domes

[lee37vernon]

coolhead domes .. what cc gives what compression out of the following ... 21.5cc 23.7cc 24 cc ??????????

what size would be best for 97 or 99 ron octane fuel .?

[punker69q]

what is your bore and stroke and how thick your head gasket is?

[lee37vernon]

68.25 bore , std stroke . head that came with the cylinder i'm using is o- ringed so came with no gasket , however i'm sure it must need one as its the std head but machined . surely without the extra thickness of the gasket compression would be ridiculous ..??!!
i have a cometic head gasket out the gasket kit but will probably be using a honda one , either whole or split upon recomendation really ..

[punker69q]

I'm guessing that your 97 ron fuel is about equivalent to 93 (r+m)/2 octane fuel, so you should keep the compression at about 11:1.

For a 68.25mm bore, your displacement would be (68.25mm/2)^2 * 72mm * Pi / 1000

72mm = stock stroke, Pi = 3.14159
In this case your displacement = 263.4 cc

To calculate your compression ratio, use the following formula :

cr= (displacement + head volume)/ head volume

In this case, 11 = (263.4 cc + X cc) / X cc

By solving this simple equation, we obtain X = 26,34cc -> that would be your total head volume needed for a 11:1 compression ratio.

Now, I don't know how your head is machined and if you could still use head gasket (don't think so, because the head had been machined for o-rings) but you could get a custom dome done, or a standard dome re-cut to get that total volume from the head without a gasket.

If the head is machined in a correct way and your squish band height is kept somewhere between ~0.04-0.05 inches you shouldn't have any problems.

[wilkin250r]

Good info. However, keep in mind that these are just calculations, and simple ones at that. There are a lot of other factors that influence actual compression. Like your squishband thickness, head and base gasket thicknesses, ect...

[punker69q]

Originally posted by wilkin250r
Good info. However, keep in mind that these are just calculations, and simple ones at that. There are a lot of other factors that influence actual compression. Like your squishband thickness, head and base gasket thicknesses, ect...

That's why I spoke about total head volume and not only dome volume...

Regardless of the base gasket thickness, squish band thickness, the shape and diameter of the head that goes to where the o-ring seals, piston distance to the top of cylinder at tdc etc, if you measure the total head volume you can't o wrong. I know of some engine builders who actually put the engine together with gaskets and everything and measure the head volume when the piston is at tdc with a seringe (sp?) and some thick oil. This way, you can know exactly what your cr is.

You could also measure everything, find the equation of the curve that the head dome follows and evaluate the integrals in cylindrical or spherical coordinates to calculate the exact head volume, but that would be overkill and out of reach for every non mathematician/engineer out there.