OK, if I am going to build a set of heads I might as well mill head heads in order to increase my static compression ratio. Stock CR is 10.0:1.

My calculations indicate that milling the the cylinder head the following amounts would yeild the following compression ratios (rounded to the nearest .01 for simplicity sake)

Milling .010" results in 10.36:1 CR
Milling .020" results in 10.74:1 CR
Milling .030" results in 11.16:1 CR
Milling .040" results in 11.61:1 CR

Can anyone confirm that my calculations were correct?

The swept volume is 498cc (2988cc/6 cylinders). The CR is 10.0:1 meaning there is 49.8cc above the piston at TDC.
The volume of the ENTIRE cylinder is the swept volume by the piston PLUS the volume above the piston at TDC, which works out to 547.8cc stock. This means the height of the cylinder is 8.07054cm (assuming the cyl is flat on top which it isn't but that doesn't matter as long as the part being milled off is flat i.e. not domed.)

Area of the cyl = 67.8765cm^2

Milling the head results in decreasing the volume above the piston at TDC by whatever amount you mill off times the area of the cylinder (67.8765cm^2).

Milling .020" =.0508cm. 8.07054cm - .0508cm = NEW CYLINDER HEIGHT of 8.01974cm.

NEW CYL VOLUME = 8.01974cm * 67.8765cm^2 = 544.342cc

Stroke remains the same so the new VOLUME ABOVE THE PISTON is 544.342cc - 498cc = 46.352cc

Resulting CR = 498cc / 46.352cc = 10.7439:1 CR

Repeat for other milling amounts

Calculations correct?

 

I believe you also have to account for the little space inbetween the piston / cylindor bore right above the top ring.

Also is this milling amount going to mess up your cam timing at all?

 

I've always heard that milling is not the safest way to increse CR. if you mill the surface too much wont that throw off the the "slack" of the timing chain on the sprockets?

 

Didn't think of that. Dang.

 

I also think you can't just divide all the CC's by 6 and go from there. You have to add all the spaces up. ie.. crank throw x piston bore + cylinder head cc area + gasket thickness + spacing above the ring/piston/cylinder wall + dished pistons or notched - domed pistons etc..

It's been years since I fooled with this stuff with my Datsun 510 L18 motor

 

Neal, if you're going to drop so much cash on this project, why don't you go all out with the 3.5 block and get pistons/rods made, have some work done on the 3.0 heads and run a stand alone engine management system? it would be a lot of money, but it would definatly be worth it.

 

Almost right - but your combustion chamber volume is wrong - c=combustion chamber volume so cr = (498+c/c) = 10, or 498/c + c/c = 10, 498/c = 9
c= 498/9 = 55.33.
Then you subtract from 55.33 the volume of the cylinder you remove (bore/2)squared x h and recalculate. (damn this is hard without proper math symbols).

 

oops - that should be (498+c)/c.

also - moderate head shaving won't change cam timing by much consider how small .020" is compared to the 12" or so distance between the head and engine pulleys.

 

But valve head to piston clearance at tdc is more likely to be the limitation.

 

You're going to slightly retard the timing on the cams if you mill the heads. Cam timing retard removes top end power.
Plus, it's more difficult than that to calculate the head cc's than that. About the only true way to do it is by cc-ing the bowl after each incremental pass. Keep in mind that the bowls are not going to be in a circumference at the mating surface or mill area. And judging by this picture it does not look as if you have that 1mm you listed.
Also, when you're calculting CR you have to include the head gasket as well. If you were to purchase a thinner head gasket your calculations would be more correct because then you could actually use pi.

 

Your total volume would not be 547.8 cc's. The formula is CR = (Cylinder Volume + Chamber Volume)/Chamber Volume. Therefore if you take your 498 + 49.8 then divide it be 49.8 you get 11:1. The way you determine the chamber volume is subtract 1 from your CR which would be 9 then divide your swept volume by that. So 498/9 = 55.333 cc's. That is your chamber volume. You can check the equation by plugging it into the formula. 498 + 55.333 = 553.333 . Now take your total (553.333) and divide it by the chamber (55.333) and you get 10:1 .

 

Nice pic Matt

Get pistons made, milling the head is a bad thing IMHO. If your factory head was never overheated or warped it has a much smoother surface than any average machine shop can cut.

 

timing chain slack should not be an issue because there are hydraulic tensioners that take up the slack. You might want to buy a new chain though.