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)
 
On mine I checked it Sunday and the readings were 79, 80, 80, 80 LBS. Wonder what that works out to?
 
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.
 
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.
 
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.
 
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
 
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?
 
CatMan1 said:
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?
Click to expand...
Looks like your numbers are right on the money - with a stock 4.2 head ;)
 
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.


 
Last edited:
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.
 
You must log in or register to reply here.
Back
Top