB "streetport" long primary exhaust system was, at WOT.

 

Hmm, interesting. The Corksport insert is similar - like an inverted funnel. Your's just has a wider mouth.

In your original post, you indicate that it cuts down on the lower RPM "drone" - and you have definitely given me some ideas for dealing with that on my FD. But for the non-street <*cough*>trailer-queen<*cough*> FC track car I need something that addresses the 5k-8k WOT range. How does your insert perform there?

Definitely thanks for the pics. :-)

-b

 

It works there, too. It doesn't take out the sharp rasp up there, though. Tuning the no-load parts of the map in the dragstrip's pit area, it would still get "hair raising" when up around the 6000rpm part of the map. The sharp rasp is what I was really trying to kill and I feel that I succeeded greatly.

I say, give it a shot. It costs about $2-4 to make the things. Worst case, you're out a few bucks and a couple hours' worth of time. The neat thing about them is, they can just be hammered in, no need to be permanent. If I really wanted, I could just drill a through hole and nut and bolt it, but it made no measurable impact on power or airflow, so why not just leave it in?

I'll try to find the video and post it on YouTube, there was a company in Japan called Anniversary Racing Factory that showed how to make a device called a "swirl silencer", cut a maybe 300hp peripheral port's exhaust from 130+db to ~120db with a basically negligible change in power. According to the video, anyway. (I can't read Japanese, but the numbers and power/sound symbols are still there)