Any way to determine compression ratio?

[rdharper02]

Looking at picking up another 93 GT. The car runs well has AFR 185s and some other goodies, but the gentlemen who owns it does not know what the compression ratio is set at after his builder asembled the motor. The builders out of buisness, so no luck calling and the current owner is more money than knowledge. Normally wouldnt be much of a concern, but I am concidering transfering my turbo and goodies from my blue 93. Any suggestions?

 

[Mustang5L5]

Well, you need to know a few things to figure it out.

Cylinder head volume
Bore and stroke
Pistons?
Head gasket thickness.

The big variables are of the engine is bored or stroked and what pistons are in it

 

[jrichker]

There is a way, but unless you have the engine mounted on a stand, it isn't practical and the measuring tool kit is expensive.

It envolves rotating the engine on the stand so that the sparkplug hole for the cylinder under test is the highest spot on the engine. You will pour light oil in the cylinder and you don't want there to be any air pockets to upset your meaurements. Start by setting the cylinder under test at TDC. Pour in light oil using a gradualted glass cylinder maked off in CCs. When the oil becomes level with the spark plug hole stop pouring the oil and note the quanity in CCs. Crank the engine to BDC and add more oil, carefully keeping track of the number of CCs you pour in. When the oil becomes level with the spark plug hole stop pouring the oil and note the quanity in CCs. Divide the BDC oil figure by the TDC oil figure and the resulting number is the compression ratio.

See Search Results for burette - SummitRacing.com for the kit with the measuring burdette for measure the oil.

 

[rdharper02]

Thanks gentlemen, definitely appreciate your help.

Question: in theory, if the compression is increased, the cylinder pressure is increased. If I do a cylinder compression check on my stock 5.0 and compare it to the PSI reading from the other motor, will it give me a reasonable basis to see if the compression has been raised? Seems like it would at least tell me (if my old rings ect are good) if the static compression is higher.