I have a differential compression tester and know how to do a leakdown
test on an airplane engine - but not on a an automotive engine. Similar
in principle I'm sure - but not familiar with the details.

On an airplane, you turn the propeller until the piston in the cylinder
in question is at top dead center on the compression stroke, then lock
the propeller in place (holding it tightly, then attach the differential
compression tester, hook up the compressed air to to it, admit 120 psi
pressure from the compressor - and watch the two gauges. When both are
equal - turn OFF the gauge valve on the cylinder side and watch for any
pressure drop. Below 90psi is unacceptable - time for top overhaul. Or -
if all cylinders are being checked.....If pressure differential between
any two cylinders is greater than 10 PSI - time to consider a top overhaul.

My Question is - on an automobile, how would you set up the engine for
the leak-down test? Never done one myself on a car.

Thanks,

John