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  1. #1

    How much compression/what dome size for 66e on pump gas.


    Not sure if you guys are the right people to ask but I値l ask anyway.

    What compression/dome size can I use and still use 93 pump gas. I知 thinking 190-200 psi since it痴 a smallish piston size (80mm) and probably conservative timing on the stock CDI. I知 also not going to run this ski wide open for more than a few seconds at a time.


  2. #2
    would I get more responses if I was asking about triple pipes or 80+ mph single pipes or rxp engines in gpr's?

    there must be a rule of thumb of acceptable high compression on pump gas with a single wet pipe...

  3. #3
    Flying Scotsman's Avatar
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    I dont know but I cant see 190-200psi.
    There are lots of informed GPR guys here who can help you give them time to find your post or pm one of the big dogs.
    I would guess 160psi but could be wrong.

  4. #4
    old school big raider's Avatar
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    33cc protec domes= 175psi or 37cc riva domes =170psi on 800 powervalve with 93 pump gas

  5. #5
    I think I'm going to go about this a different way. I'm going to machine the bottom of my cylinders to lower my port timing and releave the domes to make room for the new piston hight.


    new questions:
    • what compression ratio is acceptable for pump gas with my set up.
    • do I start calculating CR from the top of the exhuast port or from the closed power valve. I would assume the top of the port.

  6. #6
    Flying Scotsman's Avatar
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    I have run 170 psi on 93 octane have even run 170 psi on 89 octane but I was scared ****less and only got on it for short runs.
    On a 65U motor I think your safe at 170 psi somewhere around 90-91 octane.
    Short bursts can get away with less possibly.

    Mark said 170psi on 800 powervalve with 93 pump gas is the 1200 GPR the same...

  7. #7
    jdog800's Avatar
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    Quote Originally Posted by WaveDemon View Post
    I think I'm going to go about this a different way. I'm going to machine the bottom of my cylinders to lower my port timing and releave the domes to make room for the new piston hight.


    new questions:
    • what compression ratio is acceptable for pump gas with my set up.
    • do I start calculating CR from the top of the exhuast port or from the closed power valve. I would assume the top of the port.
    Are you talking about decking the cylinders? or changing the base heighth?Imo I wouldnt do either. You are basicly going to shorten the stroke which is going to rob power. You would have better luck by having the heads changed for compression.
    10:1 is pump safe. anything more and you will probably have to add octane. Jmo

  8. #8
    Quote Originally Posted by jdog800 View Post
    Are you talking about decking the cylinders? or changing the base heighth?Imo I wouldnt do either. You are basicly going to shorten the stroke which is going to rob power. You would have better luck by having the heads changed for compression.
    10:1 is pump safe. anything more and you will probably have to add octane. Jmo
    I'm not shortening the stroke. the crank is staying the same size. I lowering the ports to give me more low end. I'm going to relieve the domes to make room for the pistons. I need to calc the cc's I'll need with my new compression ratio.

  9. #9
    Moderator beerdart's Avatar
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  10. #10
    Quote Originally Posted by beerdart View Post
    good info, thanks alot. still need to know what CR to shoot for.

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