what makes compression ratio??
 

so im 17 and I'm building an engine so i dont know everything but I'm learning a lot everyday. so I'm building a race engine from a ford 302 out of a 95 f150. but I'm wondering what makes the compression ratio? is it the pistons, connecting rods, or the crankshaft? or all 3?

In a piston engine, it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.[1]

but say you want your compression ratio to be 12:1 when building an engine. does the crankshaft, connecting rods, and pistons make the cr what it is? i dont know if I'm making any sense but I'm trying..

If you think about what is in post 2, anything that influences those things will alter the CR, i.e. making the combustion chamber smaller will increase the CR, raising the crown on the pistons effectively does the same thing, increasing the stroke of the crank, increasing the bore, etc


Anything that alters the ratio of space between when the piston is up versus being down alters the CR.

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Typically, you order pistons for a specific compression ratio with the crank, rods, and heads you're using. The piston crown is larger (taking up more of the combustion chamber space) for high compression.

The combustion chamber sizes, piston dome/dish, piston pin height, connecting rod length, bore of the engine and the stroke (crankshaft) all influence compression ratio.

Best question and best answers I have seen around here for a while. Reps for everybody!

Camshaft also plays a significant role in compression ratio. To confuse you even further, there are two compression ratios. Static which is just the swept volume of a cylinder from bottom to top. And that is decided by stroke, bore, piston crown and combustion chamber size. Then there is dynamic compression, and that is the different camshaft intake valve closing events effect on static compression. The sooner the intake closes the higher the DCR the later it closes the lower the DCR. That's why a big lumpy cam in a stock engine Runs like a one legged man . Also ihate posting on my phone, it posts before I'm done and I have to constantly edit.....