Compression Test Results

[DHMag]

ok, for those of you interested, here are my results from the compression test performed today. 130,024 miles indicated.

Cylinder #4..|..Cylinder #1
Dry - 160.....|..Dry - 140
Wet - 175....|..Wet - 160
____________________________
Cylinder #5..|..Cylinder #2
Dry - 140.....|..Dry - 160
Wet - 150....|..Wet - 175
____________________________
Cylinder #6..|..Cylinder #3
Dry - 175.....|..Dry - 160
Wet - 170....|..Wet - 175
____________________________

rings look good to me.

<small>[ August 30, 2003, 06:18 PM: Message edited by: DHMag ]</small>

 

[Machspeed]

thumb Lookin good.

 

[DavidT]

Isn't that too much variation?
What does "wet" and "dry" mean?

 

[Machspeed]

Nah they look close enough. I think its as long as theyre within 10 percent of each other your ok. Wet means you spray oil into the cylinder.

 

[sdpatt]

Those numbers are substantially low and vary significantly between cylinders. That engine has seen much better and stronger days. If 10% is the variation limit, then your numbers exceed that difference. One hundred forty to 175 psig is a 25% (or 20%) difference depending on which you take as the standard number. Sorry Dale, but even my very old (279,400 miles) engine tested to 185-190 psig cold, without adding any oil into the plug hole, on all six cylinders.

 

[Machspeed]

Are you comparing between wet and dry when you say there more then 10 percent differnece? How many miles on that motor? Ya i guess those dolook a little low.

 

[DHMag]

ive always used a 25% difference between cylinders as a standard. i did get some variations on some readings, so i did multiple compression tests for a couple of the cylinders. ill run another dry test tomorrow evening to confirm my numbers. and this was done with a cold engine, 130K miles.

Scott, have you ever replaced rings ?

 

[LaTechSHO]

Ford specifies 25% as within specs....


Louis

 

[SHO_Driver]

If the compression goes up when wet then your rings do have a problem. The oil helps by sealing the worn ring to increase compression. Dry and Wet should be close for a normal engine.

 

[DHMag]

SHO_Driver:
If the compression goes up when wet then your rings do have a problem. The oil helps by sealing the worn ring to increase compression. Dry and Wet should be close for a normal engine.

also, if the rings were installed incorrectly, the end gaps could be too close to each other and cause blow-by. youll also have *some* oil at the upper ring, for proper cylinder lubrication.

 

[sdpatt]

DHMag:
Scott, have you ever replaced rings ?

No. When I replaced the connecting rod bearings last December, I had fully expected to see some vertical scoring from all those miles. Instead, I could still see the cross hatched honing marks in the cylinders. At least in the area below the piston skirts even at the highest point of travel for each piston.

 

[DHMag]

sdpatt:

DHMag:
Scott, have you ever replaced rings ?

No. When I replaced the connecting rod bearings last December, I had fully expected to see some vertical scoring from all those miles. Instead, I could still see the cross hatched honing marks in the cylinders. At least in the area below the piston skirts even at the highest point of travel for each piston.

yeah, i recall the pic you took. for the life of me, i cant remember what my cylinder walls looked like when i did my rod bearings...

 

[AutoSHO]

A couple things, I just want to clarify:

(1) Make sure you are running the test with all 6 spark plugs out; otherwise the slower rotation of the starter having to compress all 6 pistons will drop your readings. I'd also unplug the coil pack and the DIS to make sure you aren't getting spark.

(2) Make certain you're hold the throttle completely open. Not having the TB open on mine dropped the numbers 30+ psi.

My automotive class said absolutely no more than 10% variation. If you'd like me to type out the exact procedure thats in the book I can do that too, I was amazed at how much I missed when I did it the first time. Those two above are the most important though. Also, how many times are you allowing it to compress before stopping? You should turn it over 5 compression strokes (no more or less).

<small>[ August 31, 2003, 11:49 PM: Message edited by: AutoSHO ]</small>

 

[DHMag]

AutoSHO:
A couple things, I just want to clarify:

(1) Make sure you are running the test with all 6 spark plugs out; otherwise the slower rotation of the starter having to compress all 6 pistons will drop your readings. I'd also unplug the coil pack and the DIS to make sure you aren't getting spark.

(2) Make certain you're hold the throttle completely open. Not having the TB open on mine dropped the numbers 30+ psi.

My automotive class said absolutely no more than 10% variation. If you'd like me to type out the exact procedure thats in the book I can do that too, I was amazed at how much I missed when I did it the first time. Those two above are the most important though. Also, how many times are you allowing it to compress before stopping? You should turn it over 5 compression strokes (no more or less).

im about to head out to run another compression test. i guess im old school and thinking of 4 barrel V8s....i didnt open the throttle up. my dad was a mechanic for Ford back in the day and he taught me a standard of 25% difference between neighboring cylinders. is the 10% because of the higher tolerances of motors these days ?

ty for the clarification, AutoSHO

 

[93EmeraldMTX]

sdpatt:
Those numbers are substantially low and vary significantly between cylinders. That engine has seen much better and stronger days. If 10% is the variation limit, then your numbers exceed that difference. One hundred forty to 175 psig is a 25% (or 20%) difference depending on which you take as the standard number. Sorry Dale, but even my very old (279,400 miles) engine tested to 185-190 psig cold, without adding any oil into the plug hole, on all six cylinders.

I'm with him. my SHO with 153k miles on it dry was in the very low 200s.