40 commader comperesion ratio calcuation

[Studenut1915]

I have a 40 commander with a factory replacement 1950 engine in it. I have to plane the head to true it. I'm not sure how this will affect the compression ratio  

In  the past, I had seen a comment on calculating compression ratio in  either in this form or possibly Turning Wheels cooperators. Would anyone know where the information is?

Thanks in advance for your help with this,

[Gary_Ash]

If your engine is a 1950 version, then it should have a displacement of 245.6 cubic inches.  Divide the displacement by the number of cylinders (6) to get the displacement of 1 piston, or V = 40.93 cu in.  The volume in the head plus the gasket plus any distance of the piston below the top of the block gives the volume v left when the piston is at the top of the stroke.  We find the compression ratio from

CR = (V+v)/v 

Your engine should have 7:1 as the CR, so we solve the equation for v = V/(CR-1) or v = 40.93/6 = 6.82 cu in.

 

The difficulty at this point is that we don't know the area of the gasket opening that encompasses the cylinder bore, the valves, and some of the block.  The decrease in v will be approximately the gasket opening area x the amount of stock removal in planing the head.  Let's make a crude guess and say that the gasket opening area is roughly twice the area of the piston top or (pi/4)*D^2.  With a bore of 3.312 inch, the piston top is 8.62 square inches, so twice that is 17.24 sq in.  If you plane the head 0.030", the volume removed from the combustion chamber is 17.24 x .030 = 0.52 cu in, so the new combustion chamber volume = 6.82 - 0.52 = 6.3 cu in.  The new compression ratio is CR2 = (40.93+6.3)/6.3 = 7.5.  If you take a full 0.060 inch cut on the head, you'll be up to about 8.07 for the compression ratio.  I think either one would be OK and not cause a problem - except you have to worry about the valves hitting the head if you cut too much.  Normally, a cut of 0.010" to 0.020" should bring the head back to flat. Is the top of your block flat enough to avoid planning that, too?  The 2E-8E truck shop manual shows that the 245 engine had 7.5 compression by 1956, so they probably shaved the head a little at the factory.

 

We made some guesses and approximations, but the answer is close enough.

 

 

[Studenut1915]

Thanks for taking the time to reply Gary. That's exactly what I was looking for. my block is straight  but the head is slightly warped. Years ago  bought and NOS  8.0 compression 1960 truck head  and installed it on 1940 226 and  It made  a big difference on the hills. I thought I would bring up the compression on the 245 since I had to plane the head anyway  

Thanks Again