CI Formula: calling all engineers

[68X426]

It's a snowy night, so I thought I would ask the following about cubic inches.

The formula for CI (generally agreed) is [ bore times bore times stroke times cylinders times .7854 ]. It works everytime on every motor dimension. Whether stock, bored, or stroked.

But what about a Hemi? The formula is for a cylinder. The Hemi is a hemi chamber, not a traditional cylinder chamber.   Is there a "correct" formula for the Hemi? And would not the piston top also alter the formula? Or only the compression?

The formula above works on the stock dimensions of the Chrysler Hemi 64-71. So, is it just a case of a "correct" hemi formula is so close to a "standard" formula that both results are still 426 on the motor?

Spring soon.

[Dans 68]

It is the same. When you measure the displacement of an engine it is the difference between the cylinder volume at top dead center (all valves closed) versus the volume at the bottom of the stroke. The head volume does not factor in. The factor "0.7854" (or more precisely "0.785398164") is just pi divided by 4. The actual equation is ((cylinder dia)² x stroke x pi)/4. Of course pi is that 'ole constant 3.14 (circumference of a circle divided by it diameter). Don't forget to multiply your answer by 8 to get the entire engine displacement. The 426 Hemi has a bore of 4.25-in and a stroke of 3.75-in (same as the RB) which yields a displacement of 425.59 cu in.

Dan

[68X426]

Great info, thanks for sharing. I now get it.

The head volume does not factor in.

So, does the head volume matter for compression ratio?

[Dans 68]

...So, does the head volume matter for compression ratio?

Here is a good article. http://www.carcraft.com/techarticles/calculate_compression_ratio/index.html

Dan

[68X426]

Very good stuff, thank you Dan.

[Troy]

(Very simplified)
Inside an engine, the shape of the piston and the shape of the head never change so, essentially, the displacement of a cylinder is equal to the area of the cylinder multiplied by the distance the piston moves. This is true for any combustion engine. Compression is basically the ratio of the volume in the cylinder with the piston at opposite ends of the stroke. You can manipulate the compression by changing pistons or heads. You can manipulate CI by increasing the bore or stroke (or both).

Troy