830 cfm: Peaky Power
Jet Size: Primary-86; Secondary-86
We were sure the engine would stumble and rattle at idle with the 830-cfm carburetor, but we were wrong. Surprisingly, the larger carburetor ran considerably well down low. There was a definite throttle response difference between the 750-cfm and 830-cfm off-idle feeling, but both idle quality and power worked out well.
The 830-cfm carburetor posted the best peak numbers and average numbers of the entire test. While the average power gain only increased 2 lb-ft and 1 hp, it was nice to see some new peak numbers. This time around, the 383 made 22 lb-ft of torque more than its rating and made an additional 9 hp. The 500-rpm increments remained about the same as power increased with engine speed. Also, the air/fuel ratio was right on.
950 cfm: Super-Sized
Jet Size: Primary-78; Secondary-78
We were hoping to see even more power with the 950. It definitely produced a low-speed stumble, and we thought about tweaking the idle-air bleeds but refrained since we had come this far without touching any of the other carburetors. As soon as the engine passed the 1,200-rpm mark, the air speed signal through the carburetor improved and the engine ran smoothly. Typically, a large carburetor like this one is only run on an extremely high-horsepower engine, where the camshaft depletes most of the low-speed vacuum. With the idle speed set a little higher, we began the next pull in hopes of more power.
Unfortunately, the powertrain had maxed out, and we were now merely maintaining the best power curve we could. The peak numbers still posted 19 lb-ft of peak torque better than the advertised engine, and the horsepower was still up by 3 hp. In comparison to the previous test with the 830-cfm carb, the numbers reveal a loss in power at every point. The air speed signal through the carburetor slowed even at high engine speeds and caused the air/fuel mixture to become slightly less than ideal. While we could have altered the carburetor to promote a better power curve and perhaps equal the previous tested power, we would have been bandaging the situation rather than doing ourselves a favor.