Piston comp


10 replies to this topic
  • slygary

Posted July 13, 2010 - 02:02 AM

#1

According to the owners manual here in australia the comp ratio is 12.3:1 for the WR450. Has anyone put a higher comp piston in their WR450 ? Such as a 12.5:1 same as the YZ450 or even a 13:1 ? Will it be a big gain in performance ? How will it effect the electric start,will it still be able to turn the motor over ?

  • pablo83

Posted July 13, 2010 - 08:44 PM

#2

I bought my WR426 with a 444 13:1 big bore kit in it. I went back to the stock bore and compression for reliability reasons, and I did notice a power difference, but it was very minor. For trail riding, the stock bike already had more power than I could use, but for a serious racer, there are minor gains available with higher compression.

  • Mrmonk7663

Posted July 13, 2010 - 09:01 PM

#3

13:1 for pump gas. Some put a 13.5:1 but I believe they use a racegas mix. General consensus is a stronger powerband all over and works well with cam upgrades like the Yz cam.

  • tribalbc

Posted July 14, 2010 - 08:39 PM

#4

I run a 13.5-1 piston on 94 octane pump gas r+m/2, not sure if you're just research octane rating in Oz????
I did mine the same time I put YZ cams in so I can say I got good gains across the board, but can't how much is due to the HC piston. They are known though for giving more bottom and mid range.
Once again due to the YZ cams can't really say on the estart. Mine only estarts half the time, but YZ cams are known for this problem.....

  • Mrmonk7663

Posted July 14, 2010 - 08:48 PM

#5

wish we had 94 at the pumps here in the states. I wonder if 13.5:1 would work with 93 octane...[/B][/B]

I run a 13.5-1 piston on 94 octane pump gas r+m/2, not sure if you're just research octane rating in Oz????
I did mine the same time I put YZ cams in so I can say I got good gains across the board, but can't how much is due to the HC piston. They are known though for giving more bottom and mid range.
Once again due to the YZ cams can't really say on the estart. Mine only estarts half the time, but YZ cams are known for this problem.....



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  • tribalbc

Posted July 14, 2010 - 08:52 PM

#6

wish we had 94 at the pumps here in the states. I wonder if 13.5:1 would work with 93 octane...[/B][/B]


I'm sure it would. I've run a 50/50 mix of 91 and 94 which would be about 92.5 with no detonation. Depends a bit on elevation and temps as well. Cold and sea level needs higher octane.

  • Mrmonk7663

Posted July 14, 2010 - 09:03 PM

#7

Hmmm I might try a 13.5 when I swap then. I'm about 200ft above sea level but temps 90-100 in the summer. I could always put a few ounces of methanol in the tank to raise the octane if needed. Methanol shouldn't hurt any seals in the carb or motor right? It works fine in my car.

  • slygary

Posted July 15, 2010 - 01:31 AM

#8

I dont know if we are lucky or not here in australia, but we have 92,95 and 98 octane fuel at almost all of our service stations. I use the 95 in my wr 450.

  • tribalbc

Posted July 15, 2010 - 06:01 AM

#9

I dont know if we are lucky or not here in australia, but we have 92,95 and 98 octane fuel at almost all of our service stations. I use the 95 in my wr 450.


That means you are research octane only. Not sure what 98 is equivalant to, probably 92,93 R+M/2.
Maybe someone can chime in who knows for sure????

  • tribalbc

Posted July 15, 2010 - 06:04 AM

#10

Hmmm I might try a 13.5 when I swap then. I'm about 200ft above sea level but temps 90-100 in the summer. I could always put a few ounces of methanol in the tank to raise the octane if needed. Methanol shouldn't hurt any seals in the carb or motor right? It works fine in my car.


I know guys on the YZ450 forum have no problem running 13.5-1 on 91 octane......
Methanol wouldn't be my first choice of octane boosters.....

  • Mrmonk7663

Posted July 15, 2010 - 08:14 AM

#11

In the states only 93 r+m/2. They sell a 100% pure methanol at walmart in a small bottle. I used to use it with windshield wiper fluid to increase the methanol content for my meth injection on my car. I think I'll go 93 with half a cup or so of meth and I should be good to go. Thanks.




 
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