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Old 10-05-2010, 12:54 AM
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sbc 350 compression test result?

I am about to buy a 70's model camaro that the engine was built by the previous owner (2 down) so they do not know what is inside the engine. Can't I pull a spark plug and thread in a guage, turn the engine over and get if dish, flat top etc type pistns?? What numbers should represent what comp??

thanks guys!/ doug

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Old 10-05-2010, 02:25 AM
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Yeah Doug, that's a cylinder compression test. It's one of the first tests you want to do to find out if the motor is a player or not. Run the motor until it reaches operating temperature. Disable the ignition coil(s), wire the primary throttle blades (or throttle body blade(s) wide open so the motor can breathe and remove all the spark plugs. You can watch the gauge and tell when the cylinder is not going to pump any higher. Record the highest reading for that cylinder and move on to another cylinder.

There is another test, called a leakdown test that will tell you more about the condition of the motor.

You will not be able to tell what the static compression ratio is or be able to determine the cam timing from these simple tests. Some fellow believe there is a correlation, but when you understand how a motor works, you'll realize that you just can't determine these things from a compression test. For instance, if the motor was a low static compression ratio motor with a short cam in it that would close the intake valve early, it would read a certain cylinder pressure. Another motor with a very high static compression ratio combined with a long cam that would close the intake valve later might show the very same cylinder pressure.

You can get a vague idea though, by using a very strong light into the spark plug hole to eyeball the type of crown on the piston. Perfectly flat with 2 eyebrows, perfectly flat with 4 eyebrows, perfectly flat with a trough across the crown for valve reliefs, a full dish with just a very thin ring (maybe a 1/4 inch wide) sticking up around the top perimeter of the crown, a D-cup where the shape of the dish is like the letter D and there is a generous flat area opposite the dish, or there might be domed pistons in the motor. Determining the piston crown configuration and posting the casting numbers off the cylinder heads will allow some of us to begin to help you with the static compression ratio. If these are cast iron OEM heads, the casting number will be between the valve springs with the valve cover off.
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Old 10-05-2010, 05:04 AM
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Old 10-05-2010, 06:12 AM
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Thanks! great info (didn't think about the flashlight). I will post back #'s, if they let me do the test.
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