I should back up here a bit.
There are two different ways to calculate compression ratio. SCR (Static Compression Ratio) which is counting the of combustion chamber at BDC (Bottom Dead Center) and counting the same volume at TDC (Top Dead Center), and DCR (Dynamic Compression Ratio).
Again SCR takes the volume at BDC – which for sake of discussion, is 500cc. Volume at TDC is 50cc. 50 divided into 500 (500/50) = 10/1 or 10:1 to 1 compression.
This is how most everyone looks at measuring engines – but is not really a good tool.
DCR, on the other hand is more of a “real life application” of compression ratio.
Compression does not start to build, on the compression stroke, until the intake valve closes (which happens on the upwards stroke). I’m not sure what specifications are for an ’80 /6 is, but my ’77 FSM shows the intake valve closes at 48’ after BDC (or ABDC).
Note: 180 degrees from BDC to TDC and intake valve closes after 48 degrees has taken place, so 26% of the piston movement upwards is wasted.
Based off of the FSM of Intake Valve Close angle of 48’ ABDC, that would give you roughly 7.5:1 DCR (in stock condition).
I did some research on best compression ratio for you and one thing I found was consistent . . . it depends.
Factors like vehicle weight, gear ratio, Altitude, normal driving ambient temperatures, cylinder head design, length the intake valve is open, intake/exhaust valve overlap, and about 100 other factors – so I just gave up trying to get everything factored in.
MY BEST GUESS, to get to the compression ratio you desire, is to reduce the cylinder head combustion chamber volume about 6 cc (from stock measurements).
If cylinder head combustion chamber is also at 3.40” in diameter (same as cylinder bore), then you would need to shave off approximate 0.040” from an un-shaved cylinder head.
Before I was to do anything to your reman cylinder head, and after your old cylinder head was removed, I would get both cylinder heads CC’ed (combustion chamber volume checked). Then compare volumes between both cylinder heads. You want the “newer” head to have about 6 cc’s less volume than old/original head.
Then, if combustion chamber is 3.40” diameter and is round (at head gasket surface) then 0.040” milling would reduce new head down about 6 cc’s. If combustion chamber is not round or if CC difference is not 6 cc’s exactly, then the 0.040” figure is tossed out the window.
I am 99.2% certain your new cylinder head has have some material shaved off of it. The question is how much. If I was to “guess”, I would say 0.020” – but that is only a guess.
The problem with machining, is if you took off too much – you now have a boat anchor, for you can’t put the metal back onto a cast iron head. Kinda like cutting a piece of wood. Measure twice and cut once. A board cut too short . . . is useless.
Attached is a picture taken from my ’77 FSM and combustion chamber appears to be round. I couldn’t find anything to indicate what combustion chamber diameter is, for if it is wider or narrower than 3.40” – then the shave thickness will also be different.
BudW