Milled head ?

[Reg Evans]

I just installed another head on my '39 Chrysler 241 that had been milled .050.  The compression readings increased 13% compared to the original head.  Is there a rule of thumb or formula for how this affected the original 6.5 compression ratio or HP rating of 100 ?

Edited October 12, 2015 by Reg Evans

[ptwothree]

Hi Reg.....  As i understand it,  compression ratio is the difference of the capacity of the cylinder, divided by the capacity of the combustion  chamber.  So,

 

if the cylinder has a capacity of 600 cc's and the chamber is 60cc's the comp ratio would be 10:1  

 

You would need to convert cubic inch displacement into cc's and actually cc the chamber to get the numbers to figure it out.  Head gasket thickness figures in there too.

[Dave72dt]

Volume of the cylinder, gasket and head chamber at bdc versus the same at tdc. A general rule of thumb could be developed if someone had a known untouched original and was willing to spend the money milling and ccing the head after every .o10. Bear in mind the heads are castings and not every head chamber will read the same in that head or match all the other heads with the same design. Change the head design, bore or stoke and the ratios can change.

You may find it starts a bit easier, has a bit more pep and a bit better on fuel mileage as well. At least, you'll think it does whether it's really noticeable or not.