[jdean]

If you now have 9.3 with 93cc, then going down to 75cc would result in 10.92.

Above 4,500 ft. and you can basically reduce effective CR by one, so the 10.92 would be 9.92 or less.

Other considerations are whether the replacement head is aluminum. If so you can run about .5 more CR than an iron head without detonation.

According to Pat Ganahl in his Ford Performance book, the 460 heads are able to get away with a bit more CR than expected.

It appears that with your altitude, you would be safe with 75cc heads on pump gas.

Hope this helps.

John