How can I tell

G

Grumps

Member
what head I have on my Model A. When I bought the car there wasn't much information provided about its mechanical condition except that it would start and could be driven. I have been doing some work on the drive train and steering and would like to find out a little more about the motor. The first thing that I would like to know is the motor stock or has the head been replaced with a high compression head. How can I tell? I did a compression test and found the compression on cylinders 1,2 and 3 to be at 62lb and 4 was reading 64. What does this compression reading tell me? Is this a stock head or HC one? Can I even tell what it is with just a compression test. Any help will be appreciated.
 
The fact that the readings are so close is a good sign. From the pressures you got it appears to be a stock 4.2 compression ratio.
 
You can estimate the compression ratio by dividing your compression gauge readings by the ground level atmospheric pressure in your area. Example:
62 psig / 14.7 at sea level = 4.2. So a gauge reading of 62 psig indicates a 4.2:1 compression ratio at sea level.
 
I would caution that direct compression readings can vary quite a bit depending on condition of parts such as rings & valve components and the bore size of the cylinders at any given time in the life of an engine block. It can be a rudimentary estimate but I wouldn't rely on it completely. The chamber or clearance volume size will be exact with very little or no error depending on how it's measured.
 
The only way I would rely on would be to compare the outline of the combusion chamber with known and documented examples online.

Several at Vince's good site. But getting there right now from the front door might be challenged as it appears that many links on the Fordgarage site are not linked properly. This is unusual for Vince since he is so "attentive" otherwise.

One I did find is at https://www.fordgarage.com/pages/headcompressionratio.htm

Interesting his written comparison notes at least one Model A aftermarket head marketed as "High Speed" and physically marked as such - but was the same standard 4.2 compression ratio.

Joe K
 
Last edited:
Bob Bidonde said:
You can estimate the compression ratio by dividing your compression gauge readings by the ground level atmospheric pressure in your area. Example:
62 psig / 14.7 at sea level = 4.2. So a gauge reading of 62 psig indicates a 4.2:1 compression ratio at sea level.
Click to expand...
Isn't mathematics, science, and geography neat? Now I understand how there can be different psig measurements on the same engine. Elevation needs to be a part of the equation. Thankyou Bob!
 
Most Brumfield heads are 5.9 to 1. He also sold a 5.5 head for more worn motors. Also, the super heads were sold later and were 6.5 or 7.0 I believe.
 
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