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Old 12-04-2021, 02:16 PM   #1
Bob Bidonde
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Default Compression Gauge Readings

For a club presentation, I am making a chart of compression test gauge readings. For the stock 4.22:1 head, from experience I have a gauge reading of 50 PSIG minimum, 62 PSIG calculated & 76 PSIG maximum. Calculated readings = 14.7 x compression ratio.

For the 5.5:1 and 6:1 cylinder heads, what should the minimum PSIG readings be? The minimum gauge reading is your opinion of the least a touring engine could reasonably run with.

The maximum gauge readings I have are 4.22:1 = 76 PSIG, 5.5:1 = 81 PSIG & 6:1 = 117 PSIG.
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Old 12-04-2021, 02:39 PM   #2
Werner
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Default Re: Compression Gauge Readings

Guten Abend Bob,

maybe this will also help you?
Compression pressure in kg / cm² with normal cylinder head, 1000 mls after engine revision.
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Old 12-04-2021, 04:06 PM   #3
Patrick L.
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Default Re: Compression Gauge Readings

Mine has been 45# [ 3 or 4 gauges] for the past 45 yrs. But its with a 1961 engine rebuild.
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Old 12-04-2021, 04:53 PM   #4
mercman from oz
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Default Re: Compression Gauge Readings



Compression pressure in kg / cm² with normal cylinder head, 1000 miles after engine revision - Werner
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Old 12-05-2021, 09:07 AM   #5
Bob Bidonde
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Default Re: Compression Gauge Readings

Using 14.223 kg/square cm = 1 psi I see an average pressure approximately 76 psig in the chart. This would be for a newly rebuilt engine with a 4.22:1 cylinder head.


Thanks for the reply Werner. What I seek is the minimum compression gauge reading for a good running engine with several thousand miles of usage.
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Old 12-05-2021, 04:41 PM   #6
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Default Re: Compression Gauge Readings

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Bob, I know you know this but these comments are for non-Engineers and -Scientists who may wonder why your calculation differs from Werner's findings.
Your calculated 14.7 times compression ratio is for isothermal compression. We have almost adiabatic conditions in a rapid compression test.
That explains the higher 76 psi readings since the pressure is also proportional to absolute temperature ratio and the temperature rises as the air is compressed, losing a tiny amount of heat to the cylinder walls and piston as the test is done.
The other factor is that all this is for standard bores. Since the head space barely increases for an overbore but the swept volume increases a lot, the compression ratio also increases. And there will not be too many standard bore engines in the clubs now.
One of mine is 125 thou over (4 inch bore) and that makes the swept volume almost 3.5 litres against the standard, 3.3. Without measuring my engine's head space, I suspect my 5.5 to I head is nearer 6 to 1 on this block in actual fact.
Do you agree?
This engine reads 100 psi after 28000 miles and a valve grind at 25000.

SAJ in NZ

Last edited by SAJ; 12-05-2021 at 05:42 PM. Reason: Better reading order
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Old 12-06-2021, 08:29 AM   #7
Bob Bidonde
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Default Re: Compression Gauge Readings

SAJ, my objective is to provide the Low, Calculated & High gauge readings one would see in compression test. I can explain in layman's terms why the High gauge readings occur on a newly rebuilt / low mileage engine. My catchall term is "dynamic effects".
However, the Low gauge reading is not scientific, but rather subjective, so that is the basis of my post here. I seek from experiences, what are the lowest compression gauge readings for a smooth running engine.


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Old 12-06-2021, 09:38 AM   #8
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Default Re: Compression Gauge Readings

No idea if this helps but this is the data provided from the 1950 Motors Repair Tune Up chapter.
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Old 12-06-2021, 11:05 AM   #9
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Default Re: Compression Gauge Readings

Bob, don't forget the altitude factor.
"Altitude will affect the compression also, it looks like roughly 3% loss in pressure every 1000' "
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Old 12-07-2021, 08:07 PM   #10
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Default Re: Compression Gauge Readings

Quote:
Originally Posted by Crankster View Post
No idea if this helps but this is the data provided from the 1950 Motors Repair Tune Up chapter.
Great info...Thanks, Crankster
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