Can someone please tell me what "compression ratio" is?

[MoRsuS]

What does it mean when a car is said to have (eg) a compression ratio of 9:0:1

[ivymike1031]

that means that the cylinder volume at BDC is 9x the cylinder volume at TDC. If you also know what the total engine displacement is, you can calculate the TDC and BDC volumes.

Example:
CR = 9.6:1
4cyl, 1.6L displacement
displacement/cyl = 1.6L/4 = 0.4L
(vol.BDC)-(vol.TDC) = 0.4L
(vol.TDC)/(vol.BDC) = 1/9.6

vol.TDC = disp.cyl / (CR-1)

BDC volume = 0.4465
TDC volume = 0.0465

[MoRsuS]

Im sure that will make perfect sense as soon as i know what BDC and TDC stand for lol Care to explain ?

[Jay!]

Originally Posted by MoRsuS

Im sure that will make perfect sense as soon as i know what BDC and TDC stand for lol Care to explain ?

Top Dead Center and Bottom Dead Center. The 'top'most and 'bottom'most positions the piston will reach in a revolution, thus defining the maximum and minimum volume inside the cylinder.

[MoRsuS]

thanks a lot

[MoRsuS]

One more question if i could: What are transmission ratios?(ie. 3.067 in first, 1.957 in second, 1.429 in third....etc)

[Hudson]

Gear ratios are proportion of turns of the input shaft to turns of the output shaft. So a ratio of 3.067(:1) means 3.067 turns of the input (the crankshaft) will turn the output shaft one time.

[jhillyer]

It's a ratio of how much cylinder volume has been pressed at the end of the piston stroke toward the ignition end of the cylinder. A simple view: If the travel of a piston through a cylinder were 90mm long with a flat cylinder head, and the piston reached end of its travel 10mm short of the end of that cylinder, then a 10mm gap of unstroked cylinder would remain when that piston reaches end of its stroke, creating a 9:1 pressure, or 9 atmospheres, leaving a 10mm combustion chamber. Also see "final compression" or "effective compression" ratio when turbocharging. Compression ration can be adjusted by valve timing, stroke, head chamber volume, spark or glow plug, ....