CarburetorBreakdown
The Demon carbs we tested of varying cfm each arrived at that flow numberwith a different combination of venturi diameter,throttle blade diameter, andbooster design.
525 Speed DemonThe 525 Road Demon had a 1.28-inch venturi diameter, 1-11/16-inch throttle blade, and down-leg boosters of varying size in the primary and secondary sides of the carb.
650 Speed DemonThe 650-cfm Speed Demon had the same-size venturi and throttle blades but larger down-leg boosters and no choke plate, which increased airflow.
750 Speed DemonThe 750 Speed Demon had larger, 1.40-inch venturi, 1-11/16-inch throttle blades, and annular boosters in both the primary and secondary sides of the carb. The 850 had 1.562-inch venturi and 1-3/4-inch throttle blades for even greater airflow. Annular boosters helped speed up the air in the big venturi but not enough for our small test motor to receive the right fuel curve at low rpm.
The Data Don't Lie!
The relationship between carburetor sizing and the fuel curve of an engine is illustrated best using the Superflow dyno data. The upper chart shows the 525-, 650-,and 850-cfm carbs' airflow and fuel curve. As you can plainly see, the red line of the 850 carb clearly flows more air, but if you look at the bottom of the graph, at 2,500 rpm, it's moving much less fuel to the engine, which is what resulted in the lean air/fuel ratio and lower power output. The black and blue lines representing the 525- and 650-cfm carbs are much closer together, and the carbs were much closer in performance because each one offered better fuel flow.
The lower graph shows the horsepower and torque output of our 305 engine at the extreme ends of our carburetor spectrum, the 525- and 850-cfm units. The 850 made more peak power, but the 525 was better everywhere else in the curve, which is where most engines will live in our sport trucks.
The Final Word
Size is everything when it comes to choosing a carburetor, and although this article will clue you into that relationship, it's still not a replacement for a phone call or e-mail to any carb manufacturer or tuner. The bottom line is that you can tune around most carburetor sizing issues, but it's a lot easier to start with the right size of carb for your application and just make a few jet changes than it is to tweak on a carb that's too big for your engine. Like we said before, a big carb can yield a lean air/fuel mixture, and that's a recipe for disaster when your foot is planted on the floorboard. in this case, the 650 Speed Demon carb proved to have the right mix of peak power output, as well as driveability and midrange power output for our 305-cid engine.