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Compression test


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In an automobile engine, pressure at full compression should be somewhere at or above 15 times the compression ratio.  There are formulas for this, or common sense can tell you that the compression ratio tells you how many times atmospheric pressure is compressed.  Most early engines were 4:1 compression ratio, so theoretically if everything in the engine is in top notch shape, you should be seeing about 60 psi.  If you're in the 40-50 range, and as mentioned by JFranklin all cylinders are fairly close in pressure, then engine should run OK.

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There is a ton of variation in compression ratios in early cars as the combustion chamber space varies so much. Throwing a number out there would be difficult.  One way to check is to fill a cyl with oil with the piston at BDC, then turn it to TDC and capture the oil that comes out of the plug hole, assuming that the plug hole is the highest point in the chamber... With that volume and your borexstroke you could find a calculator to tell you your ratio and then you could calculate the PSI....

 

I wouldn't do all that myself (not again anyways)

 

The compression should be within 10% of each other on each cyl. On an early car, 20% of each other is likely just fine.  If the pressure is greatly different from one cyl to the next, you have an issue somewhere.  Spraying a little engine oil on top of the cyl helps the rings seal at room temp and slow movement.  Use a spray bottle, you want lots of coverage with very little oil.

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I would disagree that there's a lot of variation in early engine compression ratios.  An engine becomes very inefficient as the compression ratio is lowered.  Compression ratio in an automobile engine, gasoline powered, was limited to just over 4:1, not from a mechanical standpoint, but from a fuel standpoint.  Any higher compression ratio pre-detonated the existing fuel, and you basically couldn't run the engine due to this pre-ignition.  Much lower, and engine was very inefficient with little power.

 

Thus, until about the mid-1920's, just about every automobile engine made had a compression ration that was about 3.8:1 to 4.2:1. At 4.3:1 and above, at the time, fuel would "diesel" and engine would knock, or worse.  As fuel improved, compression ratios went up, and resultant horsepower went up.

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I would disagree that there's a lot of variation in early engine compression ratios.  An engine becomes very inefficient as the compression ratio is lowered.  Compression ratio in an automobile engine, gasoline powered, was limited to just over 4:1, not from a mechanical standpoint, but from a fuel standpoint.  Any higher compression ratio pre-detonated the existing fuel, and you basically couldn't run the engine due to this pre-ignition.  Much lower, and engine was very inefficient with little power.

 

Thus, until about the mid-1920's, just about every automobile engine made had a compression ration that was about 3.8:1 to 4.2:1. At 4.3:1 and above, at the time, fuel would "diesel" and engine would knock, or worse.  As fuel improved, compression ratios went up, and resultant horsepower went up.

I will go along with this but was under the impression that very early gasoline was of a pretty decent quality allowing for higher compression... when it was a byproduct of kerosene. Availability would be much different based on location. As need for gasoline grew and technology to go strait from crude to gas had not been developed, we got into the low octane issues. I am gearing up to go to bed so I will try to refrain from reaching for a book.  When did Ford drop from 22 to 20 horsepower?  That would likely be about the timeframe for the change.

 

As for the OP question, 1910 is likely right on the dividing line and 13 psi is very low. I would put an ear to exhaust and intake ports while hand cranking before committing to a rebuild, your issue could be small, but you are correct that 13 indicates a problem. I once found 6 PSI on the number one cyl of a model A that was running and driving... where that 6 psi came from I will never know... inspection revealed the lack of a piston!  I once found 0 psi on a chevy 6 and fixed its stuck valve with a hammer and zero wrenching aside from valve cover removal.

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The "invention" of tetraethyl lead in the early 1920's was the turning point for increasing compression ratios in automobile engines.  That additive eliminated pre-ignition, or knocking, and led to manufacturers being able to get more and more horsepower with smaller displacement engines.

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