Mike, I'd recommend that you toss that spreadsheet as
far as you can and do the math yourself. I don't know
where you got the idea that .040" = 2.25mm - either
that's a typo or it's where your error is coming from,
but either way you're FAR off.
83x64mm => 346.279cc
1mm piston deck @ 83mm bore => 5.411cc
With 45cc chambers, that's (346.279+50.411)/50.411
...that's 7.87:1, not even close to 7.1:1... you'd
need another 6.35cc to bring it that low. That'd take
.045" more piston deck, which WOULD probably present a
problem with intake manifold fit. The intake manifold
can be tweaked enough to accomodate slight variations
in engine width, not likely to be difficult until you
get to +.050" or so - but you'd be +.090".
You can safely run 87 (R+M)/2 octane fuel up to about
7.4:1 with a stock cam at sea level...The way fuel
prices are these days I'd be inclined to go to ~7.7
(that's about the max with a stock cam) and run
mid-grade gas, the cost-per-mile should be the same or
better.
Personally I don't like to run that low of piston deck
on an endurance engine - I would go with a minimum of
.045" for 77mm and .050" for 83mm. Keeping the deck as
close as possible to the lower safe limit will
increase the "quench" effect, promoting better
combustion characteristics and allowing a slightly
higher C.R. than on a "non-squish" engine. But
semi-hemi cutting also destroys the quench effect -
it's a quick & dirty way to gain chamber volume. Far
better to take the time to open the chambers up
manually by laying back the walls while leaving as
much of the flat quench surface as possible intact, or
mill a shallow dish in the center of the piston top.
I would run .050" deck which'd yield a C.R. of just
under 7.7:1 with 45cc chambers, and plan on buying
better gas. If you MUST run Regular, then stay with
the same deck and open the chambers up by ~2.5cc -
that'd bring it under 7.4:1.
Cylinder base shims ARE available, just not as common
as they are for later engines - and big-bore or not,
the shim I.D. is the same (nominally 86.25mm). SOME
1200s had the larger 90mm opening found on 13/15/1600
engines, but only late-model ones sold in other
countries. These engines have strange cylinders that
look like short 1300 jugs.
I'm out-of-stock on all but .010" 40HP cylinder shims
($7.50/set + shipping, [EMAIL PROTECTED])
Chirco lists .010"/.020"/.030"/.040" for $8.95/set
--- Mike <[EMAIL PROTECTED]> wrote:
> I'm overhauling a 62/63 stale-air 40HP for my
> beetle right now. I'm
> installing later heads (square rocker bosses) and
> 83mm big-bore pistons and
> cyls. I found that my deck measured .040" (2.25mm)
> and my stock head has
> 45cc's combustion chamber volume. This results in a
> 7.1.1 CR, which
> requires 87 octane gas minimum. I'm installing
> black cool tins under the
> cyls, and have checked the thermostatic bellows for
> proper opening @ 150*.
> If your head CC is 40 to 43cc's due to
> fly-cutting, then semi-hemi
> cutting the heads and/or 'sinking' the valves will
> increase the volume to
> lower the CR. The 77mm stock cyls don't have the
> luxury of having shims
> available, but the 83mm big-bores will use the same
> shims as a stock 1600cc
> engine with 87mm cyls.
> Boston Bob told me that the limited length of the
> intake manifold won't
> allow you to shim the cyls very much, if at all.
> I'm sending you my spreadsheet in a pmail.
> Thanks for the reminder to use later rockers,
> Marc.
>
> Mike B.
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