Do I have a high compression head?

S

SNAFU1929

Junior Member
Did some research, couldn't find a good answer on my own. Google AI says it is a high compression, but I dont trust AI.

What are your thoughts?

The only marking is a "6" or a "G".

I just ordered a Nurex automatic spark timing and want to set it right. My dad bought this car from a guy that restored it years ago, then gave it to me. He never asked the guy.
 

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SNAFU1929 said:
Did some research, couldn't find a good answer on my own. Google AI says it is a high compression, but I dont trust AI.

What are your thoughts?

The only marking is a "6" or a "G".

I just ordered a Nurex automatic spark timing and want to set it right. My dad bought this car from a guy that restored it years ago, then gave it to me. He never asked the guy.
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Hard to tell by the exterior studs look good so remove head and have a look ..Model 'A' Ford Garage ~ Model 'B' Ford Garage check this great site out may have your answers and thanks for the site.
 

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Do a compression check. If the readings are well past 65-70 psi, you probably have a higher compression head in place. To find the exact amount, you need to follow the instructions posted elsewhere on this website describing pouring oil into a combustion chamber in the upside down cylinder head with a spark plug installed. There is a formula that will convert the amount of oil the chamber took before overflowing. That will tell you the compression ratio. There have been several threads here in the past on this topic. Do a word search to find them. Good reading that you should find enlightening.
By the way - yes, you have to remove the cylinder head to perform this test. :(
Marshall
 
There are far too many "foolzum" heads (stock exterior appearance with higher CR) around to guess at the compression ratio without seeing the combustion chamber.
A friend installed the NuRex using the stock settings with a 6.0 head and had no problems.
 
Get a borescope to look inside or perhaps measure the spark plug threads as a start. Deeper threads than stock (5/8") can sometimes indicate a higher compression head.
 
If it was restored “years ago” it probably does not have a high comp head. “Back in the day” a high compression head was usually a Police head which would clearly be marked with a B or a model B head which would be marked with a C. The only way to truly tell if you have an aftermarket high comp head is to pull the head and look. The combustion chambers are different between a stock head and a high comp head so you can easily tell the difference. If you knew for certain that it was a high comp head you would want to run a bit less spark advance.
 
The traditional weak spot in the stock Model A's power curve is when shifting into third gear and then accelerating. There is a noticeable slowing down of speed and drop off of power until the RPMs begin to build up again and good speed is attained. This vulnerable weak spot is a detriment in traffic because the Model A takes so long to increase speed at this point. A higher compression head smooths out this dead area and accelerates quicker than the stock head so that the "dead spot" in power is reduced. I noticed this immediately the first time I installed a Brumfield standard head on my engine in the 1990's, an engine whose personality I was very familiar with from years of driving it. I was always disappointed in third gear by how much the car bogged down and seemed to take forever to get up to speed. With the 5.9:1 Brumfield head, however, I was pleasantly surprised at the improvement. Since most higher compression heads available for the Model A are at least 5.0:1 (the marginally better 4.6 CR Model B head excepted), I would expect that you should be able to judge whether you have an HC head by how your car responds in this window of power decrease compared to a stock Model A engine. Can you drive a local Model A that is stock and pay special attention to its power characteristics just after shifting into 3rd gear? This will tell you if you do have an HC head, but it won't tell you by how much until you perform a chamber volume test.
Marshall
 
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I doubt that you care enough about it that you will remove the cylinder head to check. But if you did, the high comp heads have combustion chambers that are markedly different from stock. Both the 5.5:1 and the 6:1 head are shaped different. The high comp heads that have been on the market for many years are sold by different retailers but they were all made by the same company (Snyder’s). Snyders website has pictures of the different combustion cylinders.
 
Once you remove a spark plug you can go in there with a small borescope and see what kind of combustion chamber you have You can buy a borescope for less than $20.
 
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