Rep Power: 0

Can someone explain what a compression ratio is? And what the #s would stand for... i.e, 10:5:1... what is it an actual ratio of? fuel? air? Thanks!

 
Rep Power: 0

Your compression ratio is the volume of the cylinder at bottom dead center divided by the volume of the cylinder at top dead center.

There is no 10:5:1, but there is 10:1 stating that at BDC, the volume is ten times greater than at TDC.

 
Rep Power: 399

Yup. so if the volume of the combustion chamber (where the cylinder is) is 550cc's when the piston is at BDC, and the piston displaces 500cc's during the compression stroke, then that leaves 50cc's left in the combustion chamber.

So 550/50 = 11:1 compression ratio.

10:5:1 isn't a possible ratio, but 10.5:1 is. [IMG]i/expressions/face-icon-small-smile.gif[/IMG] And since it is a ratio, the 2nd number will ALWAYS be 1.

and just to throw more knowledge your way, because of the way fuel/air mixes and engines work, 14:1 is about the highest compression attainable, but that is on the highest octane race gas and more than likely with detonation! [IMG]i/expressions/face-icon-small-smile.gif[/IMG] You can get higher compression than that, but you'd have some serious detonation problems!

 
Rep Power: 419

And just to add to this...

The formula for finding the effective compression is very easy: ((boost psi / 14.7) + 1) x motor compression = effective compression.

So... Let's calculate the maximum psi that we can acheive with the stock compresison ratio.

With the proper fuel system and related engine components, an effective compression of 16:1 to 18:1 should be more than workable. For heavily modified cars, effective compressions over 20:1 should be very carefully considered.

For an LX (9.5:1):
((effective compression / motor compression) - 1) x 14.7 = boost
((18 / 9.5) - 1) x 14.7 = 13.2

So 13psi is the highest that we can run (assuming an effective compression ratio of 18:1) with the stock compression ratio of an LX.

For an EX (9.9:1):
((effective compression / motor compression) - 1) x 14.7 = boost
((18 / 9.9) - 1) x 14.7 = 12.0

Just something to take into consideration when building a turbocharged D17. Most of this was re-hashed from: MotorSports Digest