Throttle Body Sizing

[Orin Harding]

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From: 	owner-diy_efi at coulomb.eng.ohio-state.edu on behalf of Paul Shackleton
Sent: 	Thursday, July 25, 1996 6:55 AM
To: 	diy-EFI
Subject: 	RE: Throttle Body Sizing


>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!

I'm using a TB from a company called TWM located in CA (I'll send the address 
if you need it). They are kind of expensive and if I had it to do again I 
think I would go to a junk yard and get one off a VW Golf or something with an 
engine size of around 2.0L.

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.

21 seconds is the correct time as the inlet (in my example) is open for 252 
deg out of 720.