Chilekesh

I've done my share of research and calculation. The factory stated compression ratio for 1989 TPI 350 is 9.3:1. The data I have:

Bore = 4.00''
Stroke = 3.48''
Compressed head gasket thickness = 0.039''
Deck height = 0.025''
Piston top volume (dish) = +8.87 cc
Combustion chamber = 63.2 cc

From these data, I'm calculating a 9.41:1 ratio. Could somebody tell me which of these numbers I have wrong? Bore, stroke and combustion chamber volume are certain, the remaining are somewhat less accurate. I got different numbers from all around. (I'm almost sure about piston dish also but still not certain.)

Second question:

Using the following link: http://www.not2fast.com/turbo/compre...pressure.shtml

I have factory internals and valvetrain. Which means my 0-0 cam duration is around 247 degrees (not between 0.05'' lifts, which is 207), rod length 5.7'', altitude 100 m and temperature 70F. According to this data, I should observe a compression of 218 psi while cranking during a compression test. Am I right? Haven't done the test yet, but I intend to do soon.

Any comments or suggestions on this? Any additional knowledge or correction?

skinny z

My calculator has a variable for the head gasket bore diameter. Using 4.10" I get 9.36:1 as a static compression ratio. The rest of the numbers appear to be OK.
As far a cranking pressure goes, using my engine as an example, I have 185 psi across the board whereas the calculator specs 190 psi. Pretty close I would say. As a backup, I tried the calculations with last years cam and the results are exact. 205 psi.
Be careful how you determine advertised duration based on the .050" spec. Cams can be wildly different in this area and the only sure way to know is to get the factory value.

Chilekesh

So is 4.1'' the factory head gasket size?

Then the volume between gasket edge and combustion chamber/cylinder bore edges makes around 0.41 cc [pi/4*(4.1^2-4^2)*0.039*2.54^3], which would make 9.36:1 as you said. I don't know the accuracy required in this anyway, maybe it is close enough. But if this was the case, they would've rounded it up to 9.4:1 instead of 9.3:1 so something seems to be amiss.

About the second issue, I can't seem to find the 0-lash duration of the factory cam, that's why I added 40 degrees to 0.05-0.05 duration. In any case, I'll have a conclusion after I check all the cylinders.

Also it is interesting to see how much effect has the height above sea level over the cranking pressure. Even 0 to 500m altitude difference translates to 13 psi decrease under 9.3:1 compression. It is even possible to read pretty different values between a sunny day and just before a hurricane.

skinny z

I don't think we can consider .1:1 ( or less) to be too much of an error.
Consider that it hasn't been exactly determined what the piston deck height is ( .025" is an average OEM value ). Have the numbers for piston dish (8.87cc) and combustion chamber volume (63.2cc) been measured?