Oh, I see. You merely forgot to mention the .008" paper gasket OR the .040" aluminum head gasket. Taking those into account your calculation would be correct. I wish you luck with the aluminum head gaskets, they are a BAD idea. The narrow contact area of the big-bore 40 cylinder wall is going to cut into it, clamping force will be lost, compression leakage will occur and it will melt down and burn away. Even steel head gaskets fail eventually, aluminum are doomed. You'd be far better off leaving those off and buying some cylinder base shims. If you don't wish to do it using one of the "right" ways I recommended before, at least consider using thicker cylinder base shims instead of the paper gaskets. .030" would give you ~7.33:1.
--- Mike <[EMAIL PROTECTED]> wrote: > 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 > ____________________________________________________________________________________ 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
