Trackmouse UltraDork
3/13/18 2:35 p.m.

And, is the inverse true? Trying to see if length plays a role in sound. I know my open headers are deafening. But there are no mufflers yet. 

kb58 SuperDork
3/13/18 2:43 p.m.

Short answer: yes.

Long answer: I copied this article below long ago from, keeping a copy in case the link broke - it did. Here's the content (apologies to the unnamed author). This is in the context of Cobra kit cars, but much of the content applies to all cars.


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.

GameboyRMH GRM+ Memberand MegaDork
3/13/18 2:44 p.m.

Length alone does play a role in sound (and backpressue), but a minor one. If you were to take two indentical factory cars, chop up one of the exhausts to remove all the mufflers, cats, and other stuff that wasn't plain tubing, and remake that exhaust into a short side-dump with all the same components, it wouldn't be a whole lot louder than the untouched car.

Trackmouse UltraDork
3/13/18 2:55 p.m.

Great posts. Thanks a ton. Long exhaust it is. 

rslifkin SuperDork
3/13/18 2:58 p.m.

Longer does mean a little quieter, especially if there's not a lot of pipe behind the last muffler or resonator.  Long tailpipes can give the echoey sound a school bus often has.  

Most importantly, longer usually means more space to fit more mufflers.  A couple of decent size straight through Magnaflows or similar in series can be surprisingly quiet.  If you can stand the slight power loss from a chambered muffler, a combo of chambered and straight through should be even quieter.  

Trackmouse UltraDork
3/13/18 3:51 p.m.

In reply to rslifkin :

So it will be quieter if I put the mufflers closer to the headers? Rather than toward the end of the exhaust?

rslifkin SuperDork
3/13/18 4:48 p.m.
Trackmouse said:

In reply to rslifkin :

So it will be quieter if I put the mufflers closer to the headers? Rather than toward the end of the exhaust?

Other way around.  Mufflers towards the end is quieter (especially because the exhaust has cooled / contracted a bit by that point, so you can run a quieter, more restrictive muffler without a performance penalty).  That's why most modern cars hide a muffler behind the rear bumper. 

Personally, if quiet is the goal, I say put a resonator or straight through muffler in the system anywhere it fits and just run as much muffler as practical. 

Trackmouse UltraDork
3/13/18 5:02 p.m.

In reply to rslifkin :

Thanks for clearing that up, I thought that didn’t sound right since all OEMs have the muff at the end. 

Vigo UltimaDork
3/13/18 9:51 p.m.

I have found that it's usually possible to put a lot more muffler under the car than it comes with if you actually care to. I've used 3 straight-through mufflers on a 3" system and had it be reasonably quiet. Did i measure it? No. But that's straight-through mufflers, so it would be easy to tamp down noise with a different muffler design if you wanted. 

loosecannon Dork
3/13/18 9:57 p.m.

On my old Suburban, I had dual 2.5" exhaust with the quietest mufflers I could get and was very disappointed how loud it was. The exhaust ended just behind the mufflers in front of the rear axle. A friend said that on wagons in particular, it's important to have the exhaust end behind the vehicle because the car acts like an echo chamber. I added tail pipes and now it's too quiet, I may need to change the mufflers to something a little more sporty.

NoBrakesRacing Reader
3/13/18 10:06 p.m.


Or you could go the other direction ; -)


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