Hi Chaz,
A 2V can provide get better milage IF the preformance factor doesn't temp you to put foot in it to often.
Basically, 2 bbl carbs come in two versions, progressive and synchronous. With a progressive carb the second throttle plate does not begin to open until the primary is approx 70%. Then, at 100% throttle both are full open. The purpose of this set up is to allow the driver to get the milage of a 1V carb while providing the performance of a 2V when needed / wanted. Another advantage of the progressive set up is that there is no loss of low end torque as a result of reduced intake air velocity. Smaller throat = higher velocity as the demand of the engine (CFM requirement) must pass through a smaller opening (unless, of course, it's too small). In my opinion, you wouldn't want to give up the bottom end on a street car. There is an old saying that I still buy..."you race HP, but you drive torque"...
The more popular progressives being used are the Holley 5200 and the Weber DGV series. As a comparison, the largest of the 200 I6 Autolite 1100's (which is the most common series on the 65 -68 Stang) had a single 1.2" (30mm) throat and a max flow rate of approx 185 CFM (stock car requires a minimum of 175 @ 4000 rpm & 75% VE). The 5200's being used have a 26mm throat and a 27mm throat, i.e. the 26/27. Bottom end velosity is therefor improved, and at the top end this carb flows 270 CFM. This carb will support a 5000 rpm engine with 85% VE (header, ignition, mild cam). The Weber 32/36 has a good bottom end and flows 320 CFM.
Syncro carbs include the Autilite / Motorcraft 2100, the Holley 2300, and the Weber DGE series (the 38/38). The 2100 is available in several versions with varied throat sezes. If you go this way the 1.08 (27/27 - 280 CFM) is the smallest of the series and would probably be the better choice for a street car (best bottom end of the bunch). The rest of the non-progressive choices would be better used on cars with material head and cam changes, at which point one has made the choice to give away the bottom for HP.
A word about mounting. The 5200 and the 2100-1.08 are the larget carbs you can place on an adapter without running into stock manifold carb bore volume issues. That is to say that the area of the 1.50 bore and the 1.75 bore are 1.77 and 2.4, respectively. The throttle bore area of the 5200 and the 2100-1.08 are 1.69 and 1.83, respectively. So, anything larger than these needs to go on a manifold modified for a direct mount if the true potential of the carb is to be achieved.
For my money, the 5200 is hard to beat unless you very serious about improving the VE of these engines. They are inexpensive, easily obtained, and easy to get parts for (tune).
Probably more than you expected, or wanted, but based on your prior carb questions (sounds like you are getting serious) I though more was better.
Enjoy the trip - Steve
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