Low compression readings

[Cooper40]

1949 Plymouth sitting for probably 30 years or so.

Tested for compression across the cylinders. Got below 30 for all of them. All the valves are moving up and down. We wire wheeled the underside of the valves. Also replaced the head gasket. The readings are still the same after all this. Put some ATF in the cylinders and tried again. Reading for 1 was 15 dry and then with ATF it was 35 or so. The others went up too but none got to 50 or above. All still very low. Would this mean stuck rings because it was locked up and sitting so long? Any tips? We have it soaking in ATF overnight. 

It is also cranking very slow. The starter sounds pretty bad. Going to take it out and rebuild it but could the slow starter be affecting the low readings?

[keithb7]

The engine does need to crank faster than "very slow" to get at least half decent compression readings. The carb needs to throttle wide open too. Remove all spark plugs when testing compression. Brushing away carbon from valve seats on a high mile engine, won't do much to help the valves seal.  When a cylinder goes from 15 to 35 after you add oil, that tells me you may have piston ring issues or cylinder wall scoring issues.

Edited October 3, 2023 by keithb7

[Cooper40]

@keithb7 Thank you for the advice! I totally forgot about the throttle being open lol. And I did not know about the plugs being out. I will do that as well. Anything else i could do to the valves with them still in the engine to make them seat better or will I have to pull them out?

[soth122003]

Since the compression went up when you did the wet check, Keep them wet and let them soak while you rebuild the starter. When I did mine when I first got my P-15 the wet readings were about 45-60. The rings, if just stuck usually need to have the engine run for a bit to get the heat and lube to loosen them up. My readings now are at 115 across the board, but my car didn't sit for 30+ years either. Try keeping them soaked and hopefully they loosen up in time.

 

Joe Lee

[Cooper40]

@soth122003 Yeah that is my plan. Keeping them soaking while we rebuild the starter. Thanks.

[vintage6t]

I'm not sure I'd jump into rebuilding the starter yet. As mentioned above take the plugs out and see if it cranks faster. With plugs in and no fuel or spark it's going to crank slow.

 

Also make sure you have the proper size 6 volt batter cable and not a 12 volt cable. A 12 volt cable is not going to carry enough current to the starter. Also make sure your engine is well grounded.

 

As far as soaking the rings the longer you soak them the better. IMO overnight is probably the minimum amount of time. Everyone has thier own favorite soaking mixture. I like a mix of ATF, diesel and a bit of acetone. The idea of the acetone is it thins the mixture so it will flow into tight spaces but then it quickly evaporates. It's also a powerful solvent.

 

After soaking and if you can get it started you may find the compression jumps up after running for a while.

[D35 Torpedo]

You can always put 12 volts to the starter to speed things up.

[dpollo]

If you can, push the car and start it in second gear.

[Booger]

what are good compression readings?  I always assumed any thing over 100 was fine and that all the cylinders be 10-15 #s within each other. (cant find squat in the bible) for a 216

Edited October 3, 2023 by Booger
spel

[keithb7]

My fresh rebuild 237 ci engine initially netted me 105 psi at 2300 ft above sea level.  Elevation makes a difference. 1000 miles later the engine made between 107-110 psi. 

[Los_Control]

24 minutes ago, Booger said:

what are good compression readings?  I always assumed any thing over 100 was fine and that all the cylinders be 10-15 #s within each other. (cant find squat in the bible) for a 216

I think it depends on circumstances, if the engine has been sitting for awhile & want to start it up .... Anything over 60psi should fire.

So If you have a 6 cylinder engine that has over 60 psi on 4 cylinders and a few others are less then that .... It should run kinda ok on 4 cylinders and as it warms up and things get oiled ..... they all should loosen up and start improving compression.

 

In my mind all the cylinders being close to each other, means all the cylinders are worn equally and behaving normal.

Low compression across the board means a worn out engine and needs refreshed.

 

If you have decent compression on some cylinders but really low on others, those individual cylinders have issue and needs investigated. Could be valve adjustment or worn valves ... just have to figure out whats going on.

 

I know some here have pretty low compression across the board, around 70-75? ... their engines run fine .... little short on horse power but running good.

[Sniper]

valve adjustment will affect compression readings too.  My 51 had around 50psi per hole when I got it, 4 years later it's still at 50.  I need to adjust my valves

Edited October 4, 2023 by Sniper

[soth122003]

13 hours ago, Booger said:

what are good compression readings?  I always assumed any thing over 100 was fine and that all the cylinders be 10-15 #s within each other. (cant find squat in the bible) for a 216

Most of the books I've read do not list a compression reading like the gage. (95 psi, 100 psi, etc.) The books list a compression number. In you case Booger, it lists as follows from the Motor Repair Auto Manual 1935-53.

1949 Special deluxe 6 P-18.

 

Bore and stroke  3 1/4 x 4 3/8

Piston displacement cubic inches  217.8

Compression ration   7.00

Max brake HP @ Rpm 97 @ 3600

Max torque lbs. ft.  @ RPM 175 @ 1200

 

Now there is a mathematical formula to get the 95 or 100 PSI, but I can't remember what it is. Using the numbers above you can find out if you so desire.

 

Joe Lee

[Sniper]

7 to 1 compression ratio. Atmospheric pressure is 14.7 PSI. Rounding up to 15 * 7 is 105 PSI.

[soth122003]

Thanks Sniper. I knew it was simple formula, but I could never remember it when I wanted to figure it out. Always had to Google it, then felt kinda dumb at the simplicity of the formula. 

They say you loose 2 things when you get older. 

1. Your health.

2, I forget what the 2nd thing is. (LOL)

 

Joe Lee

[SteveR]

28 minutes ago, soth122003 said:

They say you loose 2 things when you get older. 

1. Your health.

2, I forget what the 2nd thing is. (LOL)

2, Your youth!

and your hair stops growing out of your head in favor of your nose and ears. LOL

[Booger]

alrighty then..100 is ideal. Im just gonna throw this out there. I use autolite 306s like many do. Up here in corn country they seem to be getting scarce. Not sure why. But if you favor them have a six pack in reserve. Sorry no Champions (they usta be good)