Compression Specifications?

[srisitt]

I'm measuring the compression on a 91 ranger. I've been looking for spec information for this engine to compare the readings to, and it's been surprisingly difficult to find. All I've found just states what the cylinders should read in comparison to one another, not the general psi range the cylinders should be within individually. I've been looking online and in a Chilton manual and found nothing. I'm used to working on old honda motorcycles, where these values are clearly given as an acceptable range (i.e 160-190 psi), alongside what the cylinders should read in relation to one another. Am I missing something here? Is the individual compression of each cylinder not relevant for some reason?

 

[alwaysFlOoReD]

Wouldn't it be barometric pressure times compression ratio?

 

[srisitt]

Wouldn't it be barometric pressure times compression ratio?

That would make sense but is there no spec value cylinders should be at?

 

[Uncle Gump]

I'm pretty sure Floored nailed it...

So at sea level... 14.7 psi atmospheric pressure

Compression ratio... cylinder volume at BDC vs. TDC... lets use 8.5:1

14.7 x 8.5 = 124.95. So in a perfect world... the spec would be 125 psi. Lets remember... the sealing surfaces degrade the first time the engine rolls over. So your perfect world spec is at best a moving target.

Probably more important is the comparison of the readings between cylinders... and how the test was run to gather your compression data. Be leery of some of the stuff you see on youtube... it seems like everyone wants to be an internet star these days and feels the need to make these videos with their new tool and simply have no damn clue.

The proper way to run the test is well documented but here is how I do it..

Remove ALL spark plugs. This ensures that all cylinders are at the same free wheeling cranking speed.

Disable fuel system.

Block the throttle to WOT.

Install a battery charger... This ensures the battery doesn't degrade between the first cylinder and the last.

Ensure your tester is tight and sealed.

Crank engine over 4 complete cycles. Document reading. Repeat this for the remainder of cylinders.

You can change the number of cycles you crank the engine over to get your reading... just ensure you use the same number of cycles on all cylinders.

That will tell you the mechanical health of your engine. You can also run the test wet... adding a squirt of oil in the cylinder. If the reading goes up wet... chances are it's a lower end issue. If the reading doesn't change wet... probably a top end issue.