Well it's good to see from the answers that many of you have experience dyno testing engines.
OK,, we measured; RPM,,CHT,,EGT,,Crankcase Blowby,,Pressure tested the combustion space,, measured Torque and calculated HP.
I didn't have a way to measure intake vacuum !
The top piston ring is sealed against the cylinder wall by compression pressure behind it.
What seals this ring on the intake stroke ?? Radial ring tension to a large degree !
We installed a new well prepared ground and lapped top ring and the power returned just like it had always been.
So much is written about "Low Tension" rings,, but the consideration of how the engine draws in the precious fuel and air mixture is never mentioned along with this subject.
A leak down test that shows 3% under pressure will be much more under vacuum. And yes 1% leak down is accomplished with much attention to the cylinder, rings (OD and Face Angle) and piston ring land preparation. 1% is just about what a .003" ring gap will leak,, 300 laps??, well maybe .005". This was an unrestricted class engine, but the more the intake is restricted the greater the top ring tension should be, within reason of course.
There you have it, Best, WP