I have found the threads on compression very interesting and decided to do the calculations for my 312. What I have is...



312 bored to 3.830

3.44 stroke

6.252 rod length

.016 deck height

G heads milled .020 so estimate 67cc

Best head gaskets .043

Mummerts y-265S cam I plan to run 2 degrees advanced so valve closing @ 60.5 ABDC

1.54 rockers



I come up with a static comp of 9.238 and a dynamic of 7.583 . Is my math correct on this? Operation is @ 4000' and above, is this a premium only engine or is mid grade adequate?



Thanks Mike

1956 Fairlane Victoria (ORREO)

 

Overlooking Beautiful Rimrock AZ

I will let someone else check your math, but it looks like it should run on regular.  I have a 9.2 SCR engine with milder cam than you and it does fine on regular.  One big difference is in your quench area being .059 and mine .045.  Maybe with that increase mid grade will be necessary.

Edit:  Our regular is E10.  It runs cooler than 100 percent gas, so maybe mine wouldnt like regular in 100 percent form.

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

Using your values, I get 9.12:1 SCR and 7.49:1 DCR.  The difference in calculations could be in the head gasket volume.  For the regular bore Best Gasket head gaskets, I use 10.0 cc’s.

 

It’s always recommended to cc the heads and ideally each combustion chamber.  It’s not unusual for a variance in chamber volumes and more often than not, it will be found that chamber volumes go from small to large when examining cc values from one end to the other.  In these cases, I do the math to angle cut the heads from end to end so that the chamber volumes are equalized.

 

I’ll also add that using a measured intake closing event at the camshaft using the recommended valve lash setting rather than what’s stated on a cam card is going to be much more accurate.  Cam cards do not necessarily use the lash setting for the closing event and because the advertised duration does not state exactly what value is used, that number is in question when using it for the DCR calculation.  I will get the actual intake valve closing event from the cam when degreeing in the camshaft and while getting that value, I’ll also take some other readings around it in case I want to vary the valve lash in which to alter the DCR.  I trust that made sense.

Lorena, Texas (South of Waco)