engine compression levels.

S

sportcoupeman

Junior Member
What is the correct cylinder compression reading on a standard 40 hp Ford engine. What would be normal on a 40-year-old rebuilt engine? I'm getting 52lbs on all 4 cylinders. Any input out there?
 
Stock compression ratio was 4.2:1. So --- 90 some years later, not accounting for any rebuilds, you look about right. What's particularly good is that all cylinders are the same. That's rare and that's great. I'll say you have nothing to worry about. By the way, 4.2 at sea level would be around 62 PSI.
 
52 is pretty good.

If you have a standard head the math would be:

4.2x14.7. About 60 lbs on a new engine.

Some say the formula should be 4.2x14.7+14.7.
 
sportcoupeman said:
What is the correct cylinder compression reading on a standard 40 hp Ford engine. What would be normal on a 40-year-old rebuilt engine? I'm getting 52lbs on all 4 cylinders. Any input out there?
Click to expand...

Compression Tests are not really a good indicator. Worn rings with cylinders filled with oil can equal those numbers, -or more. What you need to do is perform a cylinder leakage test and then a contribution test.
 
Isn't a contribution test used to isolate a single bad cylinder? It wouldn't really tell you if all the cylinders were equally worn.
 
Absolute compression pressure = (4.22x14.7)+14.7.
Gauge compression Pressure = 4.22x14.7.

We work with gauges. Scientists work with absolute pressure.
 
My rule of thumb is that Model A & Model B engines with stock cylinder heads will run well on compression gauge readings as ow as 50 psig, and no more than a 5 psig difference between cylinders.
By the way, boring an engine increases its compression ratio a bit.
 
My engine has been sleeve back to the original bore size and has the original 4.2 compression head my compression readings are basically 60-65 on all 4 cylinders.
 
alexiskai said:
Isn't a contribution test used to isolate a single bad cylinder? It wouldn't really tell you if all the cylinders were equally worn.
Click to expand...

Actually, it generally tells quite a bit. When used in conjunction with a sensitive RPM gauge, it can help diagnose items like a worn cylinder(s), or too lean fuel mixture, or leaking manifold, etc. as it points you to the faulty areas to check mechanical items. If each cylinder dropped the RPMs by 25% when cancelled, that would indicate all 4 cylinders are pretty even. If #1 and #4 RPMs drop by 30% but #2 & #3 only drop by 20%
 
Here’s an example of a device that could be used to do contribution testing.

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