In a message dated 98-04-28 09:21:43 EDT, blosskf@APCI.COM writes:

<< I've searched the archives about compression testing, but haven't
 found the answers I was looking for:

 Why the open throttle requirement?  Both the Sears compression tester
 I bought and Bentley say full open throttle.  What's the deal? >>

Karl--

An open throttle allows the pistons to suck in a full "gulp" of air. With the
throttle closed, a vacuume is formed betweenthe throttle plate and the
cylinders, negating some portion of the compression that one might otherwise
expect.

A couple of other things that are good practice for comp. testing:

1) For safety reasons, disconnect the fuel management system so you AREN'T
injecting fuel during this test. Different systems have different ways, or
unplug each injector.

2) Before removing the spark plugs, loosen them and unscrew them one full, but
no more. With the distributor main lead grounded and the injectors
disconnected, crank the engine over for 10 seconds with the throttle wide
open. This will dislodge and blow away some great portion of the crud that
accumulates around the plug holes.

3) Test the motor warm, then allow it to cool, and test again. I don't have a
rational behind this. But I keep track of the differance just because I am
rediculously meticulous and think that I'll learn something. I haven't yet,
though. :)

gmbulley
cary, nc