Compression Test

[MotoArts]

Were ALL of the spark plugs out?

Carb butterflies tipped open when cranking?

How long did he crank each cylinder to test?

Did the mechanic do the test "dry" or "wet" (with oil)?

Squirting a shot of oil into the spark plug hole can help to determine ring or valve sealing potential.

Ask lots of questions..........

 

[f117rt]

Well, I have found an Engine rebuilder.

Now I need suggestions on what cam, flat or roller lifters?

Adding Edelbrock performer 4v intake with 600cfm carb.

Anything else I should ask for?

Try to get about 300+ HP.

Appreciate any suggestions.

 

[CZ-75]

I have been looking into the same thing. Here is my suggestion:

Compacts,Inc. Grind 260HR or grind 270HR.

With hydraulic roller lifters. If you download their free CamQuest software, you can ply with different settings. Then when you make your choice they detail all kits and mods necessary regarding valve train.

It will net you just over 300hp with a nice wide torque curve. Put on some wide tube headers and dual exhaust, and you hill have about 350 horses and 400 lbs of torque even if you stay at a low 8d:1 compression ratio


BTW , Edelbrock has a 2-3 week back log on their performer intake for our 2V 351c

 

[f117rt]

I have been looking into the same thing. Here is my suggestion:

Compacts,Inc. Grind 260HR or grind 270HR.

With hydraulic roller lifters. If you download their free CamQuest software, you can ply with different settings. Then when you make your choice they detail all kits and mods necessary regarding valve train.

It will net you just over 300hp with a nice wide torque curve. Put on some wide tube headers and dual exhaust, and you hill have about 350 horses and 400 lbs of torque even if you stay at a low 8d:1 compression ratio


BTW , Edelbrock has a 2-3 week back log on their performer intake for our 2V 351c

Thanks for the input.

Already have the intake in the garage, just need to get the carb.

Can you forward the link to the Cam software?

 

[CZ-75]

http://www.compcams.com/downloads/register.asp

My spellchecker wouldn't let me spell CompCams earlier


I'm going with Edelbrocks thunder series AVS 600 cfm carb

 

[ejnonamaker]

This is an old thread but once again found lots of great info. As an airplane mechanic I'd say he gave the original poster compression in percentages. jbojo covered that quite well. Thanks for the spreadsheets too. Those were awesome. I'm going to go back and do a wet compression test tomorrow. Might even toss all the plugs back in, let the engine warm up, and then do another compression check. Here's what I had today with a cold, dry engine with the throttle wide open.

http://www.7173mustangs.com/thread-our-73-mach1?page=6

 

[ejnonamaker]

So I redid my compression check by this time squirting about a teaspoon of engine oil into each of the spark plug holes. My PSI's came up an average of 20 psi from my dry check.

Here was the dry compression check numbers:

1 - 161psi

2- 129psi

3 - 126psi

4 - 147psi

5 - 131psi

6 - 124psi

7 - 129psi

8 - 145psi

I did a wet compression check and got these numbers today. By wet I mean I squirted about a teaspoon of engine oil into each spark plug hole.

1 - 180psi (Highest pressure still. Base for 100%)

2 - 145psi (19.4% lower)

3 - 155psi (13.9% lower)

4 - 160psi (11.1% lower)

5 - 150psi (16.7% lower)

6 - 143psi (20.6% lower)

7 - 148psi (17.8% lower)

8 - 164psi (8.9% lower)

It's the strangest spread of numbers I've ever seen on a compression check. Has anyone else ever seen something this far apart? What side of the fence do most of you lean on; a 10% spread, 8%, 20%? Looking at just cylinders 5 through 8 the worst on that side is almost 12% lower. The 1 to 4 side had me originally thinking head gasket until I started getting the rest of the readings. I've also never had any type of contamination though in my oil changes.

A rebuild is most likely definitely in the future. Just not this year. Body work, body work, body work. What else do you all think? Gaps all looked good from the rocker arm side of the heads. I haven't even looked at the timing chain on this thing yet. Piston rings? I guess what I'm looking for is, could I just do a few simple things without tearing this whole engine down or is it time?

As far as I can tell it should has about 120,000 miles (odometer reads 20,009) and nothing other than oil and air filter changes have ever been changed. The electric choke was disconnected from it's power source (either doesn't work or the wire is bad), the PCV and emissions were disconnected, and the power brakes were running off the carb vacuum instead of the manifold. I know now of that effects the compression check. I do wonder if the PCV was disconnected so the power brakes could run off that vacuum port vs. the manifold because the pressure is bad on the manifold. Which then makes me think about how the engine would run with just new gaskets.

 

[barnett468]

So I redid my compression check by this time squirting about a teaspoon of engine oil into each of the spark plug holes. My PSI's came up an average of 20 psi from my dry check.

Here was the dry compression check numbers:

1 - 161psi

2- 129psi

3 - 126psi

4 - 147psi

5 - 131psi

6 - 124psi

7 - 129psi

8 - 145psi

I did a wet compression check and got these numbers today. By wet I mean I squirted about a teaspoon of engine oil into each spark plug hole.

1 - 180psi (Highest pressure still. Base for 100%)

2 - 145psi (19.4% lower)

3 - 155psi (13.9% lower)

4 - 160psi (11.1% lower)

5 - 150psi (16.7% lower)

6 - 143psi (20.6% lower)

7 - 148psi (17.8% lower)

8 - 164psi (8.9% lower)

It's the strangest spread of numbers I've ever seen on a compression check. Has anyone else ever seen something this far apart? What side of the fence do most of you lean on; a 10% spread, 8%, 20%? Looking at just cylinders 5 through 8 the worst on that side is almost 12% lower. The 1 to 4 side had me originally thinking head gasket until I started getting the rest of the readings. I've also never had any type of contamination though in my oil changes.

A rebuild is most likely definitely in the future. Just not this year. Body work, body work, body work. What else do you all think? Gaps all looked good from the rocker arm side of the heads. I haven't even looked at the timing chain on this thing yet. Piston rings? I guess what I'm looking for is, could I just do a few simple things without tearing this whole engine down or is it time?

As far as I can tell it should has about 120,000 miles (odometer reads 20,009) and nothing other than oil and air filter changes have ever been changed. The electric choke was disconnected from it's power source (either doesn't work or the wire is bad), the PCV and emissions were disconnected, and the power brakes were running off the carb vacuum instead of the manifold. I know now of that effects the compression check. I do wonder if the PCV was disconnected so the power brakes could run off that vacuum port vs. the manifold because the pressure is bad on the manifold. Which then makes me think about how the engine would run with just new gaskets.

k, i'm a bit lost as to what your question is.

i looked a what i think was yur plugs on another thread and from what i can tell, it is NORT burning oil.

if it is not burning oil, there really is not much of a problem, therefore, i don't see a problem with getting another years use out of it base on what i can decipher from your info.

if the car runs poorly then obviously something is wrong but it certainly doesn't look like it would be caused by low compression or the wide compression spread between cylinders.

the manifold is for brakes . hook the brakes up correctly.

.

 

[Don C]

The vacuum at the PCV port below the carburetor and manifold vacuum should be about the same. The disconnected PCV will allow condensation and vapors to remain in the crankcase, gumming it up.

Did you run the compression test on a warm engine with all plugs removed and the carburetor propped open?

Have you checked for blow-by by seeing if fumes come out of the breather or PCV caps when you rev it?

Crud on the valves can intermittently keep the valves from fully closing.

I agree with Barnett, as long as you're not burning a lot of oil, it is generally running OK, and you have decent oil pressure there's no rush on rebuilding it and I'd keep driving it until problems start showing up. Connect the PCV (with a new valve) and connect the breather cap to the air cleaner or make sure the breather cap is filtered.

 

[ejnonamaker]

Thanks for the info. Have the PCV system hooked up correctly now. Haven't hooked the brakes up yet. The hose is there just haven't made the connection. As for oil. Maybe a 1/2 quart between oil changes. No fumes, oil contamination, runs just fine after she warms up. I just read alot of guys saying they have to be rebuilt if the compressions are more than 8-10% off. In the military we took them to 20%.

I was basically putting a feel out there to see what others thought. Do I run it until a problem arises or do I need to stop everything and yank it now. That was my subtle question.

Sent from my SAMSUNG-SM-N900A using Tapatalk

 

[barnett468]

I just read alot of guys saying they have to be rebuilt if the compressions are more than 8-10% off. In the military we took them to 20%.

nonsense, they only "have" to be rebuilt when they smoke so much you get a ticket or they simply expire.

drive it till it's dead if you need to, it won't make it much more worn out.

keep in mind, when a car engine fails, it does it a lot closer to the ground than a plane engine usually does, therefore, no parachute is required.

just make sure you have a AAA free tow card.

 

[73pony]

I just read alot of guys saying they have to be rebuilt if the compressions are more than 8-10% off. In the military we took them to 20%.

nonsense, they only "have" to be rebuilt when they smoke so much you get a ticket or they simply expire.

drive it till it's dead if you need to, it won't make it much more worn out.

keep in mind, when a car engine fails, it does it a lot closer to the ground than a plane engine usually does, therefore, no parachute is required.

just make sure you have a AAA free tow card.

Good analogy on the plane. I like it.

 

[Don C]

keep in mind, when a car engine fails, it does it a lot closer to the ground than a plane engine usually does, therefore, no parachute is required.

+1. When a car engine quits you say "oh crap, now what" and pull over to the side of the road. I've had a single engine airplane quit twice on separate occasions, both after take-off, once at about 500 feet above the ground, the other at over 1,000. It gets your attention! Too busy to say much, checking gauges, looking for landing spot, can I make it back to the runway, trying to restart, etc. Got it restarted both times...

 

[Jeff73Mach1]

If it runs okay, keep running it until it does not. A valve job often will improve the test numbers as improving the seal there helps the cylinder build pressure. Keep in mind a compression test does not simulate running conditions where much greater pressures are involved during the combustion process. You could do a leak down test, or you could just enjoy it as is, potentially for many years.

 

[barnett468]

keep in mind, when a car engine fails, it does it a lot closer to the ground than a plane engine usually does, therefore, no parachute is required.

+1. When a car engine quits you say "oh crap, now what" and pull over to the side of the road. I've had a single engine airplane quit twice on separate occasions, both after take-off, once at about 500 feet above the ground, the other at over 1,000.

ummm...NOT good, lol, however, I do believe the glide ratio of a 71 Mustang may be slightly less suited for this situation.

 

[ejnonamaker]

I think it's a 1:1 glide ratio. I'm use to atleast 11:1. 11:1 can get you that impossible 180 turn back to the runway and then some if the engine fails at 600' and above in a DV20. The DHC8 I flew sank link a rock.

Sent from my SAMSUNG-SM-N900A using Tapatalk

 

[Don C]

Yeah, any airplane that requires a lot of power on when landing have poor no power emergency landing characteristics. Many military aircraft don't even have emergency landing procedures, just attempt to restart to a certain AGL, and then eject.