It would be difficult to calculate a real figure. You could likely calculate a maximum figure relatively easily PV = nRT. However the real figure will be lower due to ring seal/valve seal and cam overlap timing which will also change with cranking rpm.
The useful thing about compression testing is the relative average reading against other engines in a similar state, and the relative reading between cylinders.
Generally when comparing average reading between engines the test method should be consistent. Like cold engine, throttle wide open. Some people will drop a little oil down the cylinder, the oil will help the rings to seal so the reading should go up. If the reading is low and doesn't go up often it is a sign of a poorly sealing valve but can be other things too.
The often touted rule is that cylinder to cylinder readings should be within 10% of one another.
There is nothing more to it really. I find it quite useful to take compression readings periodically during the break in period of an engine and plot a graph, you can see when rings are bedded in for example or find an issue early. On past cars with more expensive engines I also take a compression reading at each service. This kind of testing is THE most useful as you can maintain your test procedure. Admittedly I don't do this on the Clio because its a t**t getting to the plugs.
Using the ideal gas law with a camber vol (including piston dish, gasket thickness and head vol) of 50cc at 20deg taken from this thread:
Anyone has precise numbers of combustion chamber volume in the head of Clio 172 (F4R 732)? Or a reference to a good description how to measure it. My measurments give me numbers between 30 and 40cm3 :dapprove:
cliosport.net
You can calculate the number of mols r for the total cylinder volume. This is then compressed into 50cc and gives a pressure of 159.4 psi.
Changing the temperature to 70 deg gives 186psi
So here is likely your variation - people measuring with the engine at different temps.
