question about high compression pistons


14 replies to this topic
  • 802wrRider

Posted April 21, 2011 - 04:23 PM

#1

hey all so i rebuilt the motor in my bike this winter with a high comp piston. everyone i talk to told me i might as well. my question is, with a high comp piston, will my wr450 not like to be lugged like it used to. i took it for a couple of quick rides earlier in the week and it seemed like it was about to stall and didnt seem to like to be lugged anymore. could that be a tuning issue or is that the nature of high comp pistons? and sorry for the noob question.

thanks

  • MANIAC998

Posted April 23, 2011 - 03:57 AM

#2

High compression pistons PREFER lugging! Are you sure you've got your cam timing reinstalled properly? Maniac

  • tribalbc

Posted April 23, 2011 - 10:32 PM

#3

High compression pistons PREFER lugging! Are you sure you've got your cam timing reinstalled properly? Maniac


+1
or jetting

  • MANIAC998

Posted April 24, 2011 - 04:32 AM

#4

I think Tribal is right. It's probably a jetting issue now. Higher comp. pistons create more of an engine vaccum, so they draw more fuel up thru the carb passages, typically creating rich jetting conditions. Do some tinkering, and I think you'll get it running better than ever. Maniac

  • 802wrRider

Posted April 26, 2011 - 03:42 AM

#5

thanks guys. i appretiate it.

  • woods-rider

Posted April 26, 2011 - 06:11 PM

#6

Do you have to run race/AV gas with a HC piston in the WR or will 93 octane do?

  • tribalbc

Posted April 26, 2011 - 06:34 PM

#7

Do you have to run race/AV gas with a HC piston in the WR or will 93 octane do?


93 works fine.

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  • woods-rider

Posted April 26, 2011 - 06:37 PM

#8

Are there any other potential issues with running HC pistons? If there isnt and they make the bike run better I am wondering why they would not come like this from the factory.

  • tribalbc

Posted April 26, 2011 - 08:23 PM

#9

Are there any other potential issues with running HC pistons? If there isnt and they make the bike run better I am wondering why they would not come like this from the factory.


Really it comes down to not everyone can run 93 octane or better. I know small towns around here were you can only get 87. BTW the WR stock does fine on 87, not that I would recomend it, but in a pinch.

  • woods-rider

Posted April 26, 2011 - 08:38 PM

#10

Really it comes down to not everyone can run 93 octane or better. I know small towns around here were you can only get 87. BTW the WR stock does fine on 87, not that I would recomend it, but in a pinch.


The bike comes with a sticker right on it that says "premium unleaded fuel only" which to me means 93 octane or better. There are small towns around here also that don't sell anything but diesel and regular gas. So with a HC pistion you absolutely can't run 87 octane?

  • MANIAC998

Posted April 27, 2011 - 04:23 AM

#11

True, you shouldn't run a HC piston on 87 Octane, but Tribal said stock WR's can run on 87, and he's right. I've had to run it once, as it was all I could get, and the bike never detonated once. Maniac

  • sledhead999

Posted April 27, 2011 - 05:37 AM

#12

Around here 91 is as good as it gets for pump fuel. Typically you can run higher compression the further you are away from sea level w/ the same grade fuel.

On 2 strokes, higher compression leads to less stalling riding slow thru single track trails:)

  • woods-rider

Posted April 28, 2011 - 11:35 PM

#13

When/if I ever do the top end on my bike I will probably give a HC piston a go as I always run premium anyways. Good info guys!

  • 802wrRider

Posted May 05, 2011 - 07:43 AM

#14

just to let all know that were wondering about the HC pistons, i got to put a couple of hours on my bike in the past week and i have to save low rpms are awsome now. i putted around the field a little in second gear and was just playing around seeing how slow i could go and still pick back up and go. it was amazing. as far as power goes, i dont really think i can compare. my motor was tired and needed a rebuild so not knowing what it feels like with a fresh top end w/ std piston i cant really comment on. all i know is that it seems to pull through the rpms quicker. again that might also happen with a fresh rebuild w/ std piston. im not sure. im definately happy with the outcome though.

  • gsa102

Posted May 05, 2011 - 08:22 AM

#15

I am no expert, but I belive that low octane gas will allow detonation, or pinging, on top of the piston. This can erode the piston rather quicky, which is bad. Preventing detonation is the main reason to use higer octane gas.

Some time ago, I rebuilt a YZ 250 with an HC piston, and it pinged like crazy on regular gas. I added octane booster for some time, then as it wore in and lost some compression, it quit doing it.

Edited by gsa102, May 05, 2011 - 08:23 AM.
Clarification





 
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