Throttle Body Sizing

[Paul Shackleton]

>While trying to decide on the proper size throttle body for my MGB EFI

>snip<

>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

>snip<

What throttle bodies are you thinking of using? Are you making your own and 
if so, how? Or are you using proprietary ones? I am just about to start 
making some for the 2.0l 16v Vauxhall engine in my Westfield, but have yet 
to figure out how. Weber, Lumenition and Microdynamics throttle bodies seem 
to be very expensive. ie more than the equivalent sized carb without any of 
the complicated carb plumbing!

Any ideas on DIY throttle bodies would be gratefully received?

Also, in calculation #7, shouldn't #1 x #6 be 36 seconds? However 21 seconds 
is the answer if you say in two engine rotations (4 strokes) the inlet is 
open 252 degrees out of 720. Multiply this up to a minute (x60) and you get 
21 seconds. I'm not sure whether the answer should be 21 or 36 though.


Paul
P.M.Shackleton at hud.ac.uk