How is a compression test done when motor is out of car on the ground?

[440beep]

I'm trying to do a compression test on my 440 while the motor is on the ground. I've got the transmission hooked back up with the starter in place, and I've got a positive wire going from battery to starter positive. Have a negative wire from battery going to starter negative. Nothing happens. Starter doesn't engage.

Where is (are) my mistake(s) please?

 

[67belvedere/225]

you need jumper cables to the big terminal of starter and the negative on the engine ground
then you jump at the starter other smaller terminal with a wire from bat + to it and hold on it will crank

 

[440beep]

Yep, that worked!!

Here's something dumb too. I'm only getting like 100psi of compression. I'm guessing having open exhaust ports is a problem? So missing something here

 

[Garys1969RR]

That sounds low, especially at 1000 feet. I'm at 5200 feet and I was checking my 440 last night, readings were at 190 on # 1 and 200 PSI on #8. But I'm also right at 11 to 1 C/Ratio. Shoot some oil in there and see if it goes up. Also check several cylinders.

 

[440beep]

Yea, maybe I'm not doing it long enough. Motor is 10.3:1 compression and I'm at like 500' above.

 

[Garys1969RR]

Let it hit 3 or 4 times, with all the plugs out, throttle at least halfway open. Unless you have a very long duration cam with late closing of intake valve, you prob have worn out rings/motor. What cam do you have? Hyd or mechanical?

 

[440beep]

3 Carbs are off the intake, so the intake is open. All the cylinders are reading 100psi. The motor was rebuilt 2 years ago, so should still be fresh. The oil is out of the motor, so could that be affecting the lifters?

CAM INFO. HYD Roller
Intake Duration @50: 240
Exhaust Duration @50: 248
Advertised Intake Duration: 300
Advertised Exhaust Duration: 308
Intake Valve Lift: 0.54
Exhaust Valve Lift: 0.545
Lobe Separation: 112
Intake Centerline: 107

Let it hit 3 or 4 times, with all the plugs out, throttle at least halfway open. Unless you have a very long duration cam with late closing of intake valve, you prob have worn out rings/motor. What cam do you have? Hyd or mechanical?

 

[Fran Blacker]

I wouldn't spin it over with no oil even with the plugs out. Add some oil to crankcase. Then test a cylinder. Then squirt a little oil down the cylinder you just tested if the compression go up a lot (I don't know a number for that) I'd worry about the rings. Maybe somebody else can add to this. If the lifters aren't pumping up because of no oil adding it should help. 100 psi is low. I have cam close to the one above with 11 to 1 compression and was getting 215 psi.

 

[440beep]

Ok, filled up the motor with oil, and put some in cylinders, and now I'm getting 120psi +- across the board.

Don't know if I should he happy or sad, or time to figure out a different cam............

 

[Fran Blacker]

Put some compressed air in the cylinders to see if valves leak. You may have to pull intake? See if you can hear/feel air coming out of intake. Cylinder you check should be in firing position.

 

[440beep]

The motor's going to be rebuilt anyway. Wanted to do a compression check mainly to see if a different cam is in order this rebuild.

 

[Fran Blacker]

I know rollers aren't like flat tappet cams about mixing lifters But I'm a firm believer about putting them back where they came from.

 

[Fran Blacker]

I know rollers aren't like flat tappet cams about mixing lifters But I'm a firm believer about putting them back where they came from.

That cam probably needs 3.90's or 4.10's.

 

[440beep]

I'm not following? Do you mean for when this motor is taken apart and rebuilt?

And yes, the car is a 4:10 gear.

I know rollers aren't like flat tappet cams about mixing lifters But I'm a firm believer about putting them back where they came from.

 

[IQ52]

Lee, in my opinion your present cam shouldn't be under, say, 150 PSI at your elevation. If the lifters weren't fully pumped up with oil, the intake valve would be closing earlier and may even give you a higher compression reading than if they were fully pumped. The gain in cranking compression is most likely from the oil sealing the rings. Also, the faster you spin the engine with starter the more compression you will see. When we use the battery charger on the battery when cranking the engine we get about another 5psi gain. When the starter is pushing against an engine where the oil pump is building pressure it will turn over even slower.

 

[Garys1969RR]

The motor's going to be rebuilt anyway. Wanted to do a compression check mainly to see if a different cam is in order this rebuild.

Why are you re building if it was done 2 years ago? Did something happen like over heating? How did it run, or do you know?

 

[IQ52]

The motor's going to be rebuilt anyway. Wanted to do a compression check mainly to see if a different cam is in order this rebuild.

Going by cranking compression alone you would need to go with a smaller cam. No way would I use a smaller cam in your engine. I have a 426 small block setting here that runs 3.23 gears and uses a bigger roller cam than you have. Cranks 150+ psi @10.7:1 compression here at 5,000' elevation. You don't have the compression ratio you were told or you have rings/valves leaking.

 

[Fran Blacker]

With a flat tappet cam if you don't keep track of which lifter came out of what lifter bore you could wipe the cam. They wear in to each lobe, switching them can cause a problem. Roller cams, I understand don't have the problem, so I'm told. I drilled 2 rows of 8 holes in a block of wood and mark one end front of engine. As I removed the lifter I put them in the same place as if they were in the engine. You might not want to use wood with a hyd roller dirt and dust but the object is to put them back where they came before rebuild if your using the same cam.

 

[440beep]

Very good point you make. After adding the oil, I switched batteries and the motor was cranking faster than first times. Wonder if increase is purely based on fresher battery. So many damn variables which tells me I wasted my afternoon doing this. But at least I got a workout picking that 727 up and down getting into position.

Gary, the motor has had an oil leak since the first rebuild and I'm so sick of the everything being covered in oil!! Plus the bronze dizzy gear came apart last year so I want to have the bearing and "stuff" checked for damage, and just replacement.

Lee, in my opinion your present cam shouldn't be under, say, 150 PSI at your elevation. If the lifters weren't fully pumped up with oil, the intake valve would be closing earlier and may even give you a higher compression reading than if they were fully pumped. The gain in cranking compression is most likely from the oil sealing the rings. Also, the faster you spin the engine with starter the more compression you will see. When we use the battery charger on the battery when cranking the engine we get about another 5psi gain. When the starter is pushing against an engine where the oil pump is building pressure it will turn over even slower.

 

[IQ52]

Very good point you make. After adding the oil, I switched batteries and the motor was cranking faster than first times. Wonder if increase is purely based on fresher battery. So many damn variables which tells me I wasted my afternoon doing this. But at least I got a workout picking that 727 up and down getting into position.

Gary, the motor has had an oil leak since the first rebuild and I'm so sick of the everything being covered in oil!! Plus the bronze dizzy gear came apart last year so I want to have the bearing and "stuff" checked for damage, and just replacement.

You didn't waste your time, you are learning the whole way. It's like you never really flunk a test in school. Even if you don't get the grade you want or expect, you always learn something by taking the test. Next time it will be a better grade.