a minimum of 135. the highest and lowest should be within 15% of each other.
when you do the check make sure the engine is warm (if possible), remove ALL the spark plugs, throttle at WOT(Wide Open Throttle) and hit the compression stroke of each cylinder the same # of times (about 4 or 5).
the gauge has a valve in it so there is no time limit for holding pressure.
yea i jsut got sheet tellin me it shoud be between 134 and 250 max psi. i have a sheet that says it should be cranked at about 180rpm and should have no more than 75% variation between cylinders... do stangs typically have week top ends? id hate to buy a car and have to rebuild the top right away so im checkin it anyway