Well, I have an idea...again. Whenever I say that around here, my friends chuckle, my wife rolls her eyes and the dog hides under the porch. Nevertheless, past lessons aside, once I get one of these offbeat notions in my head I seem helpless to avoid pursuing them. Someone suggested it might help if I shared my affliction with others similarly challenged. So here you go.

I want a 2.5" exhaust system of straight-through design. From a performance standpoint, I prefer an X pipe, and I also like the "engineered" sound. I love the deep, soul-moving music of a V8, but on a street car I prefer not to play it fortississimo, frustrating my neighbors and making longer trips an exercise in masochism. I REALLY dislike ear-numbing cabin resonance that often appears around 1700-2000 rpm with aftermarket systems.

So what I think would conceptually be a good design is an X pipe followed by dual resonators close behind, and something like Borla XS Pro mufflers at the rear. Add resonator tips if you dare, and I think that could be a system that sounds appropriately menacing without being obnoxiously loud or creating serious cabin resonance.

In execution, the challenge is space for the under-car resonators. After installing an X pipe there would only be room for short, minimally-effective resonators at best, and you still have the challenge of the bottom part of the resonator body reducing ground clearance where there is none to spare.

I saw a relatively new product by Dynomax that I believe could be modified to serve as an X pipe, a full 20" long, straight-through resonator (compared to the typical 14" body mufflers used in the rear), and after the modification could, "I think," be mounted up in the tunnel, barely below the driveshaft, and only reduce ground clearance by about 1/4" or so below the exhaust pipes.

I just got under my '90 with a measuring tape. It would be a tight fit, but again, I "think" it can be done. Probably have to grind down the sheet metal lips where the seat belt attachments come through the pan, and perhaps weld a sturdy, flat plate to the tunnel inside the car to create a new mounting point. By using the proper rubber-isolated exhaust hanger mounts both front and rear on this long muffler, I think it could be located within those close confines without allowing any rattles, even when the engine is moving on its mounts, the car jostling around, etc.

Basically the idea is to slice off just shy of 3/4" of the muffler's body thickness (on the bottom) and then close up the hole by welding on a flat plate. This would allow ground clearance almost as high as the exhaust pipes themselves. Even losing somewhere between 15 - 20% of total plenum area, the 20" long unit should provide more additional muffling than one would get by adding an additional muffler of slightly larger size as is typically used in the rear. They say a picture is worth 1000 words, so check out the attached image.

Then you can either laugh at me, roll your eyes, or join the dog under the porch.

 

The FM muffler has a crossover internal. I'm extremely please with the sound. Zero drone driving down the hwy but lets it's presence be known under hard throttle. I am thinking about some cut outs post resonators for when I want to be unruly but they are far down my list of priorities. My only planned change to their system is the LT headers at some point. Maybe the cutouts will go in pre cat if I go that route and there is room.

 

Your mind does come up with interesting possibilities but one that concerned me when I had the car. Does your new system still drone?
Curious because although I do not have the Miata any longer, I will face that issue with my '66 Mustang with the new engine goes in. Having the sound I like without waking the dead or droning seems to be a difficult thing to find. Love the sound of the SBF but hate the drone with the typical flowmasters that seems to be so common. Had a 2012 Mustang 5.0 and put stock GT 500 exhaust on it and it had NO drone, did not wake the neighbors and had a decent sound when you got on it. Not sure if the 500's would do the same thing to a pushrod 302(not sure why not) but I am considering all options given my engine is close to being installed.

 

If you wanted to install the GT500 exhaust, I think it might have a slightly different timbre (tone quality) with the Windsor engine, but I think it would be close to the the same amplitude or "overall loudness" as before, and I see no reason why it should have any more resonance...as long as you installed the same components in the same positions. If you had catalytic converters before, that will mellow a system some .

Very little. I installed Dynomax Super Turbo mufflers and replaced the X pipe. The old X pipe had unusual, almost 90 degree intersection angles, which all three techs at the custom exhaust shop thought would cause weird turbulence and a deterioration in flow quality. They must have been correct because upon installation the increased flow velocity immediately flushed my wallet of excess cash, LOL. I doubt that the X pipe replacement had a major effect on resonance, but I'm mentioning it because I did make two exhaust changes at once.

The 2.25" Super Turbo mufflers still have a definite V8 growl, but are several decibels quieter than the Flowmaster 40. You can listen to them here; use decent earphones or computer speakers with a subwoofer to hear the true sound (regular, little computer speakers won't cut it): Sounds of Dynomax

I would rate the Super Turbo sound as a tad more healthy than most factory muscle car exhaust systems, with minimal resonance. Certainly there is no comparison to the brain-rattling resonance it had before around 1700 - 2000 rpm with the Flowmasters.

Turbo mufflers in general are "old" technology, employing the Helmholtz tuning chamber principle to provide decent sound control while reducing backpressure compared to a conventional reverse flow muffler. Several companies make Turbo mufflers; if you decide to go that route I highly recommend the Dynomax Super Turbos. They have larger internal baffles and curved flow directors to minimize restriction. They are not available in stainless, so that could be a deal breaker if you're doing an all-stainless system. For a mild steel system, they are well made, inexpensive, and available in all inlet/outlet configurations.

For your 340 HP crate engine, a 2.25" mandrel bent system would be adequate and give the desired performance growl without being as loud as a 2.5" system. So you'd get the same sound quality, just a little less quantity. If you have any tight bends to make and the exhaust will be fabricated on a crimp bender, you might want to go to a 2.5" system to compensate for where the pipes get squished a little.

Taking the Flowmasters off the table, the most common alternative to stock or turbo style mufflers would be the straight-through design. Several manufacturers make these. For comparison, here's another Dynomax sound clip for their Ultra Flo Welded, so probably recorded on the same equipment as the Super Turbo: Sounds of Dynomax

Straight-through models from other manufacturers sound very similar but not exactly the same. This type of muffler has a slightly deeper tone, a more definite pulsing sound at cruising speeds and, based on past experience, will create somewhat more interior resonance than the Super Turbos...although still definitely less than the Flowmasters. Most of the choices in this category will be stainless steel. I recommend Borla XS Pro; they have the best sound control in this category and are high quality. Not cheap, but still in the ballpark with other quality units.

With any straight-through design you can decrease resonance by using different length mufflers on the left and ride sides. With the Borlas, you can get the standard 14" body in any inlet/outlet configuration, and in the center/offset configuration you can get a 19" body. These mufflers can flow in either direction, making it easier to adapt the center/offset configuration. Different length mufflers will have different resonant frequencies, so, while one muffler is making some resonance, the other one is not, and vice versa. This effectively extends out the rpm range where some resonance occurs, but cuts the amplitude in half across some or all of the range to make it tolerable.

With ANY mufflers you choose, to further reduce resonance as well as that off-throttle rasp, you could install resonators in the system. This is a great plan where space permits. With a straight-through resonator, you will only reduce the "bad" noises without robbing from the quality of the deep, V8 throb we all like so much. That's what I am trying to accomplish for the V8 Miata with the suggested muffler modification in this thread, allowing it to act as a 20" resonator. I would employ this with 14" Borla XS Pro units in the rear.

Typically, if your mufflers are mounted midway under the car, you would put resonators near the exit. Conversely, if your mufflers are mounted back near the exit, put the resonators midway in the system. Resonators are usually round and 3.5" to 4.5" in diameter, made like old-fashioned glass packs. For most efficacy, choose the largest diameter that fits without creating unacceptable clearance issues, as well as the longest body size you can fit into the system. You can find them in many lengths; I've seen 6" to 24" body sizes. Don't be fooled by the "advertised" length; that will include the stubs at both ends. Compare actual body length. If you go to Summit Racing and search for "resonator" you will find a bunch of choices.

(Edited to replace that filthy "s-p-r-e-a-d-s" word with "extends." The robotic thought police caught me again).

 

Steve,
Thanks for the info on exhaust. Obviously you have spent a lot more time researching then I. Great info. The more you know then the more you worry about the outcome. I remember as a young person trying to get everything right then, it was not the sound I wanted and everyone who thought they were an expert, scratched their heads and said "well I thought that would sound right". I think the devil is in the details. Getting everything to mesh just right to one's own specific ears. I have about a month to decide on what I want before the engine goes in so will be looking at the options you suggested and talking to the guys putting the stuff together.
Thanks again...

 

A good strategy for ensuring you choose an exhaust with sound qualities you like is, of course, to listen to various systems in person. Best way I've found to do that is to attend car shows in the local area. It would be a rare car show that didn't have a few, vintage small block Fords. If you tell the owners what you're building and why you'd like to hear their exhaust, 90% of guys/gals will be glad to oblige. Who doesn't like showing off their car to someone who really appreciates it?

When comparing exhaust systems, consider more than the muffler type/brand. Bigger pipe size is going to be louder, as is bigger engine displacement and/or higher compression. An H-pipe in the system will mellow the sound some, an X pipe mellows it more, resonators reduce the rasp, and cats in the system reduce overall sound amplitude as well. With those mitigating factors in mind it should be easy to compare different muffler sounds as part of the whole system.

 

Gator, some questions, please, on the FM muffler:
1. Do you have any idea on its cost? (Didn't see it on their website)
2. Inlet and outlet sizes?
3. You are running cats, right?

Thanks!

 

1) $1650 including the short tube headers. They sell the headers for $595 so the header back cost of $1055. They have catless versions too..at least I know Keith's car is...

Flyin' Miata : Engine conversions : V8 conversions

2) 1.5" primaries and 2.5" collector and pipes front to back (I need to measure the primaries to verify, but pretty sure 1.5"

3) yes, high flow

Bert (my car) was the car they developed the system on...

Flyin' Miata : Projects: Bert the V8 Miata

My cats are now clearanced (flat bottomed) from how low he was with the previous owner. I'm running a bit higher due to the roads I drive. My pipes get sniffed for yearly inspection until he's 25 (2 more years). I'll not remove them unless I do something stupid like run nitrous. Cats help to reduce rasp and give a deeper sound quality to my ears.

 

The tubular subframe sure makes for nice exhaust routing!

 

I never could find one sound that works for all moods so I have 2 thoughts on this. 1, cut outs for when you want to go vroom and 2, internal resonators (like the car chemistry stuff) to kill the drone and give the illusion the car is civil.

That is all you can go back to an intelligent discussion now...

 

I've been toying with this idea for quite awhile. The only place I can see I have room for putting them is after the cats and the resonators. That kinda bums me out as I would not get the full effect. ...but then again...maybe that's a good thing. The hold up is a want to pull my muffler off to try it out. That should give me a pretty close idea to what it would be like.

 

I ran full effect cut outs on a Mustang once. I dumped them to the side in front of the rear wheels so it was headers, 2 feet of pipe to the Y then another 2 feet of pipe to the wheels on both sides. The problem with that set up is I rarely used it. On the track it was too loud to hear things I want to hear. On the freeway it droned like a bitch and in town it was way beyond legal noise limits. I tried putting a resonator in them but still way too loud dumping them right below the open window.

So I did a much better set up on my Camaro. It was long tubes to a Y with a car chemistry plug in it to kill the drone. Then a single pipe over the axle to the muffler with the cut out right in front of it. The cut out pipe went out the back so from behind it looked like normal duel exhaust but really one was open pipe and the other passed through the muffler.

The result was perfect. With the cut out open I got tons of sound but little drone and on with it dumped out the back I could hear tires just fine. With it closed it was flowmaster 80 plus the drone killer so it was very civil.

So that is exactly what I did with the Miata too using cats and a Flyin Miata turbo muffler with a 2 turn screw insert inside it to kill the drone. On the hwy with the top up and the cut out closed you barely hear the exhaust so it's nice for long trips. But most for my driving once I hit the edge of the neighborhood the cut out is open. I can still hear the morning news just fine but there is plenty of fine V8 sound track in the mix to make for a happy commute...

 

I get a good amount of V8 sound track at 3/4-wot. What I want the cut outs for is parking lot attention whoring and to scare off these daggun Fiat Abarth's that keep picking on me to race them. It's starting to get annoying! LOL

 

And that right there is one of the devious little joys of owning a V8 Miata. No body expects it to be anything but a limp dick. Vetts are my fav because they nearly always give me "the look" You know the aw, isn't that cute, a Miata that thinks he's a real car look. First time it happened I found out I made a terrible mistake. I put the cut out switch in a spot that I can't hit it and drop from 6th to 4th at the same time...

 

I'm thinking instead of the planned G8RB8 plates I'll go with GT3KLR. The Porsche guys are so much more fun to pick on.