Soundcheck info request

Here in Michigan for several events we must pass a 94DB sound level check. The main test criteria involves the RPM's needed to reach PEAK horsepower. That information is used to set up a harmonics tuner, and then the throttle is brought to a level that the tuning wand movement is at a minimum. The sound reading is taken at that point.

Does anyone know where I can find the RPM value at PEAK horsepower, for a factory stock 01 wr426f, and a 01wr426 with the throttle stop shortened, air box lid removed, and stinger removed.

You wont pass 94db with the insert removed.

That I know of there are only 2 exhausts in the world that will pass the 94db test.

One is made by CRD and is sold by BAJA Designs. It is rated at 91db.

The other is the made by yamaha and is on your bike with the insert in it.

I have a FMF megamax II on my yz400. Its quiter than stock but I know it wont pass the 94db test. Ive heard that if you take most of the disks out of it it will pass but then the bike wont run. so your screwed eather way.

Ive heard of a meathod of quieting a scilencer. Find a plant buld auger the size of your perforated core. Its like a large corkscrew drill bit. You may have to cut and weld a few of these augers together to make them as long as your core. weld it in place inside your core. In theary this will push the soundwaves into the packing without restrecting the flow.

good luck.

Hey Jim, how goes? There may be an inconsistency in the criteria you described for the sound test. Peak hp generally occurs near max rpms and I can guarantee you that any WR/YZ pipe you find will read well over 100 dB at those high rpms. You may have mis-heard the criteria. It is likely one of the fall-back methods for finding the test rpm for a bike where the MMIC has not published it, which could be the 1/2 of max hp rpm method. The test method is probably SAE J1287 JUN88 Stationary Sound Test for Offroad Vehicles, which the MMIC endorses. The published rpm's I know of for the 98-2000 YZ/WR 400/426 is 4500 rpm. If they've got the MMIC booklet, they've got that number. If they use another method to calculate the rpm, it will be higher, possibly up to 5500 rpm. dB's vary roughly with rpm, so you'll find higher measured sound values as the rpm's increase.

All the bike mods you've listed don't bear on the issue. It's only rpm (which is bike model and year specific) and test method that determine the outcome. They're actually intended to ensure that your bike mods don't make the bike too loud. BTW the harmonics tuner is just a mechanical tachometer to let them see you're holding the engine at the right rpm during the test reading.

So, the 01 and 02 test rpm values are probably still 4500 rpm. In case you're interested, a stock YZ pipe will blow over 100 dB at 4500 rpm. A stock WR pipe (intact!!) will maybe meet 94 dB - I measured 96.

Any Q's just ask. :-)

Originally posted by yzman400:

One is made by CRD and is sold by BAJA Designs. It is rated at 91db.

Is the CRD USFS approved yet?

Im not sure. Brian Bosh has one. Ask him.


I have the Vortip in my stock exhaust on my 01 WR426. I was measured at one of the CCC events this year and I passed the 94dB limit. I was not told how far under 94 I was but guys all commented on how quiet it was. I think it is quieter than stock at idle and gets progressively louder than stock as the RPMs increase, but passes 94 where they tested it.

Hope this helps.

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