Large valved 273

The difference in chambers is not as large as one would think. Minimum 920 chambers are 57.3 and "J" head chambers are 64.7. You have to mill .040 to get there. That means about 8cc larger chambers for each. So unmilled 920 heads are around 65 cc and unmilled "J" chambers are around 73cc. I think unmilled 675 open 318 heads are around 68cc. It is better to take the chamber volume out of the equation and think of a constant chamber and the effects of the bore area at BTC and TDC. That will show, like I tried to give a picture of the difference with the 273 and 340 pistons at the same ratio, the difference in ratio of a large and small bore. In a street engine it matters little since we are limited by the gas octane available. All 273, 318 and 340's have the same stroke and rod length. 340's are close to the top of the bore at 10.5 and have no dome.