compression reading on a 9 to1 small block

[whereyat]

what reading should i get from a 9 to 1 compression 30 over 302 with all the plugs pulled and testing one at a time? and how much difference between cylinders? thanks again

[Leadfoot]

interesting. but a crack shoot or best guess, with out knowing a little more
like a stock cam or not is really the question? hot or cold comp.............?

also most comp testers read differently depending on quality, brand, and most of all the air nipple or valve in the end.

now say a stock 5.0 non roller is about 185-190 hot, 155-170 cold. in a ball part test.

and you want or would like to see no more than 25% difference between the cylinders.

[FORDification]

Ford Technical Service Bulletin #93 - May 31, 1968
Article 1417 - ENGINE COMPRESSION CHECK
(All Cars and Trucks)

This procedure supersedes and replaces all published procedures and specifications pertaining to engine compression tests and should be performed as follows:

• 1. Be sure the crankcase oil is of the correct viscosity and make sure that the battery is properly charged. Operate the engine for a minimum of 30 minutes at 1200 rpm, or until the engine is at normal operating temperature. Turn the ignition switch off; then remove all the spark plugs.
  2. Set the carburetor throttle plates in the wide open position.
  3. Install a compression gauge in No. 1 cylinder.
  4. Disconnect the brown lead (no. 1 terminal) and the red and blue lead (S terminal) at the starter relay. Install an auxiliary starter switch between the battery and S terminals of the starter switch. Crank the engine (with the ignition switch off) at least five (5) pumping strokes and record the highest reading indicated. Note the approximate number of compression strokes required to obtain the highest reading.
  5. Repeat the check on each cylinder cranking the engine approximately the same number of compression strokes.

The indicated compression pressures are considered Normal if the lowest reading cylinder is within 75% of the highest. Refer to the following example and the Compression Pressure Limit Chart.



Seventy-five (75%) percent of 140, the highest cylinder reading, is 105. Therefore, cylinder No. 7 being less than 75% of cylinder No. 3 indicates an improperly seated valve or worn or broken piston rings. If one, or more, cylinders read low, squirt approximately one (1) tablespoon of engine oil on top of the pistons in the low reading cylinders. Repeat compression pressure check on these cylinders.

• 1. If compression improves considerably, the piston rings are at fault.
  2. If compression does not improve, valves are sticking or seating poorly.
  3. If two adjacent cylinders indicate low compression pressures and squirting oil on the pistons does not increase the compression, the cause may be a cylinder head gasket leak between the cylinders. Engine oil and/or coolant in the cylinders could result from this problem.

The following quick reference chart should be used when checking cylinder compression pressures. The chart has been calculated so that the lowest reading number is 75% of the highest reading.



EXAMPLE
After checking the compression pressures in all cylinders, it was found that the highest reading obtained was 196 psi. The lowest pressure reading was 155 psi. By locating 196 in the maximum column it is seen that the lowest allowable pressure is 145 psi. Since the lowest cylinder reading was 155 psi, the engine is within specifications and the compression is considered satisfactory.