Cylinder compression?

[stockman1]

After I complete the injector project, I will move on to the right side of the motor. New plugs... At that point I wanted to measure cylinder compression, what kind of values should I expect?

[MikeFleming]

150-155 on all of them.

Be sure to block the throttle plate wide open before cranking.

[stockman1]

150-155 on all of them.

Be sure to block the throttle plate wide open before cranking.bad idea to crank with upper intake off?

[MikeFleming]

Fine with the intake off. Then you're sure the air pasages aren't blocked by the throttle thingie.

[stockman1]

150-155 on all of them.

Be sure to block the throttle plate wide open before cranking.125 125 125 150

[blueboss]

Technically, you should be measuring cylinder compression on a warm engine but OK........as a comparison, my engine, cold was in the 150's across the board. 125 is low IMHO.

[stockman1]

Technically, you should be measuring cylinder compression on a warm engine but OK........as a comparison, my engine, cold was in the 150's across the board. 125 is low IMHO.I would agree its with in 20pct. I wasn't too worried because it had not been run for a couple of weeks. I agree a warm motor or one that had oil circulated would give better readings.

[SVOfish]

I always do a dry and wet compression test. Do a dry "regular test" write down the results then add a cap (TBS or so ) of oil to the cylinder through the plug hole. Then test again.

[MikeFleming]

I always do a dry and wet compression test. Do a dry "regular test" write down the results then add a cap (TBS or so ) of oil to the cylinder through the plug hole. Then test again. The measured pressure will always be higher simply because the clearance volume has been reduced - unless there's serious mechanical issue (where it read zero before) so you won't get accurate results for a couple reasons.

The original idea in the early 50's was to add some oil into the spark plug hole and crank the engine over a few times to help temporarily seal the rings to the cylinder. More useful on a flat-top or domed piston, in-line engine (which describes most engines at that time). Not so useful on slanted or v-style engines as getting the oil onto the top side of the piston and cylinder wall is, well, problematic.

Adding the oil on our engines with a dished piston will only fill the dish and - unless you add LOTS of oil - will not get to the cylinder walls and rings. And if you do add enough to fill the dish and reach the walls and rings, you are extremely close to hydro-locking the cylinder. Which is bad. Be sure to place rags over the spark plug holes that you added oil to unless you like cleaning oil spatter off the walls of your garage (and nearby cars and beverages).

As stout as these engines are from the factory, if you find a low hole,something is broke or worn out so take it apart and do an inspection. Don't bother adding oil - not worth the time. IMHO.

[SVOfish]

Interesting. I always did it because that how I was taught in school.

[stockman1]

The measured pressure will always be higher simply because the clearance volume has been reduced - unless there's serious mechanical issue (where it read zero before) so you won't get accurate results for a couple reasons.

The original idea in the early 50's was to add some oil into the spark plug hole and crank the engine over a few times to help temporarily seal the rings to the cylinder. More useful on a flat-top or domed piston, in-line engine (which describes most engines at that time). Not so useful on slanted or v-style engines as getting the oil onto the top side of the piston and cylinder wall is, well, problematic.

Adding the oil on our engines with a dished piston will only fill the dish and - unless you add LOTS of oil - will not get to the cylinder walls and rings. And if you do add enough to fill the dish and reach the walls and rings, you are extremely close to hydro-locking the cylinder. Which is bad. Be sure to place rags over the spark plug holes that you added oil to unless you like cleaning oil spatter off the walls of your garage (and nearby cars and beverages).

As stout as these engines are from the factory, if you find a low hole,something is broke or worn out so take it apart and do an inspection. Don't bother adding oil - not worth the time. IMHO.you sound like the guy that wrote all those engineering books that I had to read in college. That is a good thing of course! :AvatarPriceless: