How to quiet a noisy muffler

Last update: 5/21/2015  Return to home page links


After the initial excitement of actually getting the car on the road I soon realized that the exhaust sound was just WAY to loud!  In fact it was absolutely deafening above 5,000 RPM!  Something had to be done.

Plan A

I tried a racket buster on the stubby exhaust pipe that was a few inches long at the muffler output.  It helped a little but not enough to matter.

Plan B

I really wanted the exhaust from the muffler to go out the back of the car and it seemed logical that it should reduce the noise somewhat.  I used some stainless steel pool ladder tubing to fabricate the exhaust pipe from the muffler to the rear of the car.  It really didn't change the exhaust noise level at all.

What should the next plan be?

At this point I started investigating why this muffler was so loud. Since it appeared to be a stock muffler that came with the GSXR-1000 I realized that something really screwy was going on. When I had removed the end cap from the back of the muffler to install the longer tail pipe, I noticed that the short outlet pipe  in the muffler was a 9" piece of pipe that extended into the muffler to an open chamber. Most bikes that I've played with had some sort of baffle or screen with a porous material wrapped around it. This muffler had just an open pipe. I looked on Internet and found all sorts of motorcycle muffler baffles but none for the stock GSXR-1000 muffler. 

I went to the local Suzuki dealer and got conflicting answers to my question if the stock muffler in 2002 had a baffle of some sort. Basically none of the service people had ever taken one apart and the parts book didn't show one. My personal service manual doesn't show a baffle either. But I was pretty sure that there is supposed be one in there since I've never heard a Suzuki motorcycle that loud with the stock muffler.

I sometimes hang out on the LocostUSA.forums.  I posted a question about the stock muffler baffle and got various responses including the suggestion that perhaps my muffler was originally designed to be used with a catalytic converter. I can't imagine how a cat converter could be mounted on my stock looking muffler, but maybe?

The part number on my muffler is ""40F0A" and as far as I've been able to determine that it was used on the 2001 and 2002 GSXR-1000s. One of the fellow posters on LocostUSA had a brand new GSXR-1000 muffler so we compared internal measurements and my muffler is an absolutely original unmodified stock muffler.

The almost final solution

After building several space frame cars, go karts and Locouki I decided the heck with it, I'll just build a muffler baffle and see if it works. The main problem I was concerned with was that I didn't restrict the exhaust flow and cause excessive back pressure. I decided to build a tapered screen of some sort and keep the outlet end diameter of the baffle big enough to handle the rather large exhaust volume at 12,000 RPM. The pictures below show  how I made the baffle.

The real final solution

[5/10/2007 edit]  After driving the car around a bit with the tapered screen baffle, I realized that it was still too loud. I could hear the intake noise once in awhile but the exhaust was still way too loud. I removed the tapered baffle and hacksawed off about 1" of the small end of the screen (including the small washer). I then slit the screen into four 1/2" long pedals and folded them over on top of each other to form the new end. I also wrapped the baffle with some stainless steel (s.s.) wool and inserted it into the outlet pipe as before..

By shortening the screen, the whole baffle is completely contained within the 9" long outlet pipe. These changes resulted in the exhaust noise finally being lower than the intake noise. A lot of the noise I am/was hearing is caused by the intake but it is not an annoying sound. But it certainly gets your attention as it screams and yowls when you accelerate hard through the gears! With my air scoop open at both ends I think a good bit of the intake noise is directed fore and aft for my listening pleasure.

[5/18/2007 edit]  I finally have my exhaust noise under control. The last version of my baffle with the s.s. wool packing was actually too quiet. So I modified the small end of the tapered baffle by opening the 1/2" screen pedals slightly to control how much of the exhaust/noise is bypassed directly to the outlet pipe. The packing seems to be holding up OK in the exhaust heat.

I want the exhaust sound to be slightly quieter than the intake unless I really give it the gas. That way my ears will be spared but it won't be so quiet that no one knows I'm there. Of course when you really give it the gas EVERYONE knows you are around! The intake system howls like four screaming organ pipes. Then add in the enraged exhaust roar to that .......................!

[8/8/2011]  I believe I have finally figured out why my stock Suzuki muffler is so loud on my car. After I determined that my muffler is an absolutely stock muffler I reread my "Exhaust and intake systems" book to reacquaint myself with organ and tuned exhaust theory.

I have a spare stock GSXR header and the exhaust EXCV valve that came with my engine.  The normal stock GSXR has a 4 into 2 into1 exhaust header with smaller diameter pipes and the EXCV valve in the middle of the single section.  Normally this valve is completely open at ~5000 rpm.

In comparing the spare parts to the pipes on my car, I realized that the four short header pipes on the car were made from another GSXR header which are then combined into a single much larger diameter collector pipe which goes to the input end of the muffler.  The 'ol "bigger is better" theory of hot rodding.  The exhaust system was already on the car when I bought it as a roller.

I've designed tuned exhausts before and suddenly I realized what was going on and why routing the exhaust out the back of the car didn't lower the noise level. Keep in mind that the muffler was originally part of a tuned system on the bike.  The new header and exhaust system up to muffler is completely different from what the muffler was designed to work with.

The booming noise is created when the 4 header pipes dump into the larger volume of the header pipe combined with the first chamber in the muffler. And since the muffler is basically two open chambers connected by a large internal short tube they also acted as boom enhancers. The two chambers broadened the RPM range over which the noise level is at ear shattering levels!

To get an idea of what this sounds like you can do something that I did when I was in grade school (MANY years ago).  I used a half gallon glass milk bottle but something of that size and opening diameter will work as well.  Hold the bottle about 3" or 4" from the end of a lawn mower exhaust pipe.  Preferably with no muffler but with a short piece of the exhaust pipe.  We did it with a Cushman motor scooter with the engine idling.  As you vary the distance of the open end of the bottle from the end of the exhaust pipe you will hit a resonance point and the exhaust will suddenly become a very loud "boom-boom" sound.

That's what my car sounded like but much louder!  I mean loud to the point where it hurt my ears above 9,000 rpm!  The boom-boom was being created inside the large header pipe in combination with the first muffler chamber.  That is probably why adding the tail pipe out the back of the car made no difference in the sound level. The noise was coming right through the side of the muffler!  The muffler has a sound deadening layer of material around the inside of the muffler shell but just couldn't silence the increased noise at resonance that the non stock header pipe etc had created.

The following is a series of pictures that show how I made the baffle which tamed the beast.
Baffle, side view of material

This is the steel screen that I used to make the baffle. At this point I had just started forming it around a 1"diameter piece of pipe.

Baffle, end view of material

This is an end view of the same screen. The screen was originally an internal ventilated cover for a radio transmitter.

Baffle after forming

This is the baffle after I formed the material into a tapered tube. The washer on the left is a 1" diameter washer and has a 3/8" diameter hole. The larger washer on the right was machined to 1.7" outer diameter to fit snugly into the muffler outlet pipe. The hole was enlarged to 1-1/8" diameter. (This picture was made before I shortened the tube 1-1/2".)
Baffle weld
This picture shows the baffle after the washers were brazed on the ends. Actually this picture was made after I had driven the car to test the baffle. Note that the cadmium plating has been discolored from the heat after one 12,000 RPM blast!

Baffle big end
This is the outlet end of the baffle. I used some stainless steel screws to hold the screen together.

Baffle small end
This is the "inlet" end of the baffle. This is the 1st version of the baffle. Since this picture was taken, I have shortened the screen length by 1-1/2" as described above. I then created 4 screen pedals which overlap each other slightly at the end of the baffle.
All parts of muffler and baffle
Here are all the pieces that go into and on the end of the muffler. The baffle is inserted into the exhaust pipe in the direction that is shown.

The small end doesn't extend past the forward end of the muffler outlet pipe. I wrapped the baffle with stainless steel wool before inserting it into the outlet pipe.

Muffler end
Notice the small hole at the top of the muffler output pipe where a stainless screw is used to hold the baffle and the tailpipe onto the muffler. You can see the sun shining through the hole onto the inside of the pipe.

The tailpipe slips over the outlet pipe. The screw extends through the tailpipe, through the outlet pipe and finally through one of the holes in the baffle screen. This holds everything in place.
Baffle partly inserted
The baffle is partly inserted in this picture. The large washer will just slide into the muffler tailpipe. Since this picture was taken I've wrapped the baffle tube with stainless steel. wool.

The set screw doesn't allow the baffle to rattle in the muffler.
Baffle fully inserted
This view shows the baffle fully inserted into the muffler. The baffle can be removed by using a large hex head bolt to grab the large washer from inside the baffle. The baffle is then just pulled out of the muffler using the bolt as a handle.

Set screw
This view shows the tailpipe slipped over the muffler outlet pipe, the set screw that holds everything is in place and the original decorative cover that will hide the screw.

Muffler fully assembled
And here is the final product fully assembled. The pool ladder really makes a nice tailpipe. The tailpipe angles down towards the ground and then runs parallel to the side of the body towards the rear of the car. See "miscellaneous hints" for a way to keep the exhaust pipes from turning blue.

For those that are interested here are the dimensions of my stock
Suzuki 40F0A muffler

Total length........................... 17"
Muffler diameter.................... 4-1/2"
Inlet pipe dia.......................... 2"
Outlet pipe dia....................... 1.7"
1st internal chamber length..... 6-1/2"
2nd internal chamber length... 11"
Connecting pipe length........... 5"
Outlet pipe length.................. 9"

General description

The muffler is a very sturdy not too heavy a design that looks very good on the car.  The inlet has a four bolt flange that connects to the end of the header pipe that has a mating flange. The outlet pipe is angled to the axis of the muffler so that when the muffler was mounted on the bike the outlet was horizontal.

Internally the muffler has two chambers connected by a 5" length of 1.7" diameter pipe. The connecting pipe is positioned so that  more of it is within the second chamber than the first chamber.  The walls external of the chambers are made up of a perforated screen with some sort of packing between the screen and the outside wall of the muffler.

The internal end of the outlet pipe overlaps the internal chamber connecting pipe. This causes the exhaust from the first chamber to to be directed towards the rear wall of the second chamber. The exhaust then has to reverse direction to enter the outlet pipe on it's way to the atmosphere.

Even though my muffler is labeled that it meets the federal requirements for noise at 80 dBA it is VERY noisy without the baffle I made for it. There is NO way it would meet that specification without the baffle on my car.  At more than ~120 dBA noise hurts your ears.  Without the baffle the noise was way above the "hurting" point!

The muffler has a mounting tab welded to the case about 1/3 of the length forward of the outlet pipe.  My muffler is mounted so that the tab is horizontal and fits through a slot in the side of the body work.  There is a hole near the end of the end of the tab that is about 2-1/2" above the lower chassis rail.  There is a long 3/8" diameter bolt that passes through the tab, through an engine wrist pin and finally through a 3/8" diameter vertical hole in the lower chassis tube.  This arrangement clamps the muffler tab to the chassis.  The tab doesn't get unbearably warm and there isn't any noticeable noise transfered to the chassis.  My engine is hard mounted to an engine cage which is bolted directly to the chassis so the muffler is supported at both ends with no movement.

Something to try

When I feel like I just need something else to do I have an idea that might improve my exhaust system.  Rather than having the baffle at the outlet end of the muffler where it creates some back pressure in the system (which ought to lower the resonant frequency), I think a better idea would be to place the perforated baffle at the inlet end of the muffler.  This will hopefully break up the resonance of the boom at the source of the noise.  The resonance causing header pipe/muffler dimensions will be changed since the header pipe won't be connected directly to the 1st muffler chamber.  With luck I won't need to stuff stainless steel wool  into the baffle and the back pressure will be less than what I have now.

Normally you want a large chamber volume at the outlet of a tunned header pipe to simulate it dumping into the atmosphere but in this case it is causing a deep organ pipe affect that is deafening.  But if it doesn't work as hoped I can always put the original baffle back in the outlet end of the muffler.

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