So I'm trying to come up with an exhaust system for my FC TII, and I feel like finding the right exhaust is very difficult. I currently have a racing beat y-pipe with a large cat and resonator and I think that with the cat and resonator, my car sounds TERRIBLE (used to be a straight pipe, but smelled like 91 octane 2 miles behind my car and suffocates traffic). So here's the attempted goals:

Goals:
- Single Exit (Left side)
- Deep Tone (Like Apexi GT Spec)
- Little/no gas smell at standstill when warmed up (I'd like to shoot flames once in a while)
- No droning at highway speeds
- Enough back pressure to not grenade my engine (Looking at maybe 12-13PSI on stock S4 turbo)

Setup Idea:


This is just a mockup since I love the idea of the whole valved system for traffic and startup, then loud and proud for ripping it. I have some concerns with clearance for that outcrop containing the cat and resonator, because I have no clue if it would fit or not. I also don't know what thickness of stainless I should use and how that affects sound? I chose the 4 inch step-up because I love the sound the 4inch pipe has (like the GT Spec).

This is obviously best case scenario with infinite budget and I'd love to hear any input anyone might have as to the feasibility or if anyone knows of an exhaust that has that deep tone like an Apexi GT Spec (I wish I could actually find one for sale) or maybe even a corksport. Any ideas, opinions and advice would be more than welcome. Cheers!

 

well if you want 12psi of back pressure, just buy my system! with a cat mine is 12psi, without its like 7psi. its still too much backpressure, although boost control is not a problem
mine is a 70mm DP, into a cat, into a 50mm HKS sport, its completely silent at WOT.

i read a David Vizard book, and he lists in the same paragraph that the exhaust needs about 2.2cfm per hp, and later in the same paragraph he mentions that a pipe flows 115cfm per square inch.
he then doesn't do the math, but you certainly could. the stock FC exhaust, 60mm, is good for about 250hp. 70mm is about 300hp

and its really complex, but in theory a bigger pipe should make the tone more bassy/lower frequency, but may only be in comparison to the system you had before.

 

ultimately, this is a matter of what you want, so my only input here would be to tell you to:

1. edit: i spoke on backpressure and realized i forgot you are turbocharged, so i removed my comment.

2. be prepared to live with some smell. i have no idea how bad yours is, but unless it's totally ridiculous, i'm not sure there's going to be much you can do about that. it sort of comes with the territory of an uncorked rotary. your tune may be able to adjust some of it away, but i don't see a scenario where the car will smell like a big-exhaust piston engine.

3. transition your diameter changes. it's better to use a cone-shaped transition when drastically going up or down in diameter. i took your drawing a little literal, so if you already knew this, then my apologies.

what rear muffler are you using now? i had a GT Spec on my MR2 Turbo and i loved it! i never heard one on a rotary, but if it sounds as good as it did on my MR2, then i get why you'd be obsessed with it. overall, i think if you focus on getting the "right" presilencer, then your overall sound quality goals can be met.

 

If managing boost creep is a concern, could port your wastegate. Just an idea to get rid of the “boost creep” exhaust design constraint.

 

The butterfly valve will need to be able to withstand the heat. I actually run an apexi exhaust control valve which acts like a throttle body to quiet down the exhaust when I need to by closing the exhaust 70-80% so I can not **** off the neighbors or if the police are in the vicinity. I have the valve right before the single exit muffler, and the plate bent from the rotary exhaust heat. I’ve since made a replacement plate out of titanium and it has resisted the heat perfectly fine.

 

How are you getting higher boost with a cat on? I'm not sure if I made it 100% clear but I'm not looking to make the exhaust quiet, maybe a bit louder than a straight piped (with no pre silencer) racing beat exhaust and definitely a deeper-raspier tone. I feel like 50mm is too small for that high pitch, especially for a single exit.

 

I'm running on stock ECU (with future plans to go standalone) with upgraded injectors (I think? The guy who sold me the car told me they were probably upgraded when they installed the SAFC which is now unplugged). I'm prepared for *some* smell but before it would be nauseating. I've had people tell me that they think my gas tank is leaking, but it all disappeared after I had a muffler shop install a cat on it for registration inspection purposes (25YO cars are exempt from emissions testing here). I've tried locating a GT Spec, but it doesn't help that it's discontinued and also extremely difficult to find ANYONE who has one, so if you ever come across one, don't hesitate to lmk. I'll bump my ISO ad on the marketplace and hope for the best.

In terms of cats though, what's a good compromise between smell scrubbing and high boost whilst maybe even giving the occasional flame? I was thinking something like a 150-200CPI cat would be decent but its really unknown territory for me.

 

me too, basically you need enough flow to not be a restriction, and then enough cells/or cat material to actually clean the thing up.
or failing that you can look at what Mazda did to go from the FC to the FD, when they went from 205hp to 255hp... pipe got bigger, lol

https://www.rx7club.com/3rd-generati...1/#post6726734
https://www.rx7club.com/3rd-generati...8/#post9924182

 

I ran the HJS 400 cpi motorsport cat converter successfully (twice) with minimal smell (during warmup, none at operating temp) and very little back pressure. They aren’t cheap, but are rated to 950°C. The biggest long term issue with any cat on a rotary is going to be burning oil. I never ran into a problem, but there’s no guarantee how long it will last with increased premix etc.

you want the -0046 or -0064 model

https://hjs-motorsport.de/en/product...t/pe-catalysts

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I'm running an OMP with no plans to switch over to premix since it's my daily driver and don't wanna drive around with 2-stroke oil. Big question remains though; can it shoot flames?

 

Update:

I did some more research and thought up this design. Not too sure of the viability or how ECV's can hold up to rotary EGTs, but let me know what you think.



I thought of using the V-Band in order to give me the option to remove the cat a lot quicker that the regular bolt flanges so it'd give better flexibility in terms of setup. Let's say I want to go straight pipe, I throw on a test pipe or pre silencer and call it a day. Granted the ease of use isn't as good as the former design, but it's an alright choice. The ECV is there to give flexibility on the highway, morning start, etc to dampen the sound a lot, although im not sure of the validity of the flap to the high rotary EGTs after the cat.

Another idea I had was that I could run a 4inch pipe directly after the cat section (I'd need to find a cat wider than 4" diameter) and just cut and v-band further up from the taper that way it'd get a nice deep grumbly tone, whilst also having the smaller downpipe to control boost creep.

As always, input is more than welcome!

 

I recommend taking a look at the hks silent hi power catback. one of my buddies has one and it’s nice and deep tone, quiet at idle and while cruising.

I posted above about the exhaust control valve and said to change the plate material to titanium to hold up to rotary egt’s. And recommend placement by the muffler.

 

not sure where people get that idea, but Ti is not a good choice for rotary exhaust temps due to only having a service limit of 550° - 600°C, you can get by with it in the rear if the temp drop is enough and the material thickness enough (which is self-defeating). I’ve seen plenty of them crack and break over the years though.

I built a full race exhaust out of 20 Ga (0.035”/0.89mm) T321 in 2010 and it will likely still be crack free and going strong when I’m in the grave.

There’s probably more at issue in a high temp exhaust throttle valve than just the butterfly plate, but being just a round flat plate then making one from either T321 stainless steel or 625 inconel sheet material would be the better choice.
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I thought about the HKS, but I really dislike the sound at anything higher than 3000RPM. I think I'll keep trying to bump my Apexi GT Spec ISO ad until I get a bite. Then I'll pair it with the ECV and swap the valve.