Cylinder Compression Questions

[Swarzkopf]

So I did a quick compression test on my stock, 90,000 mile 95 5.0 shortblock yesterday.

I did the test on a cold engine.

Here's what I got:

1- 121
2- 126
3- 125
4- 124
5- 125
6- 122
7- 122
8- 112

According to the Hayne's manual, the cylinder compression should be over 100 and the minium cylinder should be at least 75% of the maximum cylinder, so I'm at least "okay".

However, I wonder how much performance I'm losing and how down my compression really is from stock?

Should I continue to mess with this motor, or save up and buy a new bottom end and new heads?

What do you guys have for cylinder compression? What was it stock?

Input is appreciated.

 

[SeventyMach1]

I have 135-140 on all cylinders ..... but I have a 306.


I don't think it really matters what it is .... as long as all cylinders are with a margin. Your #8 looks a little low ...... but if that 75% rule is right, I guess you're fine.

 

[Swarzkopf]

A pretty fresh 306, SeventyMach1?

 

[HISSIN50]

Shawn, warm that motor up and try again. Make sure the ignition is disabled and remove all plugs and prop the throttle blade open to WOT before testing.

Do a dry and then wet test for each cylinder. This should all be a lot more revealing (having a hot motor is pretty important, though not quite as important as if you run forged slugs).

Good luck.

 

[Stang D]

Shawn

Definitely heed Hissin50 advice. You should always test warm, with the throttle propped open, and a strong battery. Pull all spark plugs out, and then test. Be careful who you are comparing compression data with. Even a small difference in cam timing can make a pretty substantial difference in compression at cranking speed. You need somebody with the same cam, and rocker ratio. I too would recommend a wet test on #8. If it fails to improve, you have a valve not sealing too well. The standard guidance is that if it improves, your rings are bad. Be careful with that though. I can almost guarantee every cylinder will noticeably improve if you test wet. It means very little IMO, as a brand new engine will improve if you dump oil in a cylinder.

Numbers seem a little low if you have a stockish cam. But I agree with your assessment that you are "OK" at this time. 15% is the number I have always used. If I ever had that engine apart, it would get a hone and some rings at a minimum. It may run just fine for years as is though.

 

[Swarzkopf]

Thanks guys...I appreciate the advice.

I'll retest on a warm engine with the throttle held open and let you know what I get.

 

[Topdowncruise]

170 to 180 on my fresh rebuild.It was 145 to 155 prior to the rebuild.

 

[Swarzkopf]

Well, I did another compression check yesterday, this time with three changes:

1. The engine was at normal operating emperature.
2. The throttle was held wide open during cranking.
3. My battery charger was connected to the batter to maintain the same cranking RPM each time.

The results were WAY different. Cylinder #8 went from 112 when tested cold with the throttle closed up to 135 tested the correct way. It was still the lowest, but it was closer, with all of the others testing between 139-144.

Thanks for all of the help! Yet another example of why you guys rock.

 

[HISSIN50]

The closeness of the values sounds nice to me.

Glad to hear things look a little better for ya.

If #8's value went up with the addition of some oil, the ring seal is not all that it once was. But so long as the number went up, it should perform decently.

 

[Swarzkopf]

I was kind of pressed for time, so I didn't do a wet test, Hissin. I figured that since it was 93% of the value of the highest cylinder dry, it was fine...

LOL

But now you've got me wondering again.

Being that all my cylinders now seem to fall into the "normal" range for 5.0s, I'll probably continue with the work I have planned and wet test it once everything is back together.

Thanks again.

 

[Stang D]

Shawn

That sounds like real good news. Glad it worked out. You can do a wet test. I guarantee all your cylinders will go up, thereby proving your engine isn't brand new. You already knew that I bet? Provided you got reasonable oil pressure, I'd mod away if you so desire.

 

[Swarzkopf]

Thanks Stang D. I figure that I've got at least one more good year left on this bottom end. Next year I should be in much better shape financially to buy a new short block.