08 YZ450F Exhaust fix

As we all know the stock muffler on the 08 YZ450 is super restrictive. Here's an in-expensive fix, that boosts power. I took an 07 stock muffler, trimmed it back 5 inches, and installed an 06 stock end cap on it. Re-packed it of course, and on track benfits can be felt right off idle. This bike comes alive instantly now and revs out a long way, much better than stock form. Dyno runs even show a 1.5hp gain. Just wanted to let ya know if you can scrape together some old stock parts the 08 YZ450F will rip with very little work. Plus the bike definatley sounds better now, and revs much quicker. It feels like a completely different bike. Best of all it only cost me a few hours of work, and bag of 4stroke packing.

Uh huh. And how loud is it?

Just had it sound tested at local track. 98db, just as loud or quiet as most aftermarket exhausts out there.

The muffler is still 10.5" long not including the end cap. The 06 end cap is hollow which allows for more packing and helps maintain a normal exhaust tone. As we all know shorter muffler means quicker revs, which is just what the doctor ordered on the 08 YZ450F. A person could even use the 07 end cap to help quiet the system even more, however the 07 end cap is quite restrictive, and that was the problem with the 07 stock exhaust. The new exhaust I have built is still AMA LEGAL, and sounds just like all the other

450's on the track.

Just had it sound tested at local track. 98db, just as loud or quiet as most aftermarket exhausts out there.

I find this hard to believe since the 06 was blowing 101-102 Db stock. Make it 5 inches shorter and I don't see it dropping 4 Db.

... we all know shorter muffler means quicker revs,...
We do?
07 end cap is quite restrictive,
While that looks like it's true, it's debatable.

By cutting the exhaust down, is it creating better low end (noticably) and are there any negatives to this besides a louder pipe (re-jetting etc..)?

I bolted up the end section of my stock 07 exhaust on to my 08... big difference. I'm now running the Dubach exhaust that I pirated off my 07, but if I didn't have the Dubach I'd of been fine just running the stock 07 muffler.

...we all know shorter muffler means quicker revs...
By cutting the exhaust down, is it creating better low end ...
A shorter overall exhaust system length raises the harmonic range of the exhaust system to a higher point in the rev range, making it more of a top end pipe. Conversely, longer systems have a bias toward low end power.

So, then, why does the cut down '07 muffler here produce more low end? Simply because substituting any free flowing exhaust for the more restrictive '08 unit will do the same thing. The fact is that the length of the exhaust system is only a significant tuning factor in the case of pipes where there is a straight through passage along the centerline of the system, and if baffles are used, there must be an end point built into the exhaust system, such as the first chamber of a Flow Master muffler, or the crossover tube placed at the end of the headers in V-8 car applications, either of which is seen as the end of the pipe by the pressure waves traversing it. The '08 muffler has no such internal end point, and therefore can never be more than a nice quiet muffler, not a tuned exhaust system.

MX926, I would really like to believe that your exhaust is 98db, because that would be pleasant and convenient if it were true. However, I continue to doubt it on the same basis expressed by KJ790. I have no doubt at all that your exhaust system now produces more power than it did in the low/mid range, but people have obtained the same results with unmodified '06 and '07 mufflers.

Well I suppose it is possible our sound testing was off a few decibles. However my 06 YZ450F was at 100db from factory. Factory installed packing was quite minimal though and after a re-pack of aftermarket packing it tested at 98db with no problem.

-As far as why cut down the stock 07 muffler, honestly I only did it for cosmetic reasons at first, I had a few extra can's to experiment with. However after testing the stock 07 length and the cut down version, so far I like the snap of the cutdown version better and I do not notice any less pull on top. Plus with less hanging out the back thats less liabilty in a tip-over.

-Just to be clear the exhaust I am using is complete 07 muffler, and core, that has been cutdown. it contains no Baffle like the stock 08, and has an 06 endcap. I wrapped the core with fmf 4stroke packing, then covered that with 1000 degree ultra thin pipe insulation, which can be purchased at home depot.

-As far as jetting goes I am at stock 08 jetting, and made minor fuel screw adjustment. I think I'm a bit on the rich side though and probably will do some tweaking as the weather gets warmer.

-As far as why cut down the stock 07 muffler, honestly I only did it for cosmetic reasons at first, I had a few extra can's to experiment with. However after testing the stock 07 length and the cut down version, so far I like the snap of the cutdown version better and I do not notice any less pull on top.

One of the things I didn't point out is that the "rules" on exhaust length are on a "all other things being equal" basis. The '08 has a fairly large diameter header, partly as a way to v compensate for the presence of baffling, and that will influence performance of the exhaust at one length or other.
Well I suppose it is possible our sound testing was off a few decibles. However my 06 YZ450F was at 100db from factory. Factory installed packing was quite minimal though and after a re-pack of aftermarket packing it tested at 98db with no problem.

-As far as why cut down the stock 07 muffler, honestly I only did it for cosmetic reasons at first, I had a few extra can's to experiment with. However after testing the stock 07 length and the cut down version, so far I like the snap of the cutdown version better and I do not notice any less pull on top. Plus with less hanging out the back thats less liabilty in a tip-over.

-Just to be clear the exhaust I am using is complete 07 muffler, and core, that has been cutdown. it contains no Baffle like the stock 08, and has an 06 endcap. I wrapped the core with fmf 4stroke packing, then covered that with 1000 degree ultra thin pipe insulation, which can be purchased at home depot.

-As far as jetting goes I am at stock 08 jetting, and made minor fuel screw adjustment. I think I'm a bit on the rich side though and probably will do some tweaking as the weather gets warmer.

Still not biting this one. Too many inaccuracies. A silencer is not its quietest right after being packed, it gets quieter as it gets a little time on it and burns out a little packing. After a certain point too much packing gets burned out and it will get louder again. Packing a silencer tighter will make it louder, hence why the stock pipe was not packed super tight. Yet, you claim packing your stock 06 pipe super tight made it lose 2 Db. A decibel is a logorithmic scale, so 2 Db is quite a bit to lose.

You then say that you did not feel any lose of power on the top end, yet before you said that a dyno run showed a 1.5 peak HP gain. If there was a 1.5 peak HP gain, you would be raving about how much harder it pulled up top, as most aftermarket pipes are hardpressed to add 1.5 peak HP.

He never claimed to have packed it "super tight"...why not cut the guy some slack?? It is obvious the change from the stock 08' can was VERY positive (as ANY silencer other than the stock one would be). AND the price was right!!

He never claimed to have packed it "super tight"...why not cut the guy some slack?? It is obvious the change from the stock 08' can was VERY positive (as ANY silencer other than the stock one would be). AND the price was right!!

He said the stock one had little packing, so he put a lot more in to make it quieter. If you put a lot more packing in, it will be packed tight. I don't doubt that it is better than the 08 pipe, but I don't believe that it is 98 Db, and the performance gains are most likely do to his mind noticing the change in exhaust sound. Small performance gains may have occurred, but 1.5 HP increase and a 4 Db decrease seems a little unlikely.

Could I just ask a question while you guys are on the subject? My 04 can looks really long sticking out there past the number plate, what would happen if I cut say 4 or 5 inches off and replaced it with a en cap that was National Forest approved? Jetting, performance...noise I dont have to worry about...what do guys think?

Thanks

Anyone watch Sean Hamblin on Racer X video ? He is riding a stock piped yz450 and is just flying. My bad Dr.D pipe did not make me go anywhere near as fast as he is on the stock pipe. Go check it out ,it is cool to watch.

Guys there is a reason the length is what it is......it is not a mistake by yamaha, it is due to the best performance for a stock exhaust system. The length of the can is directly related to the diameter and length of the header. When you cut off sections of the cannister - you have screwed up the formula for the stock exhaust or any exhaust to be maximized for performance and sound.

Yes it may look better cut....but you have only ruined a perfectly good pipe or 2 and will have no gains and a increase in the sound.

Geez enough of these stupid threads on benefits of cutting a exhaust pipe......quit being so cheap and go buy a better exhaust if you need more power. Or take your expertise in cutting and welding and go work for a pipe manufacturer and learn about correct air velocities and exhaust gas :thumbsup:

Wow you guys crack me up. I was just letting you know what I did, and how it worked. I thought maybe there would be a few guys out there that didn't want to dump a ton of loot on an aftermarket exhaust, and wanted an alternative.

- As far as the "stock" exhaust packing, I was speaking of the 06. I took it apart with only a couple hrs of riding and it had basically no packing. I repacked it using a normal method, and it was legal as tested at the local track. "Sound testing is optional here" And it lasted 4 months of motocross racing before it started to get loud.

-The 08 I packed the same way, normal, not too tight.. I just added a thin layer of 1000 degree pipe insulation around the fmf packing.

-As far as the dyno goes, most of the gains were to the bottom and mid, the top was nearly the same as stock.

-Fortunatley I live in an area without sound regulations, so even it it was a bit loud, it doesn't matter unless I choose to race hare scrambles. Sound levels are not enforced through out the midwest, or even at Loretta Lynn's for that matter.

- Maybe my post should have read, "Cheap Mod" for 08 exhaust.

- Bottom line, its cheap, and it runs way better than stock.

-And Yes I've seen the Sean Hamblin video and he was ripping stock exhaust and all.

Anyone watch Sean Hamblin on Racer X video ? He is riding a stock piped yz450 and is just flying. My bad Dr.D pipe did not make me go anywhere near as fast as he is on the stock pipe. Go check it out ,it is cool to watch.

Yeah I saw that pretty cool track too. Should check out the Hahn and Grant clip insane!

I think you had a good idea there. The short muffler looks good and if you can wake it up, why waste $400 + on an aftermarket pipe just for the fancy name brand label or sticker on it. They're just as loud, especially 'Pro Circuit'

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