compression ratio's

[Gareth_C]

Evening all does anyone know the easy way to work out the compression ratio of an engine :confused:

[craig green]

Google it.

My Bible is not to hand.. Its a pretty lengthy sum you will need to have the specifications for.

[david dixon]

gareth you'll need to know what the standard piston top to deck height is first! (how much there is from piston top to surface) then work out how much the head has been skimmed and if you block has before then work out how much above the standard piston heights your pistons are....

[Gareth_C]

gareth you'll need to know what the standard piston top to deck height is first! (how much there is from piston top to surface) then work out how much the head has been skimmed and if you block has before then work out how much above the standard piston heights your pistons are....

That sounds a bit long winded all i wanna try and work out is that will cams for 12.7:1 to 13.1:1 ratios will be ok in my money pit lol the piston are supposed to be 12.6:1 and you have see the bottom of the head dave any guesses :confused:

[mowgli]

swept volume (cc)+ unswept volume (cc) = cr
unswept volume(cc)


swept volume is the theoretical cylinder of air that is squashed by the piston

(bore[cm]/2)squared x PI(3.1415926etc) x stroke (cm) = swept volume

unswept volume is the combustion chamber + any crown in the piston & then the bit where the head gasket goes.

no, it isn't easy to measure.

[craig green]

Put the cam in, turn it over by hand.. If it rotates twice (more ideally) without anything locking or noises, you know it clears.........

[Gareth_C]

Put the cam in, turn it over by hand.. If it rotates twice (more ideally) without anything locking or noises, you know it clears.........
I dont think the valve clearence would be the problem it just making sure the compression ratio is right to get the best out of the cams otherwise its more money wasted :thumb:

[Stuart]

see sig link for easy calculator ;)

[Ron]

easy compression test the cylinder in psi and then devide by atmosheric pressure 14.7 by mass eg 160 psi divided by 14.7 = 10.8843 so cr is 10.8.1

[burgo]

easy compression test the cylinder in psi and then devide by atmosheric pressure 14.7 by mass eg 160 psi divided by 14.7 = 10.8843 so cr is 10.8.1

seriously :confused: ive never heard of that before. surely its a tad vague though

[Ron]

that is wot i have been told and learnnt on my apprentise ship and at college :)

[burgo]

so could you reverse this theory to see how healthy your engine is. what i mean is if you know it is supposed to be 10:1 that means you should see 147psi on a compression test?

[burgo]

actually thinking about this more, that theory is complete bollocks. take a standard engine that is 10:1, and for instance produces 150psi on a compression test, now fit a longer duration cam!!! this allows more air into the cylinder and will therefore create a higher pressure on a compression test, yet you havn't changed the compression ratio at all. the compression ratio is the factor to which the swept volumn is compressed, completely different to the pressure in a cylinder.

even more of an arguement to prove its bollocks is a turbo'd engine, low compression ratio number yet massive cylinder pressure from the turbo hence lot more power

[mowgli]

burgo. the cr published for most motors is the theoretical & not the actual cr.

to get close to the theoretical, you would need a massive intake of air for every stroke, which is never going to happen in the real world.

remember the petrol engine is the triumph of engineering over design

[Stuart]

compresison test/atmosphere dosent work as it will vary with battery condition/humidity/air pressure/temperature/engine temperature/cam timing/starter motor condition

the calculated from the measurements of volumes is the 'best' and easiest way to do it.