200ci calculated static compression ratio - math check appriciated

So I used a random CR calculator on the web (http://www.gtsparkplugs.com/CompRatioCalc.html).

Six Cylinders, 3.72 bore (+.040 overbore)
3.126 stroke (from the Performance handbook)
chamber 50cc (guessing, trying to determine target for head work)
gasket thickness .050 (Felpro, from Perf handbook)
head gasket bore 3.81 (handbook)
piston/deck height .042 (per feeler gauge),
dished piston 7 cc.

It says my displacement is 203.85, 3341cc. and my static CR is 8.54 to 1.

Do my numbers look right? I need to go to work, but I plan to check another calculator later.
Is a 50cc chamber on a head actually doable?
If I shave the heck out of the head, and/or the block, will I need to get custom cupped pushrods (stock adjustable valve train). That would be a drag.

I'm not quite ready for a rebuild on my bottom end, it was recent, there's still a super nice swirl pattern in all the cylinders, it would be a waste to tear it down again.

I'm taking an engine class here at school and your 8.54:1 is correct, that's what I got by calculating it by hand. One thing you can try is a thinner gasket. I believe Victor makes one that is .045"ish thick. I noticed you have a lot of deck height. Isn't usual about .030ish? that would definitely change things. Why is yours so much higher? You can also look into finding an old FOMOCO metal gasket that crushes out at .025".

Recalculating your stuff with the .045" head gasket and your deck height at .030", your compression goes to 8.869

Recalculating with the FOMOCO head gasket (.025") and your .042" deck height, compression is 9.05

Recalculating with FOMOCO and .030" deck height, compression is 9.3.

All of these are calculated with the 50cc chamber. if you wanted a little more meat on the head, lets say 55cc, and you used the FOMOCO gasket and found out your deck height is actually .030", your compression is 8.73.

I think your limiting factor here is the huge amount of deck height you have (I would remeasure that). I don't know how safe a 50cc head is (cracking wise). Currently, with the calculations done, your best bet is to remeasure the deck height, and start looking for a steel shim FOMOCO head gasket. Or look into decking your block and really reducing the deck height. Nothing increases compression faster than decking the block.

If you decked the block down to .010" deck height, used the FOMOCO head gasket (assuming the bore on the gasket is the same), and the head is 55cc, compression is 9.13.

What kind of compression are you looking for?

I wouldn't think you'd need new pushrods. The whole point of the adjustable rockers is that they are adjustable. I think you should be fine there, unless someone else chimes in and says otherwise. If you had non-adjustable, then you'd have to shim the pedestals to get the correct pre-load.

Good luck,
Ryan

So ostensibly I’m looking for the highest cr I can get, and safely run California pump gas, which is 87. Without having to worry about cam calculations for DCR.

I’d also like to not ruin the head by taking so much off that it limits my future options. I forget the casting, but it’s a large log, one of only two that I have. The others are all early small log heads.

I’m going to buy a new feeler gauge set, my current set is straight, an angled set will be more accurate.

Basically my head is at the shop for pressure testing, getting cut for valve seals, and shaved to shrink the chambers once I decide what they should be.

Howdy back Invictivus:

You are on the right track. I need more info before I can give you exact help. What year head casting are you working with. In general, early castings- pre '70 are typically 52 cc. '74 and later are typically 62 cc. In both cases chambers can vary by 2 ccs per chamber. The only way to know, for sure, is to measure.

If you change to an aftermarket composite head gasket you will need to mill the head .025" to compensate for the difference- just to stay where you started. A stock steel shim head gasket measure .025". After market composite type gaskets range from .045" to 52".

A rule of thumb on static compression is based on the octane of the gas you are going to us- 87 octane = 8.7:1. Other variables are elevation, ambient temperature,and cam timing. Cooler temps more CR. Hotter less CR. Higher elevation more CR. IF you live on the coast, less. If you live above 3,000 ft elevation, more. If this head is going on a car with a auto trans- less CR.

Piston/deck height on pre/72' 200 engines is .025".

If you are having a head rebuilt, be sure to add a 30 degree back cut to the intake valves for better flow.

So, given what you've shared, a mill cut of .050" is a safe guess. That is .025" for the composite gasket, and .25" for added CR. That should put you very close to 8.8:1 static CR.

I hope that helps. Good luck and keep us posted on your progress.

Adios, David

Like others have said, 8.54:1 is spot on. And I agree that the deck height is your limiting factor. 0.042" is pretty big. Had it just been a piston change from the stock compression height of 1.511" to the replacement piston height of 1.500", the deck height should have only been 30 thou or so. Something isn't quite ringing up right. With that low of a SCR, you will have to be very careful with cam selection. If you have a stock cam, performance is going to be horrible with a DCR around 7.0:1'ish (assuming I have the correct values for a bone stock cam....)