From my searches of how to quite down the rx7, I haven't found anyone try a J pipe. Most have done in line resonators an/or multiple mufflers to try to quiet it down. Is there a reason I haven't seen J pipes mentioned? I know our engines give out different pulses, is that why it won't work?

A J pipe resonator from my understanding sends out 1/4 sound waves that cancel out your exhaust sound waves without creating more restriction. Isn't this a better option than inline resonators and mufflers?
https://youtu.be/ds72D4rr4hg

I'm trying to quiet my exhaust as much as possible without creating more restrictions. From my efr 9280 I got a 4" down pipe to a 3.5" high flow cat, to a 3" racing beat duel tip muffler. It's still in my opinion that it's super loud.

I thought about putting in a racing beat or vibrant 3" resonator between the cat and muffler when I came across this J pipe resonator. What are your thoughts?

 

I've seen exhausts on other cars that use something similar - a short pipe coming out of the main pipe, or a short pipe going to a sealed resonator chamber.

Big thing is there's no development that I know of for something like this with a rotary. The hard part would be testing numerous combos - size of pipe, placement, etc. - and testing to see what helps and what doesn't.

All that said, the RB exhaust is on the loud side in general, not as loud as some but louder than the design would lead you to believe. The Tanabe Medallion will quiet things down a good deal, that's a really nice muffler, but you will probably sacrifice some power.

Dale

 

From my research, you can use formulas to figure out what pipe length is needed for the frequency desired. But the formula includes a number for pulses for piston engines and not rotary.

If I was to keep the racing beat cat back and add a vibrant or racing beat resonator, would that be quieter than the tanabe cat back? Or would my best option be the tanabe itself or even Tanabe with a 3" resonator?

im not worried about noise when I'm gunning it and in boost. It's more of the idle and cruise range that I would like it to be quiet. I don't mind spending money as long as it gives me the desired results.

 

The Tanabe is a very conservative cat-back. I drove a friend's FD at Deal's Gap with it on an 8374 turbo, it was very mild-mannered sound-wise, sounded good but wasn't too much.

I can't guarantee you how it would sound on your setup, but I can definitively say that it will be quieter than the RB exhaust.

Dale

 

Can't answer which of the two catbacks (RB or Tanabe) is the quieter option once you put a good resonator or a high flow cat to the mid-pipe section, but I was in a similar predicament taming the obnoxious exhaust system I inherited from my car's PO. Solution for me was the Tanabe catback (picked up on Amazon for about $395 shipped!), and a used PFS mid-pipe resonator section I purchased from a forum member that is basically all 3" stainless mandrel bent piping with a large oval Magnaflow resonator in the middle. Retained my original downpipe which is a 3" stainless piece that was part of the single turbo kit the PO got from Turbo Source. The car is now quiet at idle and cruising, has a deep non-obnoxious tone to it with no raspy high-pitch tones to it. That deep tone just gets louder when I step on it, but it's a good kind of louder.

Now I just have to get a fabricator to recirculate the dual external waste gate dump tubes back into the down pipe, so I don't hear it scream when I punch it hard enough to make the WG's open up!

 

I'm having a tough time seeing how that doesn't create back pressure, since you're essentially diverting some portion of the exhaust flow to a dead end.

 

I could see it potentially creating localized turbulence but I'm not sure how that would act as a restriction to create back pressure.

 

Maybe we are saying the same thing, but I'm thinking if you blow air into a tube that isn't open on the other end, its going too blow back in your face at some point.

 

I'm not degreed in any form of engineering, so I can't claim any pedigree to my opinion. However, since the pipe diversion meets the exhaust at a flush 90° angle I'd think the amount of flow into it would be minimized.

Added, since the volume of the j-pipe is finite it should pressurize, and equalize, to match the exhaust pressure. There should be the exact same exhaust flow volume both upstream and downstream of the j-pipe since it's a closed off diversion.

I believe that both liquid and gasses follow the same flow properties. Adding a capped of length of short piping to a water line wouldn't necessarily reduce the flow or pressure at the exit point. There may be a momentary lag as that side piping is filled to capacity, but once that's completed it should return to normal.

As for the degree of turbulence at the junction point, wouldn't that depend mostly on the entry angle coupled with the ratio of piping diameter between the exhaust and j-pipe? I'd imagine that once the j-pipe is pressurized then the gasses in it would act as a buffer from additional exhaust entering. The sound waves would travel the length and bounce back unhindered, but they operate under different principles than gas flow.

IF there was a constant and complete exchange of exhaust gas entering and exiting the j-pipe then I could see it causing extreme disruptions. If, however, that exchange of gases were limited to tiny eddies at the junction then it would be a very minimal disruption to the exhaust flow overall.

I grew up fishing in ponds and streams, I translate the exhaust flow by recalling what happens when there is an offshoot of water from the main path. Where the offshoot and the stream meet there are ripples and swirls, but beyond that boundary the water is smooth and still.

Again, assuming that both fluids and gasses operate under the same flow principles, then a j-pipe should cause very little disturbance to overall exhaust flow.

 

IIRC, dead-end volumes like the J-pipe are resonant volumes that can minimize noise in a particular frequency range by causing the air to pulsate in a waveform that is the opposite of the sound pulses they are trying to cancel. As such, they have to be tuned to the system. Otherwise they can have an effect opposite of what you want, i.e., it could make the noise worse.

 

Yup! I did some googling when I first read the OP, the idea is new to me and it piqued my interest.

It seems that the decible range, gas temp, and j-pipe lengths have been mapped out for common (4cyl/8cyl) engines. The rotary is unique enough that I doubt anybody has gone through the trouble of conversion yet. My big question is how pipe diameter (both in the exhaust and j-pipe) would change things.

I'd love to tinker with the idea, but nothing would happen for a good 5 years on my end.

 

My understanding is that is little to no actual exhaust flow into the j-pipe as its a capped tube, so it's little to no flow restriction. Only thing that actually enters is the sound waves which bounce back at an inverted frequency canceling the frequency your targeting.

All the formulas I have found surrounds 4/8 cylinders. This video explains it very well. and actually has a spread sheet to determine the length needed but again for a 4/8 cylinder engine.