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In one part of the manual for the Universal 5424, it states a compression ratio of 21:1, which I calculate as translating to 305 PSI on a compression tester.

Elsewhere the specs state compression as 450 PSI, minimum 337.

These numbers appear to be contradictory. I thought maybe the the latter number is the effective compression due to the pre-combusion chamber's gas dynamics.

Which number should you see on a compression tester?
 

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I would use the numbers from the manual if they are what you are quoting. Looks like you took atmospheric pressure and multiplied it by 21. When the air is compressed, it is also heated which results in an even greater increase in the pressure. If I had my thermo book handy I'd calculate what the highest pressure could be for isentropic (reversible adiabatic) compression.
 

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I'd be more concerned with the even-ness of the numbers across the cylinders. ..say within a few pounds per. 450 # (as new) down to 337 nominal? I'd be happy with the 300+ *IF* all were within a few of same :D
I'd also consider a bleed-down test, giving similar times/numbers a better indicator of engine health.

One cylinder with the "book" of 450 and another with ,say,less than "nominla"?
Look out!
 

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The calculation - compression ratio vs pressure ratio is not as simple as compression ratio x air pressure.
The ratio of the 2 is a curve on a graph because of the adiabtic expansion of air. You can get that graph on Wikipedia, but don't bother because it won't help you. The biggest factor affecting the resulting pressure or engineered pressure for each engine is "valve overlap" or camshaft profile that makes use of scavenging (both intake & exhaust valves are open together for the first part of the piston's travel upward on compression). So the only number you can go by is the spec in the book. This spec will also take into acct the slight leakage across the piston rings at your approx. 200 RPM cranking / test speed. As RPM increases the efficiency of compression increases, so that 200 RPM test speed is important.
note: as a diesel mechanic for 41 yrs, I use the easy pressure gauge method, and find it to be reliable.
 
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