Engine compression test

[Tom Getz]

I need to do a compression test on my '23 Buick Model 45.  Do I warm up the engine, remove the plugs and do the test using the starter with the ignition off?  I have the equipment, but need to know what compression to expect.  Any and all advice would be appreciated.

[Morgan Wright]

You warm up the engine, remove all the plugs, and do the test with the starter and .... you need the ignition on..... but disconnect the spark by taking off the coil wire. Disconnect the fuel line too.

[Grimy]

You have a pedal (kick) starter which does NOT require the ignition to be on.  I'd leave ignition OFF.  After the engine is warm and still running, I'd shut off the vacuum tank and let the carb run out of gas--which will avoid crankcase dilution during the compression test. That's the 1923 equivalent of disconnecting a fuel line.  :-)

 

If you know the compression ratio (probably about 4.5), multiply that by 14.7 psi ambient pressure at sea level (will be less at higher altitudes) to get optimal compression pressure of 66 psi for those values.

[ROD W]

My 1920  six cylinder engines have a compression of around the late 50,s  one cylinder was 55,  a bit of oil squirted in brought that compression up a bit.   The other engine is in the middle 60,s.    

[Brian_Heil]

My 'modern' compression gauge did not come with an adapter for our older spark plug thread size.  I used an old spark plug that I drilled and tapped as a bushing.

[dibarlaw]

Brian: I did about the same. I had on old 2 piece Champion plug with the replaceable electrode. One of my compression tester adapters fit that thread. 

[Larry Schramm]

I did the same thing as Brian did.  Used an "O" ring for a seal.

[Morgan Wright]

On 11/30/2016 at 8:22 PM, Grimy said:

You have a pedal (kick) starter which does NOT require the ignition to be on.

 

A pedal starter requires the ignition to be on

 

[Grimy]

3 hours ago, Morgan Wright said:

A pedal starter requires the ignition to be on

Not on my three cars 1922-30 equipped with pedal starters.

[Grimy]

Morgan, perhaps you're thinking of the Buick accelerator pedal start from early 1930s thru 1960 or so--in that case you're correct. The OP has a 1920s car with a separate kick pedal which forces engagement of the starter and completes electrical contact to the starter, and that's what I'm talking about.

[Morgan Wright]

Good to know

[Thriller]

On November 30, 2016 at 7:22 PM, Grimy said:

I'd shut off the vacuum tank and let the carb run out of gas--which will avoid crankcase dilution during the compression test. That's the 1923 equivalent of disconnecting a fuel line.  :-)

 

Shut off the vacuum tank? I'm not as familiar with thes as I'd like and I haven't touched mine in three years, but mine has a vacuum / vacuum tank issue, so if there is something that can be turned on / off, that's something I should check. 

 

[Grimy]

Every vacuum tank I've seen in service has a brass shut off valve at the bottom of the tank in the connection to the gravity line to the carburetor.  Some with a sediment bowl at the bottom have the shut off valve incorporated into the sediment bowl.  My rule of thumb is to shut the valve if I'll be away from the car for ~20 minutes.

 

I think a Stewart Warner vacuum tank manual was attached as a pdf (or series of pdfs) to a post on this site within the last year.  Or Google "Stewart Warner vacuum tank,"  Somewhere I have an e-copy (pdf) of an article I wrote on the care and feeding of vac tanks for a local club publication.