How to Compression test on bare engine

[NHCharger]

I'm at the point on my current project where I need to start installing the wiring in the car. Supposedly the car ran fine when I bought it but I never had it running since during shipping a positive wire got pinched against the firewall and I never found it until I took all the wiring off the car. Was going to replace it all anyway. I would like to check the compression if possible before I install all the new wiring and electronics- converting to Firecore RTR system.

Engine and tranny still in car. Has not run for two years. I Can remove distributor and prime the engine. Then use a remote starter to spin the engine? If so what wiring would I need to install besides the starter relay and battery connections?

[armor64]

Are you able to spin it over with a breaker bar or ratchet? I've done it with on my old GMC with no spark plugs, a pressure tester on each cylinder (1 at a time) and write down the readings

[NHCharger]

Yes.  I can spin motor with a ratchet. based on what I read when I searched the forum it doesn't sound like you can spin the motor over fast enough by hand for a compression test.

[c00nhunterjoe]

In theory, you dont need any wiring. Make sure carb plate is wide open and you can use a jump box to power the starter. Ideally, the 1st rotation should be a minimum of 90 psi for healthy ring seal, then they should all be within a few psi of each other.

[John_Kunkel]

Leak down tester. Available from Harbor Freight.

[BSB67]

Why do you want to do a compression test?

[NHCharger]

Have never heard engine run. Figured I would try to see if there was a potential problem before I wired everything up. do not trust everything seller said.

[BSB67]

Okay.

Just consider what you will do with the info, and what numbers make a difference, and what you will do about it, and if it really matters if you find out today, or after you get the motor running.

There might be an argument to simply get the thing running, Get that behind you, then take a closer look at the motor, verse throwing more time and money at it and introducing more variables into trying to get the thing running later.  Just a thought.

[firefighter3931]

The proper way to perform a compression test is to bring it up to operating temp then pull the plugs and test away. I'd be suspect of cranking numbers achieved on a cold engine that hasn't run in a few years. 


Ron

[NHCharger]

I'm just thinking it might be worth my time to at least make sure I don't have a cylinder that's showing half of what the others are before I spend anytime putting everything back together. I know a compression test on a stone cold engine is not very accurate

[BSB67]

If you are just looking for a potentially bad cylinder, do a leak down test as John suggested.

[NHCharger]

Leak down test kit at HF. $39.95   
Results from the leak down test 
Trying to find TDC with the engine in the car by yourself was difficult. So I put the wife to work. After four cylinders tested poorly I called my engine builder for advice. After a brief discussion and 90 minutes later she's out of the car. I had a feeling this was going to happen.

[67tbird]

Leak down test kit at HF. $39.95   
Results from the leak down test 
Trying to find TDC with the engine in the car by yourself was difficult. So I put the wife to work. After four cylinders tested poorly I called my engine builder for advice. After a brief discussion and 90 minutes later she's out of the car. I had a feeling this was going to happen.

when you said She's out of car, I thought you meant the Wife gotta chuckle out of that.

[sccachallenger]

The proper way to perform a compression test is to bring it up to operating temp then pull the plugs and test away. I'd be suspect of cranking numbers achieved on a cold engine that hasn't run in a few years. 


Ron

Yep, ran a compression test on a '70 440 in a newly purchased '72 Road Runner.
Previous owner never started the engine. 3 cylinders were very low, after running through several heat cycles all three were back to normal.
What a relief! Don't be too discouraged if the cranking numbers don't look good.