If that were 100psi also the cylinder would be perfect, which of course won't happen, it will always be lower.
The bigger the difference between the two gauges the faster the air is leaking out of the cylinder.
June 2014 After 25 years I've done a compression test on Bee prior to changing the head gasket due to combustion gases in the radiator.
goes up more than the others, that generally indicates worn bores or ring problems.
If a low cylinder doesn't increase from the dry to the wet that indicates valve problems, although this is generally accompanied by a regular beat in the exhaust or intake, indicating an exhaust or intake valve respectively.
Having the piston at TDC basically checks the valves, head-gasket and the top of the bore, but if you rotate the crank to move the piston down the bore, and get a sudden decrease in pressure on the output gauge, then damage to the bore at that point is indicated.
Originally the MGB used the same breathing method as the MGA.
18V high compression are slightly higher at 9.0:1 and 170psi at 275rpm.
However 18V engines to European emission control requirements ECE 15 are quoted as 170 to 190 or 195psi, although some have said that pressure can't be reached with 9.0:1 compression ratio.
Compression Testing Crankcase Breathing Dating the Engine Dipstick!
(and sumps) Engine Mounts Exhaust Clamps Engine Number Tag Fault Diagnosis Gold Seal Engines Head Gasket Oil Cooler Oil Filters Oil Gauge Oil Hoses Oil Pressure Relief Valve Oil Pressure Warning Light Oils and ZDDP Oil Thrower Power Torque and Consumption Curves Rocker Cover Spark Plugs Tappet Chest Covers Timing Marks V8 Bearings V8 Cylinder Heads V8 Exhaust Manifolds V8 Hot Tapping V8 Inlet Manifold V8 Oil Flow V8 Oil Pump V8 Pistons V8 Starter V8 Front Cover V8 Topend Rebuild Vacuum Gauge Valve Clearances Valve Timing Water Pump Won't Start Won't Switch Off!
In February 1964 with engine 18GA a positive crankcase ventilation (PCV) system was introduced.
A PCV valve was fitted to the inlet manifold, and the purpose of this valve is to provide a source of a continuous low-level suction under varying engine operating conditions.
For engines where one or more cylinders are low go round the cylinders again putting a teaspoonful of oil into each immediately before testing it, this is a 'wet' test.