36mm venturis are fine in 40mm Webers but when you get to 38mm venturis you need the 46mm Webers since the 40mm bore with the blockage of the throttle shaft restricts air flow capacity. Both the 40mm & 46mm Webers share the same inlet diameters above the venturis. The custom bores were performed to take advantage of tailoring those two projects to be as balanced as possible for flow through the carburetor, a calculated design as opposed to a moderated approach.
Just to be clear: a 40mm throttle bore has the area of 1257 sq mm. Subtract the blockage of a 8mm throttle shaft that is 40mm long (320 sq mm) and you have a cross-sectional area of 937 sq mm. A 36mm venturi has a cross-sectional area of 1018 sq mm. So, a simple comparison of cross-sectional areas shows the 36mm venturi has a larger flow area than the 40mm exit bore. However, this is not an issue since the air flow through the venturi is at a higher velocity & associated lower air pressure than at the exit diameter so 36mm venturis are fine in 40mm throttle bodies. (A simple comparison avoiding the math associated with fluid dynamics is sufficient I believe.) However, 38mm venturis would be too large.
The issue with larger throttle bores is that the progression circuit becomes less responsive to air flow for smaller displacement engines. So a 2.0 with 46mm bores would be challenged to provide a good transition onto the main circuits. This is not an issue if the car is light and the operational RPM of the engine is routinely above 3000 RPM. This would be why the 906 (1300 pounds) was fine with 46mm Webers and 42mm venturis...not much need for below 3000 RPM operation when you are running for FIA points in 1966.
There are some tricks that I use to help tune out transition issues:
- Install tune-able idle air correction screws. These allow changing the "slope" of the transition fuel mixture curve to help prolong its effectiveness without installing overly large idle jets.
- Use tall auxiliary venturis. These help advance the initiation of the main circuit. This, in conjunction with the use of tune-able idle air correction jets allows the progression and then transition to have proper fuel mixture.
- Use full radius intake air horns. I was personally surprised in how a very minor "tick" at 3000 RPM in my engine went completely away when I installed performance air horns.
- If tall auxiliary venturis are used I recommend the tops of the throttle bodies be milled by approximately 0.010". This allows clamping of the ends of the aux venturis by the top covers which will prevent the aux venturis from loosening over time. (The tall venturis have their center of mass above the mounting interface (the rectangular end plates on the aux venturis) so the natural, rocking vibrations of the 911 engine tends to "beat" the end plates until the venturis get quite loose. The short, OEM venturis do not suffer this ill since their center of mass is at the same level as the mounting ends and no rocking dynamics are imparted to them.) When these become loose they allow "false air" to be drawn into the interface between the end plate of the hollow arm in the aux venturi and the interface with the body at the fuel transfer port from the emulsion tube well. This false air can/will delay main circuit initiation.
- And one last item: Adjustment of main jet, main air correction jet & emulsion tube will affect timing of main circuit initiation.
36mm venturis in 46mm bodies can be made to work but if the plan is to go to 2.7 liter (crank changed for later crank?) then that cost would be somewhat larger than the cost of enlarging the 40s to 46 or acquiring 46s at that time. If the 2.7 re-configuration does not happen then you end up living with compromised carb performance. Another thing to be aware of: the early 46s and those made after Porsche stopped using carbs on race engines have different progression circuit design. Those with serial numbers will have the early progression and those with alpha-numeric serial numbers will have a different pattern (3-hole or 4-hole) and are better suited for street driving.
If 40mm Webers are available then avoid IDTP bodies. IDA, IDS, IDT and IDAP are all good. If the car is to be tracked seriously then fuel well baffle plates are needed but do not vent the tops of the top covers using the PMO "anti-percolation" jig. This is a Ferrari mod as developed for the 365BB & 512BB. The venting spills fuel past the idle air correction jet which will flood your engine making hot re-starts very difficult. Also, during sustained, high-G cornering the flood will flood the idle circuit of the carb on the outside of the corner. I have developed an alternate venting that is centered on the carb that vents without flooding.