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The velocity at which the air/gas flow enters the intake valves is a very large factor in performance. Turbulence of any sort in the intakes is counter-productive. The intake ports are built horizontally into the head in the great majority of in-line motors. Air entering horizontally through side-draft carbs gets a pretty straight shot right at the valves. This produces much less turbulence and drag than you get when using a downdraft carb and forcing the airflow to make a 90-degree bend before even entering the head.
There's a secondary effect as well, although it's much less important: with the SU manifolds, the air travels the same distance to all the intake valves. With any of the existing single-carb down-draft manifolds for our motors, the airflow is longer for cylinders #1 and #4. There's no way to make all the cylinders tune exactly the same (which is why blueprinted motors sing like they do).
I have a Weber DGV downdraft on one of my cars, and it's a fine thing in its own right -- essentially a zero-maintenance device, once it's jetted correctly. But there's a fair amount of mythology out there that these produce performance equivalent to SUs. It does run better than worn out SUs, certainly -- but not ones that are properly rebuilt and properly set up (but that's a different subject).
We just un-converted a 122 back from a DGV kit back to a fresh set of HS6s -- the owner was happy with it before, but he now swears he picked up 20 HP with the side-drafts (very unscientific and anecdotal, but it does obviously zip quite a lot better).
BTW, you can have a fine rebuild done to a set of HS6s for a lot less than what a DGV conversion kit runs. Once they're rebushed, they just don't wear out anymore.
Just as a note: Don't confuse the Weber DGV or DGEV (a 2-barrel progressive downdraft) with the Weber DCO or DCOE (a 2-barrel side-draft, usually used in pairs). 4 barrels of DCOE (or "workalike" Dellorto, Solex, etc.) absolutely have more performance potential than do SUs of any model -- but they're expensive and complicated to tune.
--Phil
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