compression ratio calculations

[JackFish]

I attempted to CC one of the chambers on my head and came up with a rough number of 54, maybe 55cc's. Do I need a really accurate measurement?

I asked the shop to mill .050, taking into account a slight warpage.

So do those figures seem to jive with each other?

I punched 55 in to the CR calculator and came up with a CR of 8.8, and 104hp@4000rpm, which looks pretty good for my purposes, but maybe I should try to bump up the CR a bit?

And I have a steel shim head gasket that will compress to .020.

I also have an extra gasket if anyone wants to make an offer PM me.

 

[fordconvert]

So you have a zero deck with true flat top pistons? If not you also have to cc the block while at TDC. If you are just in the calculation stage and you plan on zero deck you can just use the published dish size of the pistons. Dont forget that if you are running say 30 over pistons that has to be included in the calculations if you are not actually ccing them with fluid.

8.8 should be fine and still run well on 87-88 gas. Most modern cars are in the low 9's because of the control EFI can offer (like knock sensors). Unless you are really good tuning a carb and have all the tools (wideband and such) much over 9 will usually require higher octane fuel.

 

[JackFish]

Haha no the block and pistons are stock 1978 vintage, AFAIK. And I'm not pulling the engine.
I'll measure the bore when I pull the old head and see if it's been rebuilt.
I suppose if it's 30 over then I would maybe consider having the new head shaved a bit more.

 

[CZLN6]

Howdy Back Jackfish:

Your calculations appear to be very close to right on. One note- a steel shim head gasket does not compress as much as composite types. We figure .024" for a compressed steel shim gasket.

I don't know what else you have planned for this engine, but 8.8:1 is a good compromise number for CR.

Adios, David

 

[Mustangaroo]

Remember is check how far down the cylinder the piston top is from the top of the block? My 1978 block was .069 not the .025 as used in the calculator on the web site. So make sure you are using the correct information, or your C/R could be much lower than expected

 

[JackFish]

Oh yeah, will do, thanks.

 

[fordconvert]

A nice thing about our inlines is they sit fairly flat while in the car so it should not take much work to get it fairly level with the head off and cc it while in the car.