According to the Hagerty formula 40 mm Webers (or Del's) will work on all but the very biggest VW aircooled engines but we know that not to be true. A pair of 40 mm carbs can sustain about 140 hp (there are instances of 40's making up to 15 or 18 hp more but use this figure as a general guideline- more on the exceptions later) which is really only a 2 liter-ish or 21something with ported 40x35 heads revving to 5500 or 6,000 rpm. 44's or 45's will support up to about 190? and 48's are required at anything higher.
That said, the carb sizes are close enough (which you will see if you look at the range of venturis available for each size) that applications overlap a fair bit, and either a set of 40's or 44/45's (or 44/45's or 48's for the bigger engines) will work on a wide range of engine sizes and combos.
Great article; but I thought for sure it was going to be about cutting holes to increase the HP to LB ratio!
Yeah, Mayo, it is a legitimate way to increase the hp to lb ratio. It is just a very, very, VERY time consuming way. But if you aren't going to increase the horsepower, it is the only way...