Throttle Body Sizing

[Orin Harding]

While trying to decide on the proper size throttle body for my MGB EFI 
project, I found the following formula for determining carburetor air flow 
requirements in "Auto Math Handbook" by John Lawlor:

CFM = (RPM x displacement) / (2 x 1728)

For my 1.8 L (110 ci) MGB engine this works out to be about 191 CFM at 6,000 
RPM.  OK fine.  A 40 mm ( about 1.5 inch) throttle body should pass about 200 
CFM with 10 inches water drop. Well I thought, this couldn't be right as the 
pre '75 MGB's had twin 1.5" carbs. and the later cars had one 1.75" carb.  
What gives here guys?

To further this thought, shouldn't the intake valves duration somehow be 
included in this?  Granted the engine may only need 190 CFM but the throttle 
body needs to pass this amount of air during the intake valve's open time so 
the  actual sizing of the throttle body would be much larger in order to pass 
the required amount of air.

Here is what I think sizing should be based on.  I've used my MGB's cam data 
in this example:

1) Peak RPM: 6,000
2) Engine CI Displacement: 110
3) Intake valve duration (Deg): 252
4) Time for 1 crank revolution: 0.01 sec. (60 sec / RPM)
5) Time for 1 deg. of crank rotation: 2.381E-05 (#4 / 360deg)
6) Intake time per 4 cycle period: .006 seconds (#5 x #3)
7) Intake time per minute: 21 seconds (#1 x #6)
8) Engine CFM requirement @ 100% VE: 222.8 CFM
    ((#1 x #2) / 2) / 1728)
9) Estimated Throttle Body steady state flow requirement: 636 CFM
    (60 sec./#7) x #8

Well, if you assume that the engine has a VE of about 65% at 6,000 RPM then 
the throttle body needs to be sized for about 413 CFM.  This is just about 
what two 1.5" carbs. will pass (steady state) which is what was used on the 
pre '75 MGB's.  This makes me think that my assumptions are correct.

Hopefully one of you will have an answer to this.