Exhaust size vs power vs noise

So, the 3400 powered nb miata is bumping noise limits where i run. And its loud enough at cruise to be aggravating,  and i have neighbors....

Anyway,  i need to do something different.

Currently it has long tube headers  2.5 primaries to 2.5 collectors. 2.5 from collectors at firewall area to y pipe at the end of the trans, roughly in between the seats. From there its three inch under the diff, to a 10 inch long glasspack. Three inch out of glasspack on drivers side with a pair of 90 degree turns to get to the stock exit location with a turndown

I have a 2.25 neon muffler and 90 to a tip that i think will fit in olace of whats currently after the glasspack. I also have a pretty large selection of misc exhaust stuff laying around, in 2.25-4 inch diameter. 

The car currently produces in the neighborhood of 200hp, 200tq. I don't anticipate it getting much more than 250-275 ever. It will also remain naturally aspirated. 

I guess the big questions are

Does exhaust diameter matter as much the further i get from the engine?

What is the realistic needs for pipe diameter in this application?

What would yall do? Anything better?

Ill see if i can find pictures of current setup 

Without doing any science, I think the 2.25 muffler might cost you some high rpm power.  As far as noise goes, you are on the right track.  Track down some sort of not-a-flowmaster oval muffler with a larger pipe size and you should be golden.

Find a specific spot between power poles and time yourself accelerating before and after, and you'll know if you ruined it.

I remember reading up on this a lot when I did the 2.4 swap in my Neon.  I kept the stock DOHC muffler, as all indications were it would not be a restriction, but that was at closer to 150 wheel horsepower.  I would think almost anything additional to (or other than) the glass pack would be quieter, keeping the same diameter.  Even just a cheap turbo muffler would help.

Based on this Hot Rod article, I think it’d be best to stick with the 2.5” exhaust at least.

Sorry if i misinformed: hp and torque is flywheel,  factory engine rating. 

I hadn't really thought about just adding another muffler to the existing stuff. 

Something like a dynomax super turbo should fit!

However. everything under this car is extremely tight. So....

In reply to Dusterbd13-michael (Forum Supporter) :

Put something light (I like sportbike mufflers) all the way at the back end, or maybe a couple of these Speedway cheapies in place of the 3" section?

Dusterbd13-michael (Forum Supporter) said:

Sorry if i misinformed: hp and torque is flywheel,  factory engine rating. 

I hadn't really thought about just adding another muffler to the existing stuff. 

Something like a dynomax super turbo should fit!

However. everything under this car is extremely tight. So....

If space is really tight, would a cheap Summit race muffler fit easily enough after the glasspack?  They tend to be pretty tiny, but won’t be as good as a bigger muffler.

2.5" header primaries sounds way too big for that engine to me.  At 200hp, I'd expect something like 1.75" primaries, 2.25" header collectors merged into a 2.5 for the rest of the run. 

In general, to quiet it down, use as little pipe and as much muffler as space allows.  Glasspack style mufflers like Magnaflows cut high frequency noise well and will make it quiet at idle, but loud under throttle.  Chambered stuff like a Flowmaster leaves more high frequency, removes more of the deep tones and gives a smaller noise range from idle to WOT in my experience. 

Something like a Dynomax super turbo will be quieter than a Magnaflow, but a bit more restrictive.  If you can fit one of those somewhere in there, do it.  And then the biggest Magnaflow type you have room for in place of the current glasspack.  If you can make it work, a combo of one chambered muffler and one glasspack or turbo type should be even a bit quieter. 

Find a decent 3" muffler with a straight through flow path, but with some volume. A 10" glasspack is a "pretend" muffler and won't attenuate sound much.Try something like a Magnaflow or Borla. I think the whole system is a bit oversized for the current power level, but will work fine with any future increases. If you were to look at and cut open the mufflers from a late model Camaro, or other pony cars, you would find significant volume and little to no packing. I've got a pair of Camaro ones in the garage and they're quite large but you can roll a golf ball right through them.

https://flic.kr/p/EcRqWn

https://www.flickr.com/photos/150940429@N02/shares/a9y0S5

I'll bet those headers don't scavenge at all. They're huge. Running a primary more like 1.625" would probably help power and may cut noise. You could also try a Dynatech "vortex cone" in the collector. 

The links may work.

Header primaries are 1.5, collector 2.5. Sorry for fat fingers....

Car runs 14.1 at 94 as it sits. and has more power than i can realistically use at this time. However,  on course it lives between 4-6k the whole time. So high rpm is important. 

rslifkin said:

2.5" header primaries sounds way too big for that engine to me.  At 200hp, I'd expect something like 1.75" primaries, 2.25" header collectors merged into a 2.5 for the rest of the run. 

That's what my gut says.  It looks like you could drop 40lbs from the car in exhaust alone.   

Glasspacks are helpful, but you need to mind the volume.  A 3" diameter glasspack with a 2.5" ID does basically nothing.  A 4" diameter glasspack would have 3.5x the muffling volume to work with.  And obviously, longer is better.

Combine that with a decent straight muffler and it should work well.

If you want to cut out the drone, you can add in a hemholtz resonator tuned for cruising RPM for almost free.

I would run no larger than 2" off the header, and then probably 2.5" single.

I usually do 2.25 to 2.5" exhaust off V8s.

I ran 2.5" off the manifold all the way back to a Magnaflow on a KA24E years ago, picked up good top end, and lost some low end, and was louder than heck everywhere. I should have gone 2" on that one.

This may help:

https://m.roadkillcustoms.com/determine-proper-exhaust-tubing-size/

Run the longest, largest diameter muffler you can.

David Vizard wrote in a book I have of his about keeping a race car quiet: perforated core glasspack in the middle, followed by a turbo muffler at the back.  Quiet, but still flows well.

As a reference point, an S2000 has a 2.5" single section (smaller after the split).  The 2.5" is capable of supporting the power (which should be similar to your target power in the future) without issue.  The mufflers aren't even that restrictive, but they are about 60lbs (!) combined.

A lot of people were doing a mod where you put a V-band on right before the split and you could swap the mufflers on and off or replace with a turndown for competition.

SkinnyG (Forum Supporter) said:

Run the longest, largest diameter muffler you can.

David Vizard wrote in a book I have of his about keeping a race car quiet: perforated core glasspack in the middle, followed by a turbo muffler at the back.  Quiet, but still flows well.

That's a good point.  Exhaust cools and contracts as it flows through the system, so putting the most restrictive muffler last in line minimizes the penalty from it.  That's one of the reasons modern cars have the big mufflers all the way at the back, rather than under the car with a tailpipe. 

We did a piece on exhaust fab and testing that might help. Here's the link. 

Dusterbd13-michael (Forum Supporter) said:

...Currently it has long tube headers  2.5 primaries to 2.5 collectors. 2.5 from collectors at firewall area to y pipe at the end of the trans...

I really hope that the 2.5" primary is a typo... Hell, how would that even fit.

Depends on noise rules. 

I used to race with an organization in the Pacific Northwest (ICSCC) and part way through my racing career they added a maximum noise reg. I was running a megaphone tip off a Norton bike at the time and that probably exacerbated my problem, but I solved it by fitting a bent tip instead of the megaphone. It was probably about a 60 deg. bend. I'd just find out where they were taking sound readings from that weekend, and then loosened the clamp on the pipe tip and rotated it so that when I passed the sound station the tip was pointing away from it.

They messed that up a couple of times by moving it during the weekend so then I'd just point the tip straight up which reduced readings.  I eventually had to go to a multi plate tip that could be adjusted for noise level.

In the old days the allowable level was 95 dB - today they are as low as 88 dB locally which makes it much harder to attain.

Here's something I copied off the web years ago. Note that the article was posted on a Cobra site, but the basics still apply:

Full credit is given to the original author, Tom Georgalos.

Summed up: the bigger the muffler and the smaller the exhaust diameter the better.

-------------------

With the sound restrictions getting tighter at Laguna Seca and for that matter all tracks, we are continually looking for better ways to quiet our cars. The sound meter requirements over the last 10 years have been tweaked more and more. Each time the meter was tweaked tighter we seemed to know what to do and how to quiet our cars just that much more. We started around 103 dba, then went to 100 dba, then 95 dba. When the meter hit this last mark of 95 dba we started to separate the men from the boys. The race cars with high strung motors were the most difficult to tame. Technology came around with the Flowmaster muffler, which seemed like a miracle. This muffler didn't cut down on power, it enhanced torque, and it made the limit. Now we are at a new low limit which is being strictly enforced, and this is as low as 90 dba. By the way these readings at Laguna Seca are taken at 50 feet. Most other race tracks take their readings at 100 feet. Quite a difference!

We don't claim to know the exact solution for quieting the car on the racetrack, but are in a panic to perfect our readings. The advice we offer below is merely some findings that we offer to you. We would love to take advice from anybody and share it with others. Many cars in our group pass with flying colors, we will try and list their formula. We are also in search of other techniques all the time which we will try for the future.

Exhaust Systems That Don't Work.
•Open exhaust, obviously doesn't work!
•Glasspack mufflers do not work!
•3 inch pipe Flowmasters don't work!
•Flowmasters with center inlet or center outlet don't work!
•3 inch pipe or larger Brand-X mufflers don't work!
•Shorter than 28 inch LoBaks don't work!
•Race style LoBaks don't work!
•Borla mufflers without catalytic converters or without a secondary muffler such as a Supertrapp don't work.
•Monza style exhaust or European high performance mufflers on Ferrari's, Porsche's, etc., don't work.
•Stinger type of exhaust on Porsche's & Mazdas, commonly known as expansion chambers, don't work.

•Diameter of Exhaust System
2 1/2 inch pipe is the absolute largest pipe that can be utilized to make strict sound limits. Naturally 2 1/4 inch or 2 inch would even be quieter. This even applies for necking down of side pipes.

•Length of Exhaust Pipe
The longer the pipe the quieter the system. Exhaust all the way out the back of the car is going to be quieter than side exhaust. Some believe routing both pipes, if dual exhaust is used, to the drivers side of the car, helps divert sound away from the sound meter. This can only be a crutch until sound is read on both sides of the track.

•Flowmaster Mufflers
Large three chamber Flowmasters (approximately 16 inches or longer) with offset in, offset out 2 1/2 inch pipe seem to get you just below the limit. On certain days this system may be right at the limit, especially if the atmosphere has any moisture present. Crossover tube is essential. Try part # FLO-42553. This is the quietest Flowmaster that they have currently. It will not sacrifice power up to 400 HP. This will satisfy most of our needs. By the way, Flowmaster has found their mufflers are quietest at wide open throttle. They seem to smooth out the exhaust flow and that's when they really do their job well. The latest versions of Flowmasters are aluminized to hold off corrosion. By the way we are on about the fourth generation of Flowmasters currently. You may have the old ones.

Flowmaster offers complete Cat-Back systems, which may interest owners of late model Mustangs & Corvettes, etc. Contact them for your application. They also have developed new bolt on aluminum heat shields which incorporate stainless steel straps. These might be just the ticket for vehicles with converters, which generate more heat. Great safety item!

•LoBak Mufflers
Street LowBaks 32 inch or longer seem to get the job done, keeping with 2 1/2 inch pipe & a crossover tube. Stay away from the race version of the LowBak. Some have reported 24 inch LowBaks working with fairly healthy motors and keeping well below the sound limit. These seem to be the answer for sidepipes for Cobra's, yet must incorporate an additional Supertrapp attached to the end. Nevertheless this is far less restrictive than just Supertrapps.

•Supertrapps & Supertrapp Mufflers
Supertrapps definitely work! They are more restrictive than most of the race style mufflers, but they allow you to stay on track with no black flag. You can tune them on the spot for the most power possible. Supertrapps seem to be the real answer as a secondary muffler system. They are the assurance you need to tune your car quiet when all else fails. Supertrapp mufflers with the plates on the tail end are less restrictive than just the plates by themselves. These may warrant looking into for side pipes and Cobra's.

•Catalytic Converters
These act more like a muffler than can be imagined. A stock Mustang GT with catalytic converters will register down in the low 80's for decibels. If your car was designed to work and run with catalytic converters, it will be just fine. Catalytic converters definitely help and work! Don't take them off! If you take them off you place yourself in the same position as the vintage cars. That is, constantly trying to find a solution. If your car is tuned for catalytic converters, stick with them! They don't give up that much power. There are some companies that offer Hi-Flo catalytic converters that claim 50 -100 % gains in flow, which mean less back pressure. This would be the avenue to pursue while still keeping sound levels down. One such company is Random Technology in Stone Mountain, GA., with their Super High Flow converters 404-978- 0264. Another source is M & M Distributing, Mesa, AZ., who carries Mor-Flow Converters 800-408-2287. Dynomax is a most recent developer of hi-flow converters. They are cheap and affordable. A final source for aftermarket high flow converters is Summit Racing Equipment at 330-630-0200.

•Stock Mufflers
Most stock street mufflers seem to work, these are somewhat more restrictive than the so-called performance mufflers. Remember, 2 1/2 inch pipe. Don't forget the crossover tube! Naturally 2 or 2 1/4 inch pipe would be even quieter!

•Walker/ DynoMax Turbo Mufflers
Turbo mufflers, as mentioned, seem to work. Aiming the exhaust to the ground in front of the rear axle causes a resonance & may cause you to be over the limit. Once again don't forget the crossover tube and out the back is the best method of quieting the car within the limit using turbo style mufflers. One of the better known brands is Dynomax, although this information could be applied generally to all brands. The Dynomax seems to be very efficient for dollars spent, robs very little horsepower.On very high strung motors or race motors, 2 1/2" Dynomax may be just over the limit. It may take 2 1/4" pipe to actually do the job. In the Dynomax line, stick with Super Turbo models, nothing less. The Ultra Flo model will not work.
Dynomax offers complete Cat-Back systems, which may interest owners of late model Mustangs & Corvettes, etc. Contact them for your applications.

•Borla Mufflers
Beware of using Borla mufflers without catalytic converters! See above. We have not seen anyone claim to conquer Laguna with Borla's by themselves. Using Borla mufflers, with 2 1/4" pipe may be the way to make them work. We know they work with catalytic converters and or Supertrapps on the ends. Let someone else try them first as a primary muffler. On the plus side these are stainless steel, carry a great lifetime warranty, and naturally these mufflers claim great power gains.
Borla offers complete Cat-Back systems, which may interest owners of late model Mustangs, Corvettes, & SHO's. etc. Contact them for your applications. We are confident that these systems with catalytic converters would pass the sound limits.

•Edelbrock Mufflers
Edelbrock has come out with a new line of mufflers. They are referred to as the RPM Series Mufflers. We don't have information on them yet, but know they have been vintage racing with two or three sets of them in the development stage. They claim good torque benefits, little horsepower loss, and many of the advantages of Flowmaster mufflers. These mufflers are stainless steel, and offer lifetime warranty like the Borla mufflers. They claim to be quieter than Flowmasters, yet we can't attest to the power relationship. We will report when more information is available.
Edelbrock offers complete Cat-Back systems, which may interest owners of late model Mustangs & Corvettes, etc. Contact them for your application.

•Bassani Mufflers
This really seems to be a popular addition to my information. This company makes an X-pipe bolt on fabricated crossover for the late model Mustangs, including the 4.6 liter variety. Their mufflers are good and adequate especially when used with the cats. Quality and fit really seem to shine with Bassani.

•Mandrel Bent Pipes
There are many companies forming mandrel bent pipes lately. This enables one to go with a small diameter tubing, utilizing quieter readings while still keeping up good flow characteristics. A normal 2 1/2" pipe system with normal muffler shop bends will reduce down to around 2.5 square inches of exhaust flow. The mandrel bent tube will maintain 3.14 square inches throughout. This is commensurate to other pipe diameters.

•Latest Attempts and Technology
The latest race muffler set-ups are proving that single exhaust systems are the most advanced. They offer virtually no penalty of power, yet quiet things down. Consult your muffler manufacturer. Along the lines of two devices, we saw a Flowmaster system with either Lobaks or glasspacks welded in- line to help cure the problem. This worked! The system was 3 inch, sort of an exception. The motor was a 377 c.i. Windsor stroker!

Harley Davidson motorcycle pipe baffles. We have seen these inserted in the end of the exhaust system to help quiet the system a bit more. Naturally you will need a straight shot of pipe at the end to incorporate this.

•Reducers and Trick Pieces
If you are fabricating an exhaust system and want the smoothest transition of exhaust gas possible.

If you are fabricating a single exhaust system and want to blend in two pipes with the smoothest transition possible.

•Some Other Factors
Noise is noise! It doesn't have to be exhaust sound to be noise and trip the meter. Here are a few of the things that might play havoc with the sound meter, while you are aiming at the exhaust system.

• Quick change rear-end, like a Franklin
• Flex fans for the radiator
• Gear drive timing sets
• Weber carburetors with velocity stacks
• Mechanical fuel injection with velocity stacks
• Turbo charger swooosh!
• Cam timing. Who can tell us what is the best type of cam to run for sound? Lots of duration or little duration?
• Compression ratio, the more you have, the more the bang.
• Exhaust leaks, holes in pipes or bad connections, etc.

That's about controlling noise, not performance. The fact that the Supertrapp is identified as a good option (you get power or you get muffling, but you don't get both) indicates this priority. I've used a slip-on Supertrapp for noise control in the past, but it's very much the "I need to pass sound at this track today" option.

Also, I'm surprised this thread has gone on this long without anyone suggesting a turbine muffler.  You could connect another turbine to it and use it to compress air on the intake side to make more power...

frenchyd said:

In reply to kb58 :

Wave theory, it actually works. 
Here's the background.  Listen to a Tuba. Then listen to a clarinet. The big air volume required to play a Tuba loud compared to the Clarinet should give you an idea of why this works. 
     Now we have to go to the camshaft. When the valve starts to open air flow is relatively small compared to when the valve is wide open. So the exhaust "note" comes in the form of a wave.  
     Headers, properly designed headers ( not simply tubular exhaust manifolds)  will pulse that wave out. To make peak power the full wave needs to hit the collector at full wave and be "pushed" out by the following wave.  That's why length and diameter are so critical. That's the horsepower part of this equation. Trust me the maths are complicated but luckily there are plenty of sites that will do the maths for you. But you do need to know things like flow rate and camshaft specs. No strip street is not a spec.  Lift and duration are.  
Stepped exhaust is what is next for muffling. 
 Primary pipes lead to collector, collector leads to exhaust pipe. Leads to muffler, leads to tail pipe, leads to outlet. 
    Each needs to increase in diameter and is most effective if the connection is after a complete wave pulse.  
The computer gave you a length for primary pipes and a diameter.  The collector is larger but shorter.  Using that volume let's say it's 22.8 fluid ounces. That is the magic number. The exhaust pipe needs to be 22.8  fluid ounces. But because it's the next larger sized pipe it will be shorter, the muffler?  You can only use the tube size and guess at how much larger the effective absorption materials are. Let's say 22" ? Plus 10%?   Then there is the practical matter of what is available.
Followed by tail pipe of the next diameter again 22.8 fluid ounces. And outlet normally a turn down again the next size or two larger. 
What?  It doesn't come out to your rear bumper or the side of your car?   Good, there is no rule that it has to.  Sound bouncing off the pavement scatters. The under chassis turbulence adds to that scattering. The more turbulence the  quieter it will go past the sound meter.  
 

On a separate note if the race track really wants to be good neighbors they will plant evergreens around the track.  Junipers have the best sound attenuation.  A double row will quadruple sound attenuation. 

Quoting this because it is fact.

Couple 49 state universal cats in those flexy tubes might do everything you need on the noise front as well as be better for other reasons.

Have you thought about Super Trapps ? We ran them on our dirt cars a few years ago when people started building houses next to the racetrack and complaining about the noise.