Is there a formula or general rule as to how many points of compression per .xxx" you gain from milling a set of heads. My project has been delayed so I am thinking of haveing my self ported heads milled for a little more torque. Possibly .030"?? I already have the cam and a set of 7.4 pushrods. The new cam has a base circle of 1.525/1.510 (so lets say 1.520) which is about .030 smaller than a stock genIII cam so I can theoreticly take .030 off the heads and retain stock valve train geometry. About what compression would this give me using the stock GM headgaskets(which I already have)?

 

The number I use is 1 cc per .006" removed from the head. Therefore, if you remove .030" that would be 5 cc which should raise your compression ratio .5 if everything else stays the same. This should get you another 10-12 hp.

 

I calculate out the change using a 66% area for the chamber compared to the bore. Basically I say that the surface area of the chamber is 2/3s of the complete bore. Milling .030 off a 4" bore is 6cc, so 2/3rds of that would be 4cc. Of course this is just a rough estimate of chamber area.

Stock pushrods are 7.38". Milling .030 will equate to .017 of cam lift without rockers. Subtract your base circle from the stock base circle, then use the .017 milling number to see how far off from stock you are.

 

who is doing the milling for you kenny? kings in pascagoula or do you have another place in mind? kings used to do some nice work.

 

I dont know exactly what you are talking about with the .017 of cam lift? Wouldn't the milling be directly porportional to the base circle as they relate to the pushrod length? New BC in my cam is 1.525 intake and 1.510 exhaust, so assume you just use an average of them. I have seen that the stock genIII motor cam's base circles are between 1.540-1.550. Using 1.520 for the new cam and 1.550 for the old cam gives me a difference of .030. Wouldn't that mean I could take .030 off the heads and still have proper valvetrain geometry?

John, I will more than likely have Matt at Professional Engine in Mobile do them.

 

No, because the pushrod is not at the same angle.



P----> /| <-----H


If H decreases by x, P will decrease as a ratio; not a number subtracted.


example

P=10, h=6
if you shorted H to 3; P becomes 5; not three less making it 7.

BUT you also have to concider that the angle changes as well so they are no longer similar triangles. Point being its not linear, its a complicated related rates problem with 2 variables; angle decreases as H decrease and P decreases as H decreases but decreases (?) as the angle changes as well.

 

Scratch the part about the .017 lift out of my previous post there KD. I misinterpreted your initial post some and was working off the 1.7 rocker ratio for valve lift.

You need to cut your base circle number in half to get the radius of the cam. If the difference in two cams has .030 base circle, that would give you .015 difference in lift. - (rephrased) - Your lifter is sitting on the cam and is closer to the center by 1/2 of the difference in base circle

Hypothetically, starting from the stock setup. Your cam is .015 shorter, you mill .030 which makes you .015 longer than stock. Stock pushrod is 7.38 and you have 7.4" pushrods which make you .02 longer still, for a total of .035 longer than stock. So ideally, you'd want a 7.35 pushrod, thats .030 shorter than stock but you were already .015 too long so you end up .015 shorter than the stock components. Basically you are .035 too long with the 7.4" pushrod or .015 short with 7.35 pushrods. Your choice.

Is all this correct?? I think so, but what do I know.

 

That makes a lot more sence! Dummy me was using the diameter of the base circle not the radius. I should have know that.
By my understanding of the valvetrain, you would rather have a slightly short pushrod rather than a long pushrod, right? How about using a shim between the rocker and rocker pedistal to compensate for a long pushrod?

 

I'm glad you edited that. I was fixing to give a math lesson.

Some folks use shims without issue. Also, the hydraulic lifter is a little forgiving in both directions. I think it would be better to be a bit short than overly long.

The best option would be to check the components during install and decide the necessary adjustments then. Whether that be a different pushrod length or shims. Thats not easy to do when you try to have everything prepped for a quick install.

 

:haha: yea, I reread my post and saw I had the math screwup.

That is why I am tring to get this all figured out now, so I can have a quick and painless install and not be parinoid about it after the install.