<VV> Varying Compression Readings?

[Larry Forman]

At 12:51 PM 4/2/2005 -0500, N. Joseph Potts wrote:
>While I follow these habits as consistently as I can, I can't say
>they are "right."
>     consistency is more important than absolute accuracy.
>  That said, I COUNT the times I turn the engine over to
>get any cylinder's compression. My number is six. Five might be just as good
>(consistent), and seven, too.
>Miami, Florida USA
>1966 Corsa coupe 140hp 4-speed with A/C

Hi Grant and Joe,
I have been doing this right (or wrong!) also for many years.  I have 
watched it over the years and see that the first one reading is always 
lowest, then slightly higher on the second one, slowly increasing less 
until about 5 or 6 is about the max.  So I am with Joe and typically use 
either 5 or 6, depending upon when I catch it.  Always remember to run wide 
open throttle and MAKE DARN sure you replace the throttle return spring 
when restarting the engine.  If you ever forget, the fastest way to stop 
the engine it to pull the coil high tension lead, knowing that you might 
get electrically bit doing that, but you might save an engine that way.  It 
is also possible you might damage an electronic ignition, but most are 
protected sufficiently.  Still a lot less expensive and easier than 
rebuilding an engine.

I also like to run this test when the engine has been warmed up.  That way 
there is consistent oiling of the cylinders and the valves and lifters have 
had time to get warmed up and not leaked down with possible erroneous readings.

I have found running WOT versus non-WOT curious.  I have had some engines 
have higher compression (or less!) running non-WOT.  I still don't know 
what is with that, but find running WOT produces more consistent 
compression readings.

While I have advocated performing a cylinder compression test as THE first 
check on engine condition, I now prefer a cylinder leakdown test.  It 
provides more definitive data on just how well the engine is providing 
compression and where the leakage is going.  Consistency for this test is 
just as important.

HTH,

Larry