1931 355A compression

[Thomas B.]

What should the compression readings be on a V-8 355A?  T. Brown

[TJ Hopland]

Are you having a problem or just doing a general health check?

A compression test is somewhat crude when it comes to coming up with a finite number.   There are a lot of variables that will effect the reading you get.   Most of the time when we are doing this at home what we are really looking for is the difference between cylinders.   Presumably the conditions and procedure will be the same between all cylinders so your numbers should be a good comparison between cylinders but may not compare to another engine using different tools with a different mechanic.   

Even the cranking speed makes a difference so keep an eye on the battery charge condition while you are testing especially if you do get some odd readings.   Come up with a procedure like test one cylinder then charge for 15 mins then test another.   You may not need 15 minutes, I'm just throwing out a number.     

   

[Thomas B.]

Just a health check.  All the readings were within 5 or 10 of each other. The lowest was 60 and the highest was 75.  Just was wondering what seemed to be normal.  She spun over pretty good.  T Brown.

[31caddy]

I got about 85 out of mine, but it has been overhauled. The compression ratio on a 355A ( 353 cu in engine) is only 5.35 :1. Your readings are decent - the 60 psi one is getting weak. When you do the test, get the engine to operating temperature and then remove all of the spark plugs. When cranking for the test, hold the throttle wide open with your heel as you depress the starter button, or better yet open it wide open using the throttle in the center of the steering wheel, if it is operable. This ensures that the maximum air charge possible is drawn into the cylinder, and it will raise the reading. Attached is a chart from an old Dykes manual showing "Nominal" compression numbers for various cars. it shows 79 for your 353 engine.

[Thomas B.]

Thank You for your input.  Very much appreciated.  T. Brown