The Ford Barn

packrat6 · 08-11-2021, 02:02 PM

what should the compression read on a warm engine ('30 four cyl.) versus a cold engine?

Werner · 08-11-2021, 04:09 PM

Hi,

on my not tuned engine I measured:
Cold: 4.9, 5.2, 5.2, 5.2 .
Hot: 5.2, 5.2, 5.2, 5.2 .
(kp/cm²)

Gene F · 08-11-2021, 04:23 PM

On mine I checked it Sunday and the readings were 79, 80, 80, 80 LBS. Wonder what that works out to?

Anteek29 · 08-11-2021, 05:20 PM

Compression numbers will depend on the type of head installed...stock or other.

packrat6 · 08-11-2021, 06:41 PM

i'm a bit confused......book says 75 or so....mine is reading 100-110 cold and 100-106 warm. so...whats up with that? the engine was rebuilt prior to my getting the car but i have no information on details.

Big hammer · 08-11-2021, 06:56 PM

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It has a high compression head on it, or your testing under sea level !

nkaminar · 08-11-2021, 07:03 PM

5.2 is in bars, I think, which is one atmosphere or 14.7 psi. So converting to psi yields 76 psi. 80 LBS is 80 psi, or pounds per square inch. 80 psi is equal to 76 psi within measurement error bands.

Patrick L. · 08-12-2021, 05:15 AM

Mine is 45# across the board, been that way since 1975 when I bought it. It was rebuilt in 1961.

old31 · 08-12-2021, 08:27 AM

For a good approximate way to get what your number should be, number do this.

14.7x the head ratio number.

Example: 14.7 x 6.1=89.67 psi or 14.7x 4.2=61.74psi.

burner31 · 08-12-2021, 10:34 AM

I have a 6:1head with the modern gasket, reading 95 psi in all tubes
This is after lapping valves, honing cylinders, new rings and pistons, also installed the adjustable lifters, did this about three months ago, no changes or issues so far

CatMan1 · 08-12-2021, 11:08 AM

Some of these look quite high! Mine was supposedly rebuilt about 1,000 miles ago, is all stock, and is reading..... #1 60, #2 55, #3 55, #4 60. That is PSI. Should it be higher?

Anteek29 · 08-12-2021, 12:20 PM

Quote:

Originally Posted by CatMan1

Some of these look quite high! Mine was supposedly rebuilt about 1,000 miles ago, is all stock, and is reading..... #1 60, #2 55, #3 55, #4 60. That is PSI. Should it be higher?

Looks like your numbers are right on the money - with a stock 4.2 head

CatMan1 · 08-12-2021, 12:22 PM

Quote:

Originally Posted by Anteek29

Looks like your numbers are right on the money - with a stock 4.2 head

Thanks! That's a relief!

Bob Bidonde · 08-13-2021, 07:54 AM

To estimate the compression ratio from gauge readings, divide the gauge reading by the atmospheric pressure in your region. At sea level, Atmospheric Pressure = 14.7 psi. In Denver, Colorado Atmospheric Pressure is 12 psi:
79/14.7 = 5.4:1
55/14.7 = 3.7:1
62/14.7 = 4.22:1

Engine power & torque vary with compression. Theoretically, gauge readings below 62 psig at sea level (stock 4.22:1 Ratio) are indicative of less than 40 hp and less than 128 ft-lbs of torque. How much less requires power & torque requires dynamometer testing.

Patrick L. · 08-13-2021, 09:00 AM

Compression is a good thing to know, for now and for the future issues.

But, as long as the engine operates as it should, who cares. Run it and enjoy it.

08-11-2021, 06:41 PM	#5
packrat6 Member Join Date: Apr 2017 Location: Smithfield, Virginia Posts: 76	Re: engine compression i'm a bit confused......book says 75 or so....mine is reading 100-110 cold and 100-106 warm. so...whats up with that? the engine was rebuilt prior to my getting the car but i have no information on details.

08-11-2021, 06:56 PM	#6
Big hammer Senior Member Join Date: Jun 2014 Location: Land of Lincoln Posts: 3,126	Re: engine compression Sponsored Links (Register now to hide all advertisements) It has a high compression head on it, or your testing under sea level ! __________________ Don't force it with a little hammer tap, tap, tap get a bigger hammer tap done

08-11-2021, 07:03 PM	#7
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 3,889	Re: engine compression 5.2 is in bars, I think, which is one atmosphere or 14.7 psi. So converting to psi yields 76 psi. 80 LBS is 80 psi, or pounds per square inch. 80 psi is equal to 76 psi within measurement error bands. __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past.

08-12-2021, 05:15 AM	#8
Patrick L. Senior Member Join Date: May 2010 Location: Largo Florida Posts: 7,225	Re: engine compression Mine is 45# across the board, been that way since 1975 when I bought it. It was rebuilt in 1961.

08-12-2021, 08:27 AM	#9
old31 Senior Member Join Date: Jan 2016 Posts: 2,092	Re: engine compression For a good approximate way to get what your number should be, number do this. 14.7x the head ratio number. Example: 14.7 x 6.1=89.67 psi or 14.7x 4.2=61.74psi.

08-11-2021, 02:02 PM	#1
packrat6 Member Join Date: Apr 2017 Location: Smithfield, Virginia Posts: 76	engine compression what should the compression read on a warm engine ('30 four cyl.) versus a cold engine?

08-11-2021, 04:09 PM	#2
Werner Senior Member Join Date: Jan 2019 Location: Germany, near Aachen Posts: 1,153	Re: engine compression Hi, on my not tuned engine I measured: Cold: 4.9, 5.2, 5.2, 5.2 . Hot: 5.2, 5.2, 5.2, 5.2 . (kp/cm²) __________________ Beste Gruesse aus Deutschland, Werner Ford Model A, Roadster, 1928 Citroen 11 CV, 1947 Hercules W 2000, 1976; (with NSU-Wankel Rotary Engine), Canadian version

08-11-2021, 04:23 PM	#3
Gene F Senior Member Join Date: Mar 2012 Location: Ohio Posts: 1,952	Re: engine compression On mine I checked it Sunday and the readings were 79, 80, 80, 80 LBS. Wonder what that works out to?

08-11-2021, 05:20 PM	#4
Anteek29 Senior Member Join Date: Dec 2014 Location: Santa Maria, CA Posts: 985	Re: engine compression Compression numbers will depend on the type of head installed...stock or other. __________________ Alan 1929 Special Coupe 1941 Pick-Up 1955 Victoria

08-12-2021, 10:34 AM	#10
burner31 Senior Member Join Date: Dec 2014 Location: Shawnee, Ok Posts: 3,471	Re: engine compression I have a 6:1head with the modern gasket, reading 95 psi in all tubes This is after lapping valves, honing cylinders, new rings and pistons, also installed the adjustable lifters, did this about three months ago, no changes or issues so far __________________ Keith Shawnee OK '31 SW 160-B

08-12-2021, 11:08 AM	#11
CatMan1 Senior Member Join Date: Jun 2021 Location: Pella, IA Posts: 326	Re: engine compression Some of these look quite high! Mine was supposedly rebuilt about 1,000 miles ago, is all stock, and is reading..... #1 60, #2 55, #3 55, #4 60. That is PSI. Should it be higher?

08-13-2021, 07:54 AM	#14
Bob Bidonde Senior Member Join Date: May 2010 Location: Long Island, NY Posts: 3,462	Re: engine compression To estimate the compression ratio from gauge readings, divide the gauge reading by the atmospheric pressure in your region. At sea level, Atmospheric Pressure = 14.7 psi. In Denver, Colorado Atmospheric Pressure is 12 psi: 79/14.7 = 5.4:1 55/14.7 = 3.7:1 62/14.7 = 4.22:1 Engine power & torque vary with compression. Theoretically, gauge readings below 62 psig at sea level (stock 4.22:1 Ratio) are indicative of less than 40 hp and less than 128 ft-lbs of torque. How much less requires power & torque requires dynamometer testing. __________________ Bob Bidonde Last edited by Bob Bidonde; 08-13-2021 at 09:27 AM.

08-13-2021, 09:00 AM	#15
Patrick L. Senior Member Join Date: May 2010 Location: Largo Florida Posts: 7,225	Re: engine compression Compression is a good thing to know, for now and for the future issues. But, as long as the engine operates as it should, who cares. Run it and enjoy it.

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