i think its going to be alot more than .4 psi on the compression gauge.Your not talking about 12.9 to 12.5 psi for static compression test.I think compression psi should be alot more like 20 to 30 psi difference,but i could be wrong.i found this on net but its about car engine,there is so many variables i think without getting your actual cylinder volume,piston dish or dome volume,cc head,headgasket volume,etc.. i think its going to be a guess. Also there is static an running compression, on a engine u can check compression by kick or turning starter over with throttle open which will give u static compression(100psi to 175psi etc... U can also turn engine over faster like checking one cylinder on a car while it is running and get a running compression,usually to check leaking valves,blowby,bad cam lobes ,etc...Numbers on a running compression test are a lot lower like 40 to 70psi. i'm saying this cause on top of the guessing what exact volume combustion chamber u have u also have to take in account how the camshaft and cranking speed change numbers. Ohhh i'm getting to deep with this here is what i found. if what i have said is wrong please lead me to the light
So What Cranking Pressure Can We Expect To See Then?
For an average road engine we don't need to calculate the effect of all the above factors because the expected cranking pressure will be quoted by the manufacturer. Haynes manuals and other tuning guides also quote the figures. If the data is not available you can make a good estimate of the expected cranking pressure from a healthy engine just from the compression ratio. You can expect the cranking pressure in psi for a road engine with a standard cam to be about 17 to 20 times the value of the compression ratio. So our engine with a CR of 10:1 should generate between 170 and 200 psi. The better the condition of the engine the closer the figure will be towards the top end of this range. If the engine has been modified by fitting a different cam then you need to make some allowances. Fast road cams will reduce the expected figure by 10 psi or so. The best thing to do is take a set of compression readings as soon as the engine has been built so that you know what it generates in good condition. For competition engines it's even more important and I recommend to all my customers that they do a compression test after every race to make sure that nothing inside the engine is going wrong. The vast majority of them take no notice of course because it involves a bit of work. Finding a potential problem before it leads to an engine blow up can save a lot of time and money in the long run though. Copyright David Baker and Puma Race Engines
The cranking pressure can also be used as guide to setting the compression ratio to suit competition camshafts. The longer the cam duration the higher the CR needs to be to make it work properly. As explained above, the later inlet valve closing of the long duration cam will reduce the cranking pressure but when you raise the CR this compensates and the cranking pressure comes up again. My rule of thumb for normally aspirated competition engines is I don't like to see less than 200 psi cranking pressure regardless of cam duration if the CR has been matched to the cam correctly. Anything less than 180 psi and the cam won't be working at its best, especially at low rpm.
An equation I use for estimating cranking pressure from static compression ratio (CR) for engines in good condition with standard road cams is as follows.
Cranking pressure (psi) = CR^1.3 x 14.7 x 2/3
CR^1.3 is the Compression Ratio raised to the power 1.3. On pocket calculators you can do this with the y to the x function.
For modified engines, or more specifically engines with long duration cams you need to then apply the following approximate factors.
Fast road cam - multiply by 0.95
Rally cam - multiply by 0.90
Full race cam - multiply by 0.85
Example. A rally engine with 11:1 static compression ratio.
11^1.3 = 22.58
22.58 x 14.7 x 2/3 x 0.90 = 199 psi
