Compression ratio for hemispherical combustion chamber, 2V, aircooled, NA, gas

[Evil Tweety]

Any opinion on optimal CR for an air cooled twin, 2 valve/cylinder running in Bonneville?  Running gas, naturally aspirated.
Thanks!

[panic]

Not enough info.
80 VE, good quench, small R/S ratio: 15:1.
105 VE, large bore with single plug, large R/S ratio, bad quench: 12:1.

Tell us more?

[Evil Tweety]

Not enough info.
80 VE, good quench, small R/S ratio: 15:1.
105 VE, large bore with single plug, large R/S ratio, bad quench: 12:1.

Tell us more?

I'm running in the middle of that range right now.  It's a single plug, bore around 80mm (3.14") - had a bad quench with the last head - but I've got one where I can take a lot of the dome off the piston and improve it.

Sorry - don't know what you mean by R/S ratio . . . but please let me know if this info helps.

[John Noonan]

R/S is Rod to Stroke ratio..

[Evil Tweety]

R/S is Rod to Stroke ratio..

Thanks John!  So measuring the rod (center of wrist pin to center of big end) it worked out to 2.3:1 or 2.44:1, depending on which crank I am able to use.

[Evil Tweety]

Thought I'd give this a bump and see if anyone had any comments . . . any more input?
Thanks!!!!

[panic]

Rod ratios that high are more sensitive to knock than for example 1.7, etc. but as to actual numbers IKBA. There isn't much diff (if any) between your 2 ratio choices.
Remember than the weak density/atmo of Wendover is your friend in designing in a static CR, since some people plan on closer to sea level and can't use such big numbers.
Not sure how much effect, but the narrower the RPM range the engine must pull near top speed (viz., close-ratio with high progression is better than street ratios) the higher the static CR, closer to max power the jetting, more aggressive the spark setting. In any event, does your drive-line allow keeping the engine above 5,000 RPM under power?

[Evil Tweety]

Rod ratios that high are more sensitive to knock than for example 1.7, etc. but as to actual numbers IKBA. There isn't much diff (if any) between your 2 ratio choices.
Remember than the weak density/atmo of Wendover is your friend in designing in a static CR, since some people plan on closer to sea level and can't use such big numbers.
Not sure how much effect, but the narrower the RPM range the engine must pull near top speed (viz., close-ratio with high progression is better than street ratios) the higher the static CR, closer to max power the jetting, more aggressive the spark setting. In any event, does your drive-line allow keeping the engine above 5,000 RPM under power?

With the electromotive EFI we can do whatever we want with spark and gas.  RPM is higher than 5,000, more like 8,000 plus.