How many psi for compression test 318)
- Thread starter Rosetti
- Start date
from the 1970 FSM
It's not as much how high the compression, but more importantly how even they are.
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What is the concern that a compression test might help to diagnose? Is the engine missing? Is it not running smoothly? Are the spark plugs fouling?
Some people like to look at "the numbers", but those "numbers" can vary depending upon the altitude the compression test is done at. The factory numbers are stated to be at sea level, which a lot of the country is not, fwiw.
Please advise,
CBODY67
Some people like to look at "the numbers", but those "numbers" can vary depending upon the altitude the compression test is done at. The factory numbers are stated to be at sea level, which a lot of the country is not, fwiw.
Please advise,
CBODY67
Is it the original engine for the car? Do you know how to identify a polysphere vs LA 318? You haven't said much about your experience or history with this car - have you had the engine running, or has the engine run at some point in the past week, month or year?
To prepare for a compression test, you need to have the battery fully charged. If the battery is weak or old you may need to charge it up during the test. The battery needs to turn the engine crank at a consistent speed for all cylinders. Hold the throttle plate open and stick a screwdriver down the carb to hold it open during the test. Disconnect one of the wires at the spark coil (one of the the low voltage wires). Don't let the loose wire touch anything, tape it up if necessary. Remember that all the spark plugs come out first before you start testing.
If the engine has been run in the past week then I think you can do a cold test without spraying any oil into the cylinders. I've gotten good numbers on my 318 (LA block) doing a cold test. You only want to do 4 cranks, you'll see the needle jump each time, with not much difference between the 3'rd and 4'th crank. The 3'rd crank should get you over 100 psi. If you have 130+ PSI on 4 cranks, call it a good result. The absolute best you could see is 150 to 160.
When connecting the compression-tester hose, just screw it in and finger-tighten it, don't use any tools. It should have a big o-ring on the threads to seal it.
To prepare for a compression test, you need to have the battery fully charged. If the battery is weak or old you may need to charge it up during the test. The battery needs to turn the engine crank at a consistent speed for all cylinders. Hold the throttle plate open and stick a screwdriver down the carb to hold it open during the test. Disconnect one of the wires at the spark coil (one of the the low voltage wires). Don't let the loose wire touch anything, tape it up if necessary. Remember that all the spark plugs come out first before you start testing.
If the engine has been run in the past week then I think you can do a cold test without spraying any oil into the cylinders. I've gotten good numbers on my 318 (LA block) doing a cold test. You only want to do 4 cranks, you'll see the needle jump each time, with not much difference between the 3'rd and 4'th crank. The 3'rd crank should get you over 100 psi. If you have 130+ PSI on 4 cranks, call it a good result. The absolute best you could see is 150 to 160.
When connecting the compression-tester hose, just screw it in and finger-tighten it, don't use any tools. It should have a big o-ring on the threads to seal it.
It's a Canadian car. Original engine. It runs and drives (however, not very well, due to the fact that it was on a farm for the past 12-15 years or so). It was running about 2 months ago. Also, I don't know how long that gas has been in there. Before I replace/ rebuild the engine, I'd like to see how she runs with newer gas, and after a compression test.
I am not a mechanic, but it comes to these kinds of things, I'd like to be as knowledgeable as possible. For example, I did not know that the battery had to be fresh, and I thank you very much for letting me know that part.
Just so you know, I think this forum is awesome, and everyone on here is just wonderful. Thank you so much for all the help, and all of the details you've given me. If there's anything else you can think of, please let me know, I would GREATLY appreciate it.
I am not a mechanic, but it comes to these kinds of things, I'd like to be as knowledgeable as possible. For example, I did not know that the battery had to be fresh, and I thank you very much for letting me know that part.
Just so you know, I think this forum is awesome, and everyone on here is just wonderful. Thank you so much for all the help, and all of the details you've given me. If there's anything else you can think of, please let me know, I would GREATLY appreciate it.
In addition, pull all the spark plugs. You are testing one cylinder at a time and you don't need the drag of the other 7.
I can't just do it one at a time? Is there a benefit to pulling all of them at once?
Do the valve covers look like this? If so you have a Poly 318 and not a LA. It doesn't matter really. Just curious.
Maybe... They're pretty rough. I know that the distributor is at the back, against the firewall (if that makes a difference).
Having the polysphere in the car will tell you (and us) that this is almost certainly the original engine. If the lower edge of the valve cover is wavy then it's a polysphere.
Yes you take out all the plugs first, then screw the tester into each spark plug hole. Then when you're done you put all the plugs back in. You want the engine crank to spin at a good clip during the test, and to not drain the battery while you're cranking. The engine will turn very freely without plugs.
It's too late now, but you should have fully drained that old gas out before running the engine. And you should have changed the oil (you didn't say if you did or didn't).
Yes you take out all the plugs first, then screw the tester into each spark plug hole. Then when you're done you put all the plugs back in. You want the engine crank to spin at a good clip during the test, and to not drain the battery while you're cranking. The engine will turn very freely without plugs.
It's too late now, but you should have fully drained that old gas out before running the engine. And you should have changed the oil (you didn't say if you did or didn't).
I will check the valve covers sometime today or tomorrow, and get back to you. Thank you again for all of the support!
The Poly valve covers have the scallop at the bottom. Obvious for sure. The only Mopar ones like that.
A compression test at this point in time might not be the best way to determine the engine's condition. As noted in a recent thread about resurrecting a 1957 or 1958 Plymouth proved. After an engine has been inactive for many years, it will take some time for things (like the rings in their grooves on the pistons) to free back up and seal decently good again.
As an old Chrysler service manager told me years ago, that a compression test will only check the seal on the top compression rings and little else. Other than a burnt valve, which would be obvious when the engine is running.
Not everybody has the good fortune of resurrecting an engine after a lot of years and it suddenly runs just like it was just parked. The main thing is that it runs smooth and the exhaust flow out of the tail pipe is similarly smooooth.
Put some fresh motor oil and a new oil filter in it. Get the old gas out of the tank and replace it with fresh gas, too. With the spark plugs out, you can look at them to see what their electrodes look like, as to deposits and color thereof, plus the ceramic color. You can run a gap gauge through them to verify the plug gap at this time.
When you gap the plugs with a bent-wire gauge, it will not only check the gaps but also clean the contact areas so the spark can jump easier. So of the .035" wire is loose, go up to the .040" wire and check things, which will probably take some effort. Once that has been done, then set the gap to a "loose" .035" and you're done with that part of things.
With the new gas and oil, run the engine for an hour or so at rpm levels between idle and about 2000rpm. THEN, it should have enough run time to get a good compression test. Plus help get the battery more toward "full charge", so it will take less time on the battery charger.
The NICE thing I like about LA and Poly-A motors is spark plug location and accessibility.
Keep us posted on your progress, please,
CBODY67
As an old Chrysler service manager told me years ago, that a compression test will only check the seal on the top compression rings and little else. Other than a burnt valve, which would be obvious when the engine is running.
Not everybody has the good fortune of resurrecting an engine after a lot of years and it suddenly runs just like it was just parked. The main thing is that it runs smooth and the exhaust flow out of the tail pipe is similarly smooooth.
Put some fresh motor oil and a new oil filter in it. Get the old gas out of the tank and replace it with fresh gas, too. With the spark plugs out, you can look at them to see what their electrodes look like, as to deposits and color thereof, plus the ceramic color. You can run a gap gauge through them to verify the plug gap at this time.
When you gap the plugs with a bent-wire gauge, it will not only check the gaps but also clean the contact areas so the spark can jump easier. So of the .035" wire is loose, go up to the .040" wire and check things, which will probably take some effort. Once that has been done, then set the gap to a "loose" .035" and you're done with that part of things.
With the new gas and oil, run the engine for an hour or so at rpm levels between idle and about 2000rpm. THEN, it should have enough run time to get a good compression test. Plus help get the battery more toward "full charge", so it will take less time on the battery charger.
The NICE thing I like about LA and Poly-A motors is spark plug location and accessibility.
Keep us posted on your progress, please,
CBODY67
Jakter
Active Member
Don't be in a hurry to rebuild/replace the engine.
Get it running and put several thousand miles on it at all speeds then assess the condition of the engine.
I'd love to be able to test it for a few thousand miles, but, it's winter time here, and I don't know if I'm going to be able to be driving around that long. The idea was, the restore this guy over the winter, then drive it in summer. Anything else I can try?
An awful lot of discussion over what engine... It really doesn't matter.
Just do the compression test and then worry about the numbers. All the plugs out, warm engine (if possible) with the choke and throttle open. If you have large variances between the cylinders, that's a problem. If it's low, let's say under 100PSI, squirt some oil in the cylinders and see if it comes up.
It's really that simple... Don't overthink it or let someone try to make you overthink it.
Just do the compression test and then worry about the numbers. All the plugs out, warm engine (if possible) with the choke and throttle open. If you have large variances between the cylinders, that's a problem. If it's low, let's say under 100PSI, squirt some oil in the cylinders and see if it comes up.
It's really that simple... Don't overthink it or let someone try to make you overthink it.
And grab a service manual from MyMopar
Jakter
Active Member
How many miles are on it?
This is going to be a bit of a downer post, and being interested in the shape of the engine is natural, but if the car is new to you, and this being an Ontario Canada car, you really should at least have a look at the condition of the frame, check for rust, CHECK THE BRAKE LINES and the condition of various front end suspension joints, and then take each wheel off one at a time and have a really good look at the brakes. If the brake shoes are old, doesn't matter how much material is on them, they can break free of the shoe plate on a hard stop.
Unless you know the car is safe to drive, those things are more important than an engine that runs.
For the Americans reading this, unless the car changes hands (changes ownership) there's no requirement to EVER have a car safetied in Ontario. It might even be the case that an ownership change from a parent to a daughter/son might also not require a safety certification. A car can be in quite bad shape and still be legally drivable (ie have legal license plate). Insurance companies won't and don't give a hoot either.
Unless you know the car is safe to drive, those things are more important than an engine that runs.
For the Americans reading this, unless the car changes hands (changes ownership) there's no requirement to EVER have a car safetied in Ontario. It might even be the case that an ownership change from a parent to a daughter/son might also not require a safety certification. A car can be in quite bad shape and still be legally drivable (ie have legal license plate). Insurance companies won't and don't give a hoot either.
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