Downdraft Common Plenum Setups

For the best performance, I tend to think one of the sidedraft setups are the way to go, though I guess if you were trying to keep an engine looking reasonably stock or you just want to keep it simple a downdraft setup might fit the bill. For mild daily drivers the Weber from the 3.3l XE Falcon works very well with a 12 port head and stock manifold, though it's a bit small for very high performance work. Even the stock Varajet flows more than a 350 Holley, and can make reasonable power but the downside is that there are few people familiar with these carbs, and jets and rods aren't as readily available as they are for other carbs. If you're willing to spend some time with them though, they can work well.

For higher outputs, some people have had good results with a small 4 barrel carb on a 9 port motor, but whatever you do steer clear of the 350cfm (or 500cfm) two-barrel Holleys. A Holden 6 with a well sorted Holley 350 will start and idle well, run cleanly and produce a moderate amount of power. But it will also use more fuel than the other small two-barrels, without any power advantage. They obviously have a pretty severe problem with mixture quality and/or distribution. I suspect that this is due more to the orientation of the carb rather than any problem with the carb itself - a peculiarity they have compared to the other 2 barrels is the butterflies open together, and they swing open on an axis that's at right angles to the engine. Maybe this contributes to the poor distribution, and it would be interesting to try one rotated 90 degrees on a four barrel manifold. I know there are a squillion blokes out there with Holley equipped sixes, and they all swear that their engines run just fine. Which they do of course, but I'd bet good money that they would make significantly more power and use less fuel with a different carb.

While we're on the subject of Holleys, be aware that the two barrel carbs are rated at twice the pressure drop of the four barrels (3"hg versus 1.5"hg), so the 350 (and even the 500) two barrel will be undersized on anything but a fairly mild motor.

Although downdrafts mightn't be the ultimate in performance, a decent two-barrel on a stock 12 port manifold is a cheap and simple solution for a mild street engine. For higher performance a four barrel will provide sufficient bulk flow for good peak power but it will be difficult to match the mixture quality and distribution of a set of triples. Good results have been had from 390 and 465cfm Holley 4 barrels, and for a full-on drag engine it might be possible to use anything up to a 600cfm carb.

Various Carbs Compared
Various throttle bores compared. Clockwise from top: Stromberg single throat from red 202, Rochester Varajet from black 202, Weber from XE Falcon, 350 Holley. Varajet flows a bit more than 350, the Weber just a tad less.