Posted By: domingo
How much to mill stock hemi heads to gain half a point? - 04/27/11 04:17 PM
Hi!
How much would a set of OEM HEMI heads would need to be milled in order to achieve an extra half a point of compression?
How much milling is considered safe?
Domingo
Posted By: mbogina
Re: How much to mill stock hemi heads to gain half a point? - 04/27/11 05:27 PM
3 basic variables to work with, Volume of cylinder (Vcyl), volume of everything above that (Vtdc), and Compression ratio (CR). Example: for a 4.31 bore, 4.15 stroke, 10.0 comp ratio, Vcyl= 992cc, the unknown variable is Vtdc, the general equation reduces to 992= 9.0 x Vtdc; so Vtdc= 110.2. To increase the comp ration by .5, the equation changes to 992=9.5 x Vtdc, so Vtdc= 104.4, a reduction of 5.8cc. Milling a hemi chamber is about .0041"/cc, so 5.8 x .0041= .024".
All hemi heads have a register on the deck near the exh face at the center of head (about .5" x .5"), once the register is gone, that is the factory safety limit for milling (usually about .080 deep when untouched.)
Posted By: racingblues1426
Re: How much to mill stock hemi heads to gain half a point? - 04/27/11 07:36 PM
From past experience I would never remove anything more than necessary to true the hemi head. Alignment issuses are a huge problem once you start milling the heads.
Posted By: RUNCHARGER
Re: How much to mill stock hemi heads to gain half a point? - 04/28/11 03:27 AM
I was going to bring up the alignment issue as well. It's not a huge deal but it has to be right.
Sheldon
Posted By: mbogina
Re: How much to mill stock hemi heads to gain half a point? - 04/28/11 02:12 PM
at 4.15 Stroke, 4.28 bore, Vcyl= 978cc, reduction needed is 5.7cc for 0.5 CR increase (-.023"). Minor amounts of milling such as this will not cause other alignment problems. Our SS hemi's are decked about -.500", the heads another .040 to .065"- combine that with angle milling and milling the intake face- THEN you have some engineering to work through.