Could someone accurately and fully define this for me..?

[yellow400ex05]

What is the difference between static and dynamic compression?

which one has which between the z400 and 400ex?

I think its kind of weird how a 400ex only has a 9:1:1 compression ratio and a z400 has a 11:3:1 compression ratio...

[zrpilot]

I'm no expert here but I'll do my best.

Static compression is simply the ratio between the volume of the combustion chamber when the pistion is at BTDC and the volume when the piston is at TDC.

However dynamic compression (or cranking compression) takes into account many things, primarily the cam specs. Cams specs do change cranking compression. The longer the intake valves stay open the lower the dynamic compression.

Other things that effect dynamic compression: the quality of combustion chamber sealing and elevation.

IMO the dynamic compression is more important then static.

So why does the Z400 have a higher static compression ratio then the 400EX, yet still run fine on 89 octane? It probably has a high lift/duration cam and there is more opportunity for air to escape through the valves as the piston begins its compression stroke. With an aggressive cam some dynamic compression can be lost. This must be made up for with a higher static compression piston.