Quote:
use the 0.04 and 8.12:1 r.
[snip!]
what it all really boils down to is doing a compression test in when the engine been broke in. all the diagrams and excel sheets in the world wont tell you EXACTLY what your compression will be.
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I dunno, ricekiller. I may be mis-reading the above (and if so I apologize), but it looks to me like you are saying that a compression test will tell you your compression, as in compresion ratio.
I do not see how you are getting the same readings (and 185 PSI is
very high!!) cranking with the starter and cranking by hand. I'm not saying you can't be, just that I find it tough to see how. Perhaps a limitation on my part.
Other people have observed different compression pressure readings depending on if all the plugs are out or not (if they are the starter cranks quicker), the health of the battery (poor battery or connections == slower starter), if the throttle is wide open or not (WOT means less resistance to air getting into the motor) and so on. And there is a rule of thumb for compression readings and altitude--I think it's something like 5 PSI lower per every 2000 feet? I don't remember exactly, but there are folks driving 914s around the Denver area (5000+ feet) as well as the SF Bay area (~20 fteet).
The compression pressure is related to the compression ratio, but the relation is indirect. There are other things that affect the pressure, including how well the rings and valves are sealing. But the compression ratio of a given engine is a pretty decent indicator of what grade of fuel you want to run in that engine. It also varies with a lot of factors internal to the engine (combustion chamber shape, number and location of spark plugs, number and location and orientation of valves, how efficiently the cooling system removes heat from the cylinders and chambers, etc.) but for a given engine the CR gives you a decent idea of the kind of fuel you can use.
Getting the CR even across all cylinders is also important to help the engine have a long and happy lifespan. Compression pressures will change as the components wear-in, so you those numbers are tougher to use when initially setting up an engine anyway.
Compression pressures are a good indicator of the current state of the top end of the motor. But compression ratios are a more useful quantity to know about when you are building the motor, IMHO.
--DD