Heart head compression question-gear head math

[cars2cool]

I just did a compression test on my fresh "A engine today. The block is bored .060 oversize and has a consistent gauged compression in all 4 cylinders of 65 psi. The elevation in my driveway is 1200 feet above sea level and I don't know what my atmospheric pressure currently is. The heart head I have on my A is an unmarked type. I tried to cc measure the head before installation but failed at it. Would like to know a ball park figure of what the compression ratio is using gear head math. And as my 7th grade math teacher, Mr. Etzel said "Mr. Knudson you need to show your work so I can see where you made your mistake." The new NAPA compression gauge is made in CHY-NA, so I have my doubts right there. Thanks in advance

[cars2cool]

Added note, engine was cold, all 4 spark plugs removed and carb. removed.

[CWPASADENA]

From the numbers you have, it looks like only about 4.5:1.
Has the engine been run or is it just as assembled. Unless the engine been run and the rings seated in, you may not have a valid reading.
I would also suspect the gauge if it is made in China.

[cars2cool]

Originally Posted by CWPASADENA

From the numbers you have, it looks like only about 4.5:1.
Has the engine been run or is it just as assembled. Unless the engine been run and the rings seated in, you may not have a valid reading.
I would also suspect the gauge if it is made in China.

Yes the engine has been run a few times for heat cycles to retorque the head and fine tuning carb. and adjustments to the generator output. Plugs are sooted as are the tops of the valves and pistons.

[old31]

CW is correct.

4.5x14.7=66.15

[nkaminar]

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To be absolutely correct you need to know the atmospheric pressure during the test. At 1,200 feet, lets assume it is 14.6 psi. So the math would be 65/14.6 = 4.45 to 1.

But, the intake valve does not close at bottom dead center but after the piston has started up, so the actual compression ratio will be a little higher. The only way to get a truly accurate number is to measure the volumes. The way to do this is to turn the engine over by hand until the timing pin falls into the detent in the cam gear, take the #1 spark plug out, and then pour engine oil into the cylinder until it comes up to the bottom of the spark plug hole. Measure what you pour in. A graduated glass cylinder is a good tool to use. The compression ratio can then be calculated. Rotate the engine by hand to push out the oil through the exhaust valve before starting the engine, or suck it out through the spark plug hole. If you push the oil out into the exhaust manifold, start the car outside because a lot of oil smoke will come out the exhaust pipe.

The compression ratio is the displacement of the cylinder divided by the volume in the combustion chamber. In your case it will be a little larger than 201/4 cubic inches divided by whatever you measured in cubic inches.

[Gene F]

Originally Posted by nkaminar

To be absolutely correct you need to know the atmospheric pressure during the test. At 1,200 feet, lets assume it is 14.6 psi. So the math would be 65/14.6 = 4.45 to 1.

But, the intake valve does not close at bottom dead center but after the piston has started up, so the actual compression ratio will be a little higher. The only way to get a truly accurate number is to measure the volumes. The way to do this is to turn the engine over by hand until the timing pin falls into the detent in the cam gear, take the #1 spark plug out, and then pour engine oil into the cylinder until it comes up to the bottom of the spark plug hole. Measure what you pour in. A graduated glass cylinder is a good tool to use. The compression ratio can then be calculated. Rotate the engine by hand to push out the oil through the exhaust valve before starting the engine, or suck it out through the spark plug hole. If you push the oil out into the exhaust manifold, start the car outside because a lot of oil smoke will come out the exhaust pipe.

The compression ratio is the displacement of the cylinder divided by the volume in the combustion chamber. In your case it will be a little larger than 201/4 cubic inches divided by whatever you measured in cubic inches.

OK, calculated how? I can use a vial from a rain gauge, which will work perfectly. So what is the formula for the math to get this exact number? Of course the volume of the oil will be different for each head, because the amount of oil will end up varying.

[katy]

Do you have picture(s) of the combustion chamber side of the head?

[cars2cool]

Yes I do in a post from a few years back after I acquired the head. I don't know how to repost the pics, sorry.

[Bob C]

Here's the head.

[cars2cool]

That's the one, thanks!

[Fullraceflathead]

That looks like a standard head to me.

[cars2cool]

Look at the area above the cylinder.

[Bob Bidonde]

!4.7 psig is the atmospheric pressure at sea level. To estimate the compression ratio car2cool's needs to divide 65 by the local atmospheric pressure in PSI.

[Bob C]

Stock head.

[nkaminar]

To clarify my comments in Post #5. The intake valve closes after the piston starts up to take advantage of the momentum of the incoming air. It is still flowing into the cylinder as the piston comes up when the engine is running at a good clip. So the pressure you get in a compression test will be lower than the pressure that is in the combustion chamber when you are running the car wide open at 50 or 55 mph. There are other factors too. If there is some restriction in the intake, such as the throttle being partly closed, then the pressure will be less than measured during a compression test. If you have a carburetor and intake system that flows more than the standard Zenith, then the pressure will be higher.

There is a thing called "brake mean effective pressure" (BMEP) which is a measure of the average pressure in the cylinder. It is calculated based on the torque that the engine is putting out. More BMEP, more torque. The BMEP can be used to compare two engines of different displacement as it is independent of displacement.

The compression test is a good tool for looking at the health of the engine. A leak down test is also good and sometimes will tell you more than the compression test.

[updraught]

>>Mr. Etzel said "Mr. Knudson you need to show your work so I can see where you made your mistake."

Mr Ricecom used to say "You are like zee blind chook, looking for zee corn, scratching, scratching, all day long"

[Fullraceflathead]

With the added picture of the stock head I can now see the difference would that possibly be the slightly higher compression police head?

[cars2cool]

Originally Posted by Fullraceflathead

With the added picture of the stock head I can now see the difference would that possibly be the slightly higher compression police head?

I can't say for sure if it is a "police head" or not. When I picked up my engine from Herm Kohnke a few years back, I showed him the head and asked him if he knew what it was or who might of made it and he had no comment on it but was not impressed with the quality of the casting.

[nkaminar]

Gene, The displacement of each cylinder is 201/4 or 50.25 cubic inches. It will be a little more if the engine has been bored, but not much and not enough to worry about. Lets say you measure the oil to fill the combustion chamber to be 7.7 cubic inches. Then the compression ratio would be 50.25/7.7 or 6.5 to 1. If you measured 12 cubic inches to fill, then the compression ratio would be 50.25/12 or 4.2 to 1. There are lots of errors that can creep in so one decimal place in the compression ratio is all that can be expected.

Make sure the piston is at the top and both valve are closed. That would be cylinder 1 with the pin in the detent in the cam gear. Also, fill the oil to the bottom of the spark plug hole, not the top.

It would be best to suck the oil out rather than let it escape out the exhaust manifold. Some will escape anyway and will create a lot of smoke when it starts to burn off.