COMPRESSION CHECK TIPS

I'll differ with you because your lengthy explanation will be misleading to many... mostly because it is wrong. The reason the reading increases with each stroke is because of the equipment itself. It is because the displacement of the measuring equipment is added to the combustion chamber displacement. Each stroke begins with that extra displacement at the pressure of the previous stroke. After 4 to 6 strokes that difference is very small and the reading will not rise higher with 50 strokes.

One thing that should be mentioned is that the absolute maximum reading is much less important than the comparitive readings between cylinders. There are simply too many variables that affect absolute readings. Ambient temperature, altitude, starter condition, battery voltage, oil weight, cylinder deposits, equipment used, gauge calibration, etc.

I recently took 8 sets of readings on the same engine (not Ford) using the same equipment. Wet/dry/hot/cold/throttle wide open/throttle partially open, in various combinations. The max readings varied widely but the relative readings were similar. The best cylinder gave high readings from 82 to 136. The engine will get rings.

George Willer

Later Roland (AL)

1) remove all the sparkies so its eazier to turn over with starter motor
2) jamb or screw the rubber end into the sparkie hole under test
3) crank with the starter motor at least 5 puffs
4) write the highest number down
5) after testing all the cylinders, evaluate the results

If checking a worn engine, take a 2nd reading with a squirt of oil (about 1 oz or teaspoonfull) down the sparkie hole to seal the rings, this will evaluate the condition of the piston rings and the exhaust valves. if'n reading goes up more than 20 psi, worn rings, if'n reading goes up just a little bitt, leaky/burned exhaust valve; if'n 2 adjacent cylinders are low, blown headgasket

N-Ford flathead specs: 90 psi min (dry)..... ..respectfully, Dell

Since I recommend you pull ALL the sparkplugs before you start your compression tests and since the carb throttle plate is closed by the governor, there is no advantage to holding the N-Throttle open for compression tests, but you can if'n you want to. On engines that you don't pull the sparkies, then yes, you do need to open the carb throttle to allow maximum suck thru the carb.

How to convert compression ratio to compression psi? Since air weighs about 14.7 psi, multiply by CR and add 1 atmosphere. ...ie... 6 X 14.7 = 88.2 + 14.7 = 102.9 psi.

The other reason I could think of to be sure the throttle is fully open is to ensure repeatability of your compression test results. If you were to take a compression test and record the readings for comparison later, and happened to have the throttle wide open for the test, and then at a later date, took another test, and happened to have the throttle closed, your readings might be a little less, even though there was no change in the engine. By making it a point to be sure the throttle is wide open for every test, you remove that variable from your recorded test results.