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Discussion Starter #1
I purchased a JE Big Bore kit for my 03 Polaris Predator 500. I was forced to get a 12.5:1. I don't want to run race gas and was wondering how much I would have to have the piston machined down to lower the compression to 11:1. Please help parts are almost done at the shop and I want to get it put back together.
Don't know if it matters but the shop is installing +1 valves. As long as I can get away with running 93 octane I'll be happy.
 

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Have you talked to the machinist that would be doing the work? If its a machinist that specializes in engine components he or she would likely know how to figure it out.

Beyond that, I do not know anything about calculating compression and machining, so I'm not of much use.
 

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Discussion Starter #3
Yea I asked everybody I've talked to but they all go on a rant about how many variables there are and conclude that there is a formula to figuer that out but they don't know it.
 

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They used to shave the heads in the auto race engines to raise the compression. Maybe the machine shop can fabricate a thicker head gasket for you, which would increase the area of the combustion chamber and therefore lower the compression. This would eliminate the need to modify the engine or any internal components.
 

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Discussion Starter #5
I suggested that to Rossier in Plover. He is supposed to be the best in the area. He told me that his concern with that is it might screw with the timing. I already have two sets of head gaskets and base gaskets. If somebody has more expertise on that please let me know. I will just glue two base gaskets together and go that route if possible. But then I have the same ? of how many mm would change to the compression I'm looking for.
 

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My Outlaw is 12.5:1 and I run 93 octane with no problems what so ever.
 

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Discussion Starter #9
Would you like to enlighten me there poleman? This is no joke, I don't want to have to run race gas and if you know of a simple, reliable way to avoid that please feel free to say something.
 

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How long have you been running it like that?
Been running it like that since it was new (after break in), it's an "08", Full FMF pipe, jet kit (160 main I think) K&N, modified airbox. Goin on 6 years old, and it will still rip your arms out of the sockets!
 

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They could machine the piston top down a hair and drop the compression that way. Or enlarge the combustion chamber in the head.
 

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If you are intent on lowering the compression, they make concave top pistons for exactly that purpose.
 

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Discussion Starter #13 (Edited)
I've already purchased the JE 560 big bore kit because the jug was junk anyway. It was gonna cost me about the same to go with OE so why not. JE kit said it would require race gas. I just want to make sure it is reliable and runs on pump. The head and jug will be done at the shop this week so If I don't figuer out a cost effective way to bring it down a little bit I will just run it like that regretfully and very carefully.lol

Oh and remember I just want to know how much I would have to take off the piston or add to the base gasket to aquire my lower desired comp. ratio? I know all the options I have (I think) just need someone who know the mathmatical formula or the answer. Thanks to anyone with an idea.
 

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Discussion Starter #15
I just posted my ? on there so I will see if they got any info on the subject. Thanks for the lead. Doesn't mean I giving up on here though if anyone has any more ideas on this let me know. And if I get a solution I will post it.
 

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Discussion Starter #17
So I got alot of the same opinion on the other forum. Almost all of them running 12.5:1 or more are running pump premuim gas 92 and up. No problems with theirs. So I'm gonna run mine on pump 92-93 and just make sure I keep it a little on the rich side.
Oh and thanks for that calculator. I have no idea of how to figuer the cc volume of the head without maybe filling it up with a liquid or playdough and dumping it out in a beaker or something.lol
 

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I'm running a 12.5:1 on 93 octane no problem. Mine is a 400L (2stroke) but I don't think that makes that big of a difference.... I say go for it!
 

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Just keep an eye on your plug for signs of a Lean condition. I don't think you'll have any problems, throw in a few ounces of Startron to keep fuel stable.
 

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fast, I didn't mean to sound like a smarty. I just never heard of someone building a big bore high compression engine and then trying to LOWER the compression. It will run just fine on 93 octane with a little booster in it. If you wanted to run regular fuel why did you get a high compression kit?
If you want reliability, I wouldn't be adding any extra gaskets or anything. IMO you'd just be adding the potential for problems.
 
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