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  1. #1
    FLjoyrider's Avatar
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    highest compression on 93 pump gas?

    Getting my newly rebuilt 780 motor assembled, planed the 780 heads down about .040". I went ahead and checked the compression before putting the motor in the hull and I'm getting 175psi PTO 185 Center and 175 Mag.. a little disappointed with them not being perfectly even, probably due to the machine shop, but is this going to be ok without detonation I'm running the pro cdi box and v-force reeds.


  2. #2
    dhoad's Avatar
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    I don't think the compression is likely to be accurate if you have not broken in the motor and testing it cold can be wildly inaccurate.
    just my 2 cents

  3. #3
    FLjoyrider's Avatar
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    Also, ring end gap is about .025" which is out of spec arrording to the book.. If I remember correctly it calls for .012 to .018". SBT said its fine, what do you guys think?

  4. #4
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    What's the squish measurement? I'm running .047 at 165 psi on a stock 780 bore. I'm still mixing 50/50 91 and 100 octane with no problems. I feel I should be able to reduce the octane, but I need to get her dialed before doing so.
    Your compression will go up as the rings seat. I'd guess 190+. With that ring gap, I really wonder how tight the squish is to get 175+

    I have some spare jugs that I won't use till they are bored because the ring gap measured .019+ I don't think I would feel good about the ring gaps you have.

  5. #5
    Hayabusa's Avatar
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    Quote Originally Posted by FLjoyrider View Post
    Getting my newly rebuilt 780 motor assembled, planed the 780 heads down about .040". I went ahead and checked the compression before putting the motor in the hull and I'm getting 175psi PTO 185 Center and 175 Mag.. a little disappointed with them not being perfectly even, probably due to the machine shop, but is this going to be ok without detonation I'm running the pro cdi box and v-force reeds.
    I think that compression is going to be way to high to run on pump gas and the Pro CDI is only going to add to the issue.

  6. #6
    FLjoyrider's Avatar
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    I plan on running the first tank on avgas, havent checked the squish.. i'll do that this weekend. I wasnt comfertable with the ring end gap either.. which indicates my machinist over bored. I checked the clearance however, and measuring from 1/2" below the wrist pin the cylinder is .0055" bigger than the piston.. maybe im measuring in the wrong place?

  7. #7
    FLjoyrider's Avatar
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    May be in the market for some new heads.. I dont want to be spending a ton of $$ on avgas all the time

  8. #8
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    155-160 MAX would be borderline for 93 octane.

    You squish clearance will tell if you will have deto issues also.

    Avgas isn't the same as pump gas. There's a few threads about this on here already.

    PRO CDI will definately add to a little deto problem with that high of a compression.

    Ring and piston clearance will also vary depending on the heat created by the engine.

    Advanced timing (PRO CDI) and high compression heads will create more heat, and may need larger gaps to accomodate for this.

  9. #9
    AWA Member 32DegH2O's Avatar
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    93 octane is good for up to around 155psi...maybe 160psi if you know what you are doing and do a few other thngs...otherwise anything over that you need to start mixing. At 175psi you are looking at pretty much all race gas.

    X...ya just bet me to it!

  10. #10
    FLjoyrider's Avatar
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    ahh, ice cold beer! ok, checked the squish, its .028 mag .026 center and .028 pto. Funny thing is I have an extra head, and swapped it for my pitted one.. but after the machine shop removed the ridge (shaved it till the edge was removed on the sealing surface = about .040") the one odd cyl had 140psi and the other two 175. so I was forced to use the pitted cylinder since it was the only one I had that matched. I've heard of people running 200psi VP110.. which you cant beleive everything you hear but ive got a co worker who runs 180psi on his kawi's with no problem on 93.. (again, really can't beleive it till you see it) so anyway I guess theres only one way to find out if i'll get deto. I always have the stock cdi to fall back on.. and maybe i'll pick up some extra heads too for security.

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