Jet Size: Primary-84; Secondary-84
After experiencing the 950-cfm carburetor, we weren't too sure of what to expect from an even larger piece. Usually, once you reach the limit of beyond, it's common sense to head in the opposite direction. However, since we're talking about carburetor sizing, we thought it would be interesting to see what happens way beyond big. Now, into Dominator cfm, we figured the engine would hardly run at all below 1,500 rpm.
Again, the Holley HP series amazed us with its ability to meter incoming air speed and adjust for the proper amount of fuel needed to run the engine. Of course, the idle was extremely choppy below 1,000 rpm, so we raised it to 1,200, where it seemed happy. The touch of the dyno handle had changed as well. Throttle response went out the window from anything below 2,000 rpm, and at that point, we just wanted to see what the extra cfm would do to our previous power curves. Amazingly, we saw one more peak lb-ft of torque and still managed to stay at a 5hp peak above the advertised horsepower rating. Reviewing the 500-rpm incremental test points, the 1,000-cfm carburetor did better than the 950-cfm unit but was still making considerably less power than the 830-cfm piece. Perhaps the air speed signal through 1,000-cfm carburetor boosters was stronger than that of the 950-cfm, which is why it did a little better. Either way, the 950-cfm and 1,000-cfm carburetors were too big for our engine.
The biggest difference in power was the change from a restricted 390-cfm carburetor to a larger 600-cfm unit. This part of the test showed just how much power was lost with too small a carburetor. The most interesting part of the test is how little difference in power there was out of the box between a 600-cfm carburetor through to a 1,000-cfm unit. As mentioned before, this had a lot to do with the air speed signal being picked up from the boosters. All of the tested carburetors featured down-leg boosters, which means the air speed is calibrated farther down in the venturi to help improve the signal. An annular booster design sits farther above the venturi so incoming air is not blocked, but at low engine speeds, the signal becomes extremely weak if the carburetor is too large. Judging from the test results, we'd say that the 750-cfm carburetor seemed to provide the best power curve and throttle response. In the event that the engine made 50 hp or more and was dedicated for racing, we'd be willing to say that the 830-cfm piece with down-leg boosters might add a few extra horsepower and still be livable on the street. As you can see, carburetor sizing isn't a science. As long as you are not all-out heads-up racing, simply balance throttle response, low-speed driveability, and peak power as close as you can, and the carburetor will do the rest.