Biff,
Bob's right, the volume of your combustion chamber is the key ingredient here.
On the 12G295 it's 28.3 cc. , compared to 24.5 cc on the standard bugeye head.
You can get a rough estimate by levelling the head on a table with the
combustion chambers upward, placing a piece of plexiglass with a small hole
drilled in the middle over one of the combustion chambers and by using a
pipette and a burette introduce a known quantity of fluid, like Marvel Mystery
Oil, into the chamber through the hole in the plexi (you probably have some
left over from your windblocker) If you start out with 28.3 cc of Mystery Oil
and can't get it all into the combustion chamber, you know it's been skimmed.
If you get less than 28.3 cc into the head, you can plug that amount into some
formulas I have to figure out what your compression ration will be with the
various mods done to your engine. Again, if you do hit 28.3 cc, some other
formulas will let you determine how much material needs to be removed from the
head to reach your target CR. Of course, depending on the cam you use and
whether you have oversize valves in the head, excess skimming may cause your
valves to hit your piston tops or top of the block (an expensive ouchey). I
have a 12G295 head on my 948, bored forty over with flat-top pistons and am
running the 88G229 Cooper 997 cam. The head has standard valves and I
purchased it new. I never milled the head to boost the CR, but the engine
produces plenty of power and runs fine on pump grade premium without any
additives. You might want to go more wild with your motor so boosting your CR
might be in the cards. Avoid going too high though, because using octane
booster is a real PITA.
Best regards,Rick
In a message dated 10/18/2003 1:20:48 PM Eastern Daylight Time, "Biff"
<biffjones@erols.com> writes:
>All,>>Recently I acquired a 12G295 head that I want to refurbish, Vizardise,
>and install on my 948. One pleasant surprise was that it has rimflow valves
>installed and they are clean and the guides are tight!>>Anyway, my question:
>How can I tell if the head has or has not been shaved/milled? I'm might be
>making an 'assumption' that since it has already been upgrade with the rimflow
>valves that it might have been already shaved/milled. But I don't know what
>it was installed on before my purchase. If it was on a 1100cc it might not
>have been shaved, however, if it was on a 948 it probably was. I don't want
>to install it and find my compression down and have to remove it and have it
>shaved to bring the compression back.>>Is there a measurement somewhere on the
>head that will tell me something?>>Thanks,>Biff Jones>'59 Bugeye, 'Kermit'>'62
>Sprite, 'Ole Gray'>'71 Midget,
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