Physics\design of the '08/'09 shorty can horsepower

6 replies to this topic
  • FriscoKLR

Posted October 21, 2010 - 04:16 PM


Just picked up a 2009 450f, have it in the garage next to my clapped out '06. What in the design of the shorty muffler on the '09 450f's can cost that much horsepower compared to the '06?

  • Steel Panther

Posted October 21, 2010 - 05:04 PM


Its got a bunch of cones in it, thats all I know, if you look someone had a picture of one cut open a while back.

  • KJ790

Posted October 21, 2010 - 05:20 PM


The 06 silencer (and pretty much every other mx silencer) is just a straight through core, which provides very little resistance to the exhaust and thus little back pressure which equals good power. The 08-09 silencer has perforated cones inside, cause resistance to the flow and back pressure to the exhaust system, which is bad. Here's a cutaway of the 08-09 silencer:

Posted Image

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  • grayracer513

Posted October 21, 2010 - 05:26 PM


It's restrictive, simple as that.

If it were a well designed baffled muffler, it would have been much less so. There is resonant chamber technology in common use the would have possibly made it quieter AND increased the power output, but they ignored it for the frankly antiquated approach they chose:

Posted Image

  • FriscoKLR

Posted October 21, 2010 - 06:38 PM


Thanks for the quick replies. Does the yellow restrictive cone fit into the cylinder below it or is the cylinder from an older silencer for comparison?

The stocker came with a brand new ProMoto Billet spark arrester end cap on it. Think I'll give cutting it out a shot before dropping $350+ plus on a new slip on.

  • grayracer513

Posted October 21, 2010 - 07:19 PM


The can slides over the core below it in the same manner as any typical silencer does. The yellow cone runs down the center of the perf core.

So whereas in a normal fiber packed silencer the exhaust flows straight out with very little restriction, the "shorty" puts two perforated cones in the path of the exhaust and forces some to flow through the perforated ends of the cones, some to go around them, and some directly into the packing due to the pressure created ahead of the cones. It's the kind of thing that was done in cheap custom mufflers for British bikes in the '60's. :excuseme:

Quite often, the add-on end cones like PMB, DRD, etc., that add a spark arrestor require the removal of the second (yellow) cone. The spark screen itself does not impact performance negatively in any way that I've ever noticed.

  • davcon

Posted October 23, 2010 - 06:27 AM


I have the PMB SA also.
http://www.thumperta...7517&highlight= Recently I bashed out the inboard cone. Thanks to 'ray' for the idea.
I swear it was a huge improvement. I also repacked the silencer and replaced the stock air filter with a twin air at the same time so those improvements may have contributed.

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