390 cfm: Big Power, Little Carb
Jet Size: Primary-65; Secondary-65
So long as the appropriate supporting items are used, the Smeding-built 383ci is rated at 440 lb-ft of torque and 440 hp. By following even the roughest formula, it's safe to say that 390 cfm wouldn't cut it with this application, but we wanted to see how much of a difference a small carburetor would make. The smallest four-barrel carburetor Holley offers in a 4150 HP-style mechanical secondary configuration is this one. We bolted the NASCAR 390 cfm on the Smeding piece and made a few pulls.
By reviewing the dyno results, it's easy to see that the small bore and venturi of the carburetor definitely restricted airflow. Compare this test to the later 750-cfm and 830-cfm tests, and you'll see that power is down considerably across the entire curve. By using the smaller carburetor, we restricted airflow and hurt the potential power the engine could have made. However, the torque did reach its advertised 440-rating even with the teeny-weenie carb. Surprisingly, the EGT and O2 sensors both showed a near perfect oxygen/fuel content in the exhaust, and carburetor tuning was not required.
600 cfm: Bigger Gets Better
Jet Size: Primary-70; Secondary-70
The next step up in the line is a 600-cfm carburetor. Had we used something smaller, it would have been a vacuum secondary unit and we didn't want to alter any factors in the testing procedure other than the cfm rating. We bolted on the 600-cfm carburetor but made no other adjustments.
As you can see, the engine produced considerably more power than with the small-bore 390-cfm unit. Average torque numbers jumped by almost 20 lb-ft, and the horsepower hopped up 17 more than before. As you follow the 500-rpm increments, it's easy to see horsepower gains from nearly 10 hp down low to upward of 25 hp or more higher in the rpm band. Again, carburetor jetting was near perfect out of the box, and we didn't alter anything.