650 cfm: Changing of the Guard
Jet Size: Primary-70; Secondary-70
Though power was on the rise, we still hadn't reached the level we sought. This time, we swapped on a 650-cfm pot. Surprisingly, more cfm only netted an average gain of 2 lb-ft of torque and 3 hp. However, the larger carburetor developed 444 hp and was 21 lb-ft of torque stronger than the advertised number. Reviewing the 600-cfm and 650-cfm horsepower curves at 500-rpm increments will again show an increase of approximately 2-3 hp down low and 5-8 hp higher in the rpm curve. Again, the air/fuel ratio was right on out of the box.
750 cfm: Still Gaining
Jet Size: Primary-73; Secondary-73
The important thing to mention here is that of all the carburetors tested, only the 750-cfm HP Holley came with a vacuum port below the base of the carburetor. We would have liked to include engine vacuum in the entire testing sequence, but due to a lack of accessible portable vacuum sources, we were unable to do so. Although the Air Gap manifold does have a tapped source for vacuum, it is inaccessible unless there is a spacer below the carburetor.
To our amazement, the average numbers were almost negligible: 1 lb-ft of torque and 1 hp. Peak torque was still up by 21 lb-ft and horsepower remained the same at 4 hp above the advertised peak. This test also showed optimum air/fuel ratio results, which revealed how well the HP Holley carburetors are calibrated from the factory. If you are wondering how the 390-cfm carburetor was able to supply as good an air/fuel ratio as the 750-cfm unit using smaller jets, it's because the smaller carburetor created a stronger airflow signal, causing the fuel discharge ports to deliver the maximum amount of additional fuel. The 750-cfm carburetor may have had an equally good air/fuel ratio with larger jets, but the air speed signal was slower; therefore, it drew less fuel from the jets and delivered nearly the same air/fuel readings.