How does the quadrajet carburetor perform any better than the others?

Answer 1

The Quadrajet (Q-Jet) carburetor is better on a street/daily driven vehicle for a couple reasons. One is that the primary throttle bores are pretty small, so at part throttle, like most driving, you won't be using the entire carb, and hence you'll use less gas.

Take for example a mechanical secondary Holley four barrel. At part throttle, all four barrels are open and spraying fuel. In a Q-Jet, only two barrels are open and spraying fuel.

One other reason is that it's a vacuum secondary carb. This means that the secondaries open a bit slower, so if you need to gun it, you'll have the power, but won't use all the fuel at once, saving you a bit.

Again, take a Holley for example. Go from 25% throttle to 100%. You go from four barrels spraying some fuel to all barrels spraying maximum capacity. This could bog your engine down from too much fuel for too few RPMs if not tuned correctly, and will immediately use more fuel.

On the other hand, the Q-Jet would open the secondaries slower, allowing the engine to come up into the higher RPMs it would need for the extra fuel. Say you go from 25% to 100% throttle. You go from two barrels spraying some fuel to two barrels spraying maximum capacity, and then in addition, two bigger barrels spraying 25% capacity, 50% capacity, 75% capacity and if you stay in it long enough, 100% capacity.

This would use progressively more and more fuel, so for a quick spurt, you would save some fuel you would have used with the Holley, say only going up to 50% capacity on the secondaries compared to the Holleys 100% capacity immediately. But, you do lose some responsiveness.

This all still happens pretty quickly, as in within a split second for the Holley to go 100% compared to a Q-Jet taking 1 second or so to go fully open.

Other than that, the Q-Jet is basically on par with other carbs. They can be harder to tune, but if you know what you're doing, it will just take a bit more time and effort, not much.

In a cruiser or daily driver, you'd be better off using a Q-Jet. For a race car or weekend toy, just go for the Holley since you won't be driving it all that much, and they are easier to set up and tune for most people.