Which do you think would work better on a hot 250,
These
Solex 40) or These (DHLA 40 Dellorto)
If it were me, neither! I'd go for 45 DHLA or DCOE's.
In theory, though, any independant runner intake, even with a 40 mm carb, may get you around 190 hp tops. I think there may be a severe loss in usable rev range with these carbs because you can't get near to the ideal venturi size Weber recomemds. Info I have from David Vizard says a 40 mm carb is no better than about 75 hp each, but that
only happens when venturi size is below 34 mm on a one carb per four cylinder basis.
Because I respect your judgement, I'd like to tell you that an engine reving to 6000 rpm needs about 250/23 or 11 square inches of venturi area. That means a minimum of 1.83 sq" per cylinder, or 1.53" or 39 mm venturis minimum. That rules out 40 mm carbs unless you a making a very mild 250 engine. The biggest venturi size to go to with these 40 mm carbs is 34 mm, and over that it is very poor functioning carb, with insuficient signal and poor air flow. About 175 cfm per barrel. With 45's and 34 chokes, it is over 180 cfm per barrel.
This means a maximum rev range of about 4500 rpm with these 40 mm carbs.
The proportionality is shown by the 24 and 26 Oz. Datsun's. A set of 3 DCOE 40's are set up with 29 mm venturis on the basic kits Weber offered. This worked out to a factor of between 23.7 and 26.35 cubes per square inch for the little 146 and 162 cube OHC six. In Australia, 265 Hemi Six Pack Valiants had 38 or 40 mm venturis for the E37/38/48 to E49 vehicles, with various gross hp ratings from 248 , 280 and 306 hp from 4800 to about 5400 rpm. The lower power engines had mild 256 degree camshafts, and the other engines had screamer 280 and 302 degree cams (25.1 to 22.7 cubes of engine per square inch of carb venturi).
Weber also published a guide on venturi area for engines between 100 to 700 cc per cylinder, and from 6000 to 10000 (!) rpm. The higher the revs, the lower the engine cubes per square area of carb venturi. For any carb to function well, it must have between 42 and 18% signal, down to 10% only if you are prepared to sacrifice low end tractability.
That is, the throttle and venturi can be, say
* a DHLA 40 with a 28 venturi, or a DHLA 40 with a 34 mm venturi, 36 at a pinch. (Okay with a 200 cube engine, a limit to a 250, in my opinion)
* A DHLA 45 with a 34 or 36 mm venturi out flows and outperforms a 40 mm carb with a 34 or 36 venturi. A 45 DHLA can take a 40 mm venturi.( Same comments)
* A DHLA 48 could live with a 34 choke, or up to a 41 comfortably.
