Is 140 - 150 PSI good for a compression test of a 318 LA?

[MoPar~Man]

Back in '86 when I was restoring my dad's '67 Monaco (Canadian car with poly 318) we replaced the engine with a re-man engine from a local Chrysler dealer. I've only put about 10k miles on the car since then. At the time, my dad said the new engine gave the car more pep then he ever remembered the original having. The 10k miles that I put on the car all happened in the 10 years after that, basically no miles since about '93 or so, and not driven or started at all between '99 to 2022.

Since about a year ago I've burned about 7 gallons of gas either idling the car or driving it short distances in the city.

After warming it up last night, I took the plugs out and did a compression test. Three cylinders were showing 140 - 142, three were showing 150, two showing 145. Using about 4 cranks for each cylinder, first crank always gives 100 or more PSI.

Then I slapped my forehead after realizing the throttle was closed. I jammed it open and repeated the test on a few cylinders but it only seemed to add 2 to 4 PSI to the numbers - ?

So my questions are - are these numbers ok, and should there be more of a difference between closed throttle vs WOT?

 

[kmccabe56]

I'd say you have a perfectly healthy engine, especially given that it sat unused and unprotected for 23 years.

 

[sprice]

I believe a rule of thinking is the variance should be within 10%, your good.

 

[commando1]

I'd be thrilled with those numbers.

 

[413]

All sounds great, and it didn't need the help of the open carburetor to get the compression up.

Note: please don’t let your classic car idle for extended periods, drive it or don’t start it at all. It will shorten the life of the exhaust system. You can’t get the moisture out idling in the driveway.

 

[CBODY67]

It takes a good 10 miles of driving for the oil temp to approach coolant temperature, Then it takes more time for the oil to cook-out the various condensates in the oil. PLUS, seems that LA motors run cooler than B/RB engines, so you can factor things up from there. ALSO, that "10 miles" is in weather with an 80 degrees F ambient temperature.

Similar with the automatic transmission fluid. It gets up to temp quicker, though.

Then there is the condensate which happens in the exhaust pipes and muffler. When I'd see some of the late-model Toyotas in the pre-delivery inspection activities, after they'd been idling in the shop and the techs drove them outside to put some gas in them, as soon as they'd hit the throttle to head to the new car parking area, they'd spit out about 8 oz of water out of the exhaust pipe.

Think - - - hot gas meets cold pipes (or internal crankcase walls) and makes "rain", just like in the weather realm of things, rain stops, then humidity happens when all of the moisture evaporates in the hot sun. There's a reason the factory maintenance schedule lists "Short trips of 10 miles or less" as "Severe Use" and requires shorter oil change intervals.

The mentioned idle time is better than nothing, as to keeping the battery charged and engine internals coated with oil. Just change the oil much more often.

Enjoy!
CBODY67

 

[CBODY67]

Back in '86 when I was restoring my dad's '67 Monaco (Canadian car with poly 318) we replaced the engine with a re-man engine from a local Chrysler dealer. I've only put about 10k miles on the car since then. At the time, my dad said the new engine gave the car more pep then he ever remembered the original having. The 10k miles that I put on the car all happened in the 10 years after that, basically no miles since about '93 or so, and not driven or started at all between '99 to 2022.

Since about a year ago I've burned about 7 gallons of gas either idling the car or driving it short distances in the city.

After warming it up last night, I took the plugs out and did a compression test. Three cylinders were showing 140 - 142, three were showing 150, two showing 145. Using about 4 cranks for each cylinder, first crank always gives 100 or more PSI.

Then I slapped my forehead after realizing the throttle was closed. I jammed it open and repeated the test on a few cylinders but it only seemed to add 2 to 4 PSI to the numbers - ?

So my questions are - are these numbers ok, and should there be more of a difference between closed throttle vs WOT?

As to the compression test results . . . the compression test can be good, but it's as much about the balance between the cylinders as it is about the numbers. Considering that later 8.2 CR B/RB motors spec compression test numbers were just over 100psi at WOT, your results sound very good.

Rather than worry about how far open the carb throttle might be, squirt some engine oil into the cyl and see how much the compression might rise. The lack of rise means things are still good and not too worn.

There are other ways to check compression. One is to unplug a plug wire at a time (with the enging idling) and check the rpm drops. Weak cyl = less rpm drop, and vice versa. Just MAKE SURE your body is well-electrically-insulated from the car body!

Enjoy!
CBODY67