Vacuum at WOT means there is a restriction at the carb; the engine wants to pull more air than the carb can deliver. Think of watching a vacuum gauge as you are going down the road at 60 MPH, and then suddenly you let off the gas. Vacuum goes way up, because the engine wants more air than the carb can deliver with its butterfly's closed. At WOT, the presence of vacuum means that the carb is still restricting the airflow somewhat.
Since the carb was the same on all three tests, what this means is that with the Performer RPM, the engine wanted to pull the most air, and with the PI, the engine wanted to pull the least air. This is a function of the efficiency of the manifold. The PI is the least efficient of the ones tested, because with it installed, manifold vacuum at WOT was lowest.
Hope that helps - Jay
Jay Brown
1968 Shelby GT 500 Convertible, all aluminum 489" 1030 HP Supercharged FE
1969 XL Convertible, 460
1969 R code Mach 1, 706 HP 511" all aluminum FE, 10.457 @ 127.47, 2005 Drag Week Winner, Naturally Aspirated Big Block
