working out compression ratios

[Ian Williams]

Here are the variances I have found Jon, I am sure if I went through all the heads in my shed I could possibly find more, all these heads have not been skimmed with exception of the IA38 which has 10 thou off. What is interesting is the variation in the plug depths, very early having 1/2" reach then all mid production 5/8" reach until we get to IA684 then back to 1/2". There is quite a variation in chamber depths, I have marked these up in thousands if an inch and measured them all in exactly the same places, the IA684 plainly has the highest CR but it also has the greatest restriction in the all important transfer port.