40s are nice, 45s offer greater tune-ability if you have the motor for them. We've run both, our guru preferred the 45s.
That's a little misleading. The choice between 40mm and 45mm carbs comes down to how much air your engine pulls through them, which is a function of displacement X peak RPM. There is an optimal air velocity you want to target; put a 45mm carb on a small, low rpm engine and it wll run far worse than it would with a 40mm because the air speed will be too low.
The general rule of thumb is that the venturi diameter in mm = the square root of (single cylinder displacement in cc X RPM where maximum power is produced / 2500). Then the carburetor's butterfly diameter is 1.25X the venturi diameter.
So if your "3 liter" BMW engine displaces 2,985 cc, that's 497.5 cc/cylinder. If it makes peak power at 5,000 rpm, that works out to (497.5 X 5000 / 2500)^1/2 = 31.5mm venturi. The closest common size would be 32mm. Then 32 X 1.25 = 40 mm, so a Weber 40DCOE would be correct.
In the above example, if the peak power rpm was increased to 6,500, then the venturi size comes out to 36mm, and the carb size should be a 45DCOE. So while 45's might be right for a race engine, you're probably better off with 40's for the street.
All depends on your motor.
Now that I'll agree with!
