"How To Quiet Down That Obnoxious, 4-Stroke Muffler" or "Stealth Me, Baby!"

By Tom Niemela

Now we've gotten serious.  We're saying, "Shut Up!"  Too many riding areas are getting systematically closed down, so we have officially imposed a 94 decibel maximum exhaust limit at our events.  Would prefer 92, but 94 should be easily attainable to start with.  Are we cruel?  Are we picking on people that have aftermarket muffs?  Are we just trying to force everyone to ride stock muffled bikes?  No (well, maybe on our AA sections!), no, and no!  



Being active in keeping our riding areas open, we encounter many situations and opinions.  Our club encounters a smattering of our riding friends who continue to ride with loud pipes.  Emotional horsepower.  Noise annoys.  Period.  Most of the 4-strokes nowadays have so much power, why would you want to go up another 10 decibels for a small amount of ponies?  We also are around land owners, public and private.  Many don't understand our sport, and we ALWAYS get hit with the noise issue.  If we were on closed-course areas, I wouldn't writhe over the issue, but our events are on public lands where people use the lands in other various ways too.  That's what they remember - the noise.  Noise annoys, and I don't buy into the "loud pipes save lives" slogan, although that's primarily for the street.  I buy into the Team Stealth concept.  Over the years, our events have slowly lost access to more areas due to loud exhausts.

In this cloud-of-loud, here are some options that work.  You could try some of the following:

Alan Roach's website at Baja Designs has a nice, real-world description of the issue and possible solutions:

"Muffling a big bore four stroke motor is a real pain. Anything you do to make it quiet seems to have the same effect as stuffing a potato in the exhaust pipe. Iíve come to realize, however, that if there is going to be anywhere left to ride, its up to us to make our bikes as quiet as possible. Iíd rather put up with a little horsepower loss than have closed signs on my favorite riding areas. If you can put up with the stock muffler, leave it on. ďBut it weighs 23 lbs!Ē one customer told me. Try 7 lbs, 3 ounces (XR 600). Itís lighter than it looks. The lightest aftermarket mufflers shed a whooping 2 lbs (Big Deal on a motorcycle that already weighs 285 lbs). Thatís why one of our favorite ways to get more power out of your XR or KLX is to use a Thumper Racing exhaust baffle." 

"It replaces the restrictive baffle in your stock muffler, increasing power output considerably while only increasing the noise level a few decibels. If you want to go to an aftermarket muffler, there are three choices currently that are acceptable sound output wise. They are the new Yoshimura TEC, and the new FMF Megamax or Performax. They provide decent horsepower gains, about 4 to 5 horsepower on the top end. All give the bike better throttle response and cleaner carburation, especially if youíve previously freed up the intake. All are louder than stock but are not obnoxious like a standard Supertrapp, Cobra ISDE (definitely not stealth) or Megalloy. If you already have a Supertrapp, do everyone a favor and retrofit the EAR baffle to it, while we still have some places left to ride."

This is the solution I personally use: the stock muff and the TR down snout tip.  I'm still running stock jetting and baffle, although I plan on modifying those.  The exhaust is still quiet and the bike rocks (XR650).  I have to agree with Allan - this combo works and doesn't have kids in campgrounds covering their ears when I ride by.  Worked on my XR600 too.  I cannot attest to the db level of the Yoshimura and FMF yet, but I can attest to the horrible Supertrapps and the Cobras.  Unacceptable.  I've taken many stock pipes and did the Swiss cheese effect in the end of the pipe. That works too.  However, you still want to retain your spark arrestor qualifications.  I installed a White Brothers E-series exhaust on my Yamaha YZF426 race bike and, although it's considerably quieter than the disgusting stocker, I'm down to only five disks (it still revs out fine) and plan to try only four. 

So you don't have a decibel meter and you  don't know how loud 94db is?  Testing has shown that a current, stock Honda CR250 or Yamaha YZ250 (with a freshly and properly repacked muffler) is almost on the mark for 94db.  That should give you a baseline.  Be aware that the 4-stroke exhaust carries further (due to its lower note), but not as much of a pitch as a 2-stroke exhaust.

Another option that has been used at tech inspections of many ISDE enduros is to cut up a piece of thick cycle inner tube about a foot long.  Slide the tube over the end of the pipe, slip a hose clamp on to hold it at bay, and enjoy the quieter exhaust.  Be sure to point the tube downward.  This lasts awhile, but will eventually wear out, or burn off the tube, so it's only a temporary fix.  If you can get it to fit, car radiator tubing lasts much longer.  If you are aware of any other tips/tricks to help quiet things down, please let me know!

The AMA has a standard for measuring exhaust sound. It is posted in their competition book each year titled AMA Sports Rules, and is posted at:  http://www.ama-cycle.org/rulebooks/. The AMA standard for performing the test is by using a calibrated decibel meter 20 inches away from the exhaust tip at a 45 degree angle to the exhaust flow. Most events have a piece of string 20" long, tied to the end of the db meter. They just hold the end of the string to the end of the exhaust and pull the meter 45 degrees to the side (of the direction of the exhaust outlet). Once the exhaust is revved up, the db meter will show the actual db level. Make SURE that the db meter being used has been, and is, calibrated often!  Otherwise, the results cannot be reliably used.  If you just purchase a db meter from Radio Shack, it cannot be trusted, since it has no way to calibrate itself.  Intercomp Racing Products offer about the cheapest one that calibrates. You can find them on the web at: http://www.intercomp-racing.com/  The sound meter they offer can be seen by clicking here.   The cost of this unit is $209, part #360015. A sizeable chunk, but is about the cheapest quality unit and is very accurate and can be quickly calibrated.   Technically there are specified rpm's for all engines, where the exhaust should be measured (half max RPM), and that is the point to be measured. Many bikes don't have tachs, so another option is to use an oscillating needle (Briggs & Stratton market one) that will oscillate to its widest point at specified RPMs (when set on the engine case).  It is manufactured in W. Germany and sold/imported by B&S under the name of 'Treysit' sirometer or vibratach. You can see it here: http://www.perr.com/images/bs_19200.jpg  Cost is about $17. Just set the knob to the proper RPM setting and  the wire arc will get wider and wider - that's the point to measure (at its widest arc, no matter what the shape of the arc is).   This is when the engine is at half its maximum RPM.  Only problem with this needle method is if an engine uses counter balancers, it can be harder to see, then you must use an inductive tachometer instead.  If using either tach, then you will need a booklet (available from Motorcycle Industry Council) that has the proper rpm listing for each brand and size of engine for testing sound. BE SURE to get the latest, downloadable updates and addendums from MIC's site!  Most events just have you roll up your scoot, one person holds the string on the exhaust tip with one hand, and the db meter in the other. The other person revs the throttle briefly (usually around 1/2~3/4 throttle) and they record the db level.  Done.  What's interesting is the amount of noise many engines emit (not counting the exhaust). Nowadays most bikes are water cooled, which absorbs the majority of engine noise, but in the 'old days', many engines had huge fins on the cylinder that radiated the noise outward. 

Here is another great article that was passed around that kind of hits straight to this issue. 

Bottom line: it's up to us to take care of us, before we have yet more laws imposed on our sport, or continue to get closed out.  With that, please understand our reasoning for this requirement.  Yes, we like a sweet-sounding scoot.  We just know that non-riders don't, and we want to offer you all MORE places to ride.  Yes, we race too.  We just prefer to have loud pipes in their place.  What we all should do is adopt a policy of getting a quiet pipe and try to make it quieter.

Be a part of the solution or continue to be a part of the problem.