flycutting for compression ration

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Re: flycutting for compression ration

Post by dragvw2180 » Wed Sep 04, 2013 7:23 pm

you like livin on the edge,LOL

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Re: flycutting for compression ration

Post by Devastator » Thu Sep 05, 2013 8:52 am

Max Welton wrote:Why not have the cylinders cut down to reduce deck? Any competent machine-shop should be able to do that.
Any competent machine shop should also be able to deck the heads.....
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Max Welton
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Re: flycutting for compression ration

Post by Max Welton » Thu Sep 05, 2013 9:13 am

True. And to get the rest of the way to 8.5 he will still need a little flycut anyway.


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Re: flycutting for compression ration

Post by dragvw2180 » Thu Sep 05, 2013 1:23 pm

Ding Ding Ding we have a winner !!!! Just flycut the heads. I do like the idea of putting all the CC's of fluid ( I use tinted rubbing alcohol) you need into the chamber of the head and marking or measureing it to get a fairly precise number for the flycut. Mike McCarthy

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Re: flycutting for compression ration

Post by Marc » Thu Sep 05, 2013 6:32 pm

.050-.055" deck is a sensible figure with a large bore (the bigger the bore, the more the piston-to-head clearance is reduced by the same amount of piston rock - and any increased piston-to-cylinder clearance allows even more rock). Even with a stock bore, .040" is pushing your luck. Missed shifts happen.
As for shortening the would only make sense if you couldn't achieve your ends by decking the case and/or flycutting the heads - otherwise, it's just one more "custom" component that'll need to be duplicated should you ever need to replace one for some reason.
I'm more conservative when it comes to street C.R. than arnolds64, I encourage you to stay with your plan and shoot for 8.5:1. Have you done any mockup to see what your valve-to-piston clearance looks like? With flattop pistons you'll need to flycut the heads ~.070" for 8½, so you're going to want to see at least .160" clearance with the heads as they are right now (.190" would be better, at least for the exhaust). That'll give you ~.090" or more clearance after the heads are cut.
Clearance is at its minimum as the piston approaches TDC on the exhaust stroke; should you ever want to experiment with advancing the cam timing that'll reduce the intake clearance, so IMO it's wise to make the check with the cam advanced a couple of degrees even if you plan to install it straight-up. If you end up needing to mill any valve reliefs out of the piston tops they'll drop the C.R. slightly so more flycut will be needed to compensate.

Probably not a concern on this mild of an engine, but perhaps worth mentioning for someone reading this thread in the future:
Note that as the heads are cut or the piston deck height is increased, the valve contact points move "up" on the piston. If you mark the locations for valve notches before the heads are cut, the notches end up low which gives less gain in clearance for a given depth (and C.R. reduction).

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