There's nothing wrong with the Engine CR calculation spreadsheet I've been using. It is my piston/ cyl. deck hieght of .040", plus a stock .008" paper cylinder gasket plus a special .040" aluminum head gasket (to compensate for previous fly-cutting of the heads....total deck equals 2.25mm, so my CR calc is correct.
Mike B. ----- Original Message ----- From: "marc vellat" <[EMAIL PROTECTED]> To: "Air-Cooled Volkswagen Discussion List" <[email protected]> Sent: Wednesday, April 30, 2008 2:24 PM Subject: Re: [vintagvw] 40 horse expert 1961-1965 beetles > 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. > > > > > ____________________________________________________________________________________ > Be a better friend, newshound, and > know-it-all with Yahoo! Mobile. Try it now. > http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ > > _______________________________________________ > vintagvw site list > [email protected] > http://lists.sjsu.edu/mailman/listinfo/vintagvw > _______________________________________________ vintagvw site list [email protected] http://lists.sjsu.edu/mailman/listinfo/vintagvw
