What if you don't want to toss that stock dual snorkel you scoured the swap meets for? For our next test we tried the dual snorkel with a K&N replacement filter inside, but with the old-timer's trick of flipping the air cleaner lid. The result was a power curve identical to that shown in Test 2. A K&N #E1530 element is a direct replacement for the stock filter in the stock dual snorkel can. The stock housing can be run, and with a flip of the lid it'll move air as well as a wide-open element. We used the K&N open air cleaner for the rest of our tests.
Carb SwapsSo far we had stuck with the stock '71 AVS in purely original form. The 440 Magnum AVSs were of a square bore design, with 1111/416-inch throttle bores all the way around. The standard 440s, 383s and 340s made do with a smaller version of the carb, with 171/416-inch primaries. Our 440 Magnum's AVS was, of course, the large version, with a flow capacity of 750 cfm. Since AVSs are getting old and are increasingly difficult to find in the large version, we thought we'd test a couple of alternatives. Indeed, our own AVS had suffered from a stripped fuel inlet fitting, which we repaired by tapping to a pipe thread. With other types of damage, the repairs may not be so straightforward, so let's look at the most logical replacement alternative.
Closely related to the AVS, although the design pre-dated it, is the Carter AFB. Although it's an older design, the AFB is still being produced under a couple of different labels, and new carbs as well as parts are readily available. The two designs are so similar that the AVS is typically mistaken for an AFB. The biggest difference is that the AVS uses an adjustable spring-loaded air door above the secondary, with a fuel spray bar in the secondary side, without a conventional venturi. In contrast, the AFB uses a conventional booster and venturi in the secondary, with a non-adjustable counterweighted air valve below the booster. The AVS also has three-stage metering rods, as opposed to the AFB's two-stage rods. Although the AVS three-stage rod set-up can be fitted to the AFB if the AVS's rod covers are used, unfortunately no one makes them anymore.
Since it is the most logical replacement carb, we picked up a new Carter Competition Series AFB in the same square bore 1111/416-inch 750 cfm size as the old AVS. The swap couldn't have been simpler, owing to the similarity of the carbs: the fuel inlets are in the same place (and even interchange); the flange patterns are the same; the linkage clears all of the manifold obstructions; the factory throttle and kickdown linkage bolts up in the same position; unlike some other aftermarket carbs, it doesn't interfere with the coil in its stock manifold-mounted location; the vacuum fittings are in the same place; and the stock air cleaner housing fits. The only thing closer would be to buy a brand new AVS.
We just did what the typical Saturday enthusiast would do when shelling out for a new carb. We took the AFB out of the box and bolted it on. The result is shown in Test 3-definitely down some on the old AVS. The AVS had the advantage of being factory calibrated specifically for the stock 440 Magnum, while the AFB has less specific "universal" calibration. No doubt with some jet changes the AFB's results could have been improved, but we decided to let the chips stay where they fell.
While we had the AFB bolted on, we decided to try a 1-inch carb spacer. Not because we saw any specific benefit with regard to the AFB in particular, but we wanted to see what it would do on a stock manifold on a stock engine. The open carb spacer will increase the plenum volume and more freely join the left and right sides of the essentially divided stock manifold (the thick stock carb base gasket acts as a short spacer essentially doing the same thing). Typically, open spacers will help increase top end power. On hotter engine combinations, the effect can be drastic, but we are dealing with a stock 440 here, so we didn't expect miracles.