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Discussion Starter · #1 ·
whats up guys like the title says will a cp 12.75 piston be fine running on 91 pump gas in a 2008 yfz? im at sea level thanks for any help my bike is a duner and i don't want to run race fuel ive heard you can because it will drop your compression down to around 12.5 with the crank differances.
 

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ive heard the 3008s runs on plutonium via. a flux capacitor. once you hit 65 mph it generates 1.21 gigwatts and you go from the start to the finish line in NO TIME!.. but i run 110 mixed 50/50 with 93 none ethanol for my 13:1 .. pump gas safe is right around 12.5:1 ive heard but i've also seen people run straight 93 on 13:1 i would play it save.. too much investment to worry about..
 

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Discussion Starter · #4 ·
thanks for the reply...

im asking if the 12.75 is safe on pump gas because of the difference in cranks between the 04-05 yfz's and the 06+ yfz ive heard that this piston will be safe on pump due to the fact that the 06+ yfz's crank has a shorter rod..is this true? if so im guessing that it would put this piston right around 12.5?

and if this is the case then what would the venom 12.4 piston be on a 06 plus bike?
 

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the 06 and up bikes have more compression with same piston as the 04-05, due to the stroke being longer. I just put a cp 13-1 piston in my 07 and on the box it said 04-05 was 13-1 and the 06-09 was 13.25-1
 

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Discussion Starter · #6 · (Edited)
that clears things up! thanks

what specs are the venom pistons advertised at? 06+ or 04-05?

or even the wicked 13.5:1 piston? is that advertised at 06+ specs?
 

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Discussion Starter · #7 ·
the 06 and up bikes have more compression with same piston as the 04-05, due to the stroke being longer. I just put a cp 13-1 piston in my 07 and on the box it said 04-05 was 13-1 and the 06-09 was 13.25-1

also yfz guy what fuel are you running? 50/50 mix or ?
 

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i'm not sure what year the companys advertise the compression for, you would probley have to give them a call to find out. and for fuel i'm running 94 at about 2200 feet, but i did go to seal level a couple weeks ago and ran it on 94, i heard it detonate once or twice at realy low rpm with a good load on it. i would like to run race gas and mix it 50/50 but in canada where i live it is hard to come by and vp is almost 30 bucks a gallon.
 

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Here's the deal with aftermarket pistons:

The longer stroke of the 06+ cranks adds to the compression. The "key" is setting the squish properly.

If you just slap in a piston on the newer 06+ stroke without adjusting the deck height, you will loose compression.

04/05 vs 06+ with the same deck ht. and squish setting, and same combustion chamber volume will add 0.25'points of compression to the 06+ versions.

This on the stock bore.
 

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ok 06+ yfz's have lower compression than 04-05 models. i believe 04-05 have 11.9 where as 06+ have 11.4. that being said 06+ have a longer stroke but a shorter rod which causes the piston to sit lower thus decreasing compression. cp has there boxes marked wrong and have never bothered to correct it. if you run a piston that is for an 04 05 model in a 06 model you will have .5 less compression due to the shorter rod. as for venom pistons i have no clue which year they are speced too, only venom can answer that.
 

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If you do the math on CP pistons, AND set the squish the same withe same deck withe the same combustion chamber volume, the box labels are correct.

Oh, and you can run pump 91 octane on 12.75:1.
We've done for 3+ years on 3 yfz's now.
 

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the box labels arnet correct. you would have to adjust squish a lot to gain .75 of a point. when cp designed those pistons they based them off the 04 05 crank. a 12.5 piston spec'd for an 04 05 in a 06 yfz will be 12:1, unless you put a thinner base or head gasket and adjust squish it wont be 12.75:1 as advertised.
 

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Discussion Starter · #13 ·
So the CP piston for the 04-05 yfz that is marked 13 to 1 will really be 12.5 in a 06+ yfz and pump gas safe?
 

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Yes that is correct. I went through all this a while back with my build. I though I was running a 12.75 the whole time but after enough research on here I figured out it was only 12:1 since the piston was spec'd for an 04 05. And yes you will be able to run pump gas on 12.5:1
 

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Discussion Starter · #15 ·
Ok cool thanks! And mixxer pmed me back and said that his piston is based off the 04-05 yfz but he said it would be .2 higher in a 06 which Dosnt make any sense
 

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I'm not sure why mixer would say that. You can't take the same piston thats spec'd 12.5 for 04 05 and put it in an 06 Which is already .5 lower and gain compression there is no possible way with the stock cranks. 91 should be fine.
 

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your first post is correct, my 08 has a CP "12.75:1" but is actually a 12.5:1
 

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i dont wanna start a big argument about this but the 06 and up bikes will make more compression with the same piston as an 04-05, when the piston is at top dead center it is at the same height on the 04-05 as it is as the 06 and up, they have to put a shorter rod on the 06+ beacause it has a longer stroke, so if it had the same lengh rod as an 04-05 it would hit the head. so now that all the years sit at the same top dead center, the 06 and up bikes piston sit lower at bottom dead center, beacause of the longer stroke. so thats why it has more compression on the newer ones.
 

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Look up the info on the quads. You will see for yourself that 04 05 made more compression than 06 and up. If you guys don't believe me ask terse.
 
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