Throttle Body Sizing

[Michael D. Porter]

Orin Harding wrote:
> 
> 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)
> 
>
> What gives here guys?
>As you've guessed, this is a simplistic formula which does not take into 
account gas speed, volumetric efficiency or pressure drops. 
 
> To further this thought, shouldn't the intake valves duration somehow be included in this?
>Duration is only one aspect of what you need for the answer--since the 
valves open progressively, the rate of flow changes across the 
duration--and the absolute flow changes with manifold pressure.  If a 
carburetor choke (or a throttle body) were sized for average flow, it 
would present an undue restriction on peak flow.  Second, overlap must be 
figured into the equation, since the gas going down the pipe is doing 
useful work (if the tuning is correct) creating pressure waves, even 
though it has absolutely nothing to do with displacement.  Both of those 
factors change the CFM calculated at absolute pressure.  

As far as the best means of determining correct throttle body size, there 
are probably too many variables for the ordinary plug-in formula to 
consider, many of which would have to be empirically derived.  But the 
best means of making assumptions is to begin with the torque peak.  By 
definition, that is the point of maximum volumetric efficiency for any 
engine.  Unless your torque curve is flat up to 6000 rpm (if it is, tell 
us how to do it-naturally aspirated, of course!), the volumetric 
efficiency drops progressively at higher rpm, and you can make estimates 
from that with your revised formula.  Finally, since you're already 
limited to throttle bodies in the marketplace, the first rough step might 
be to make sensible estimates of horsepower, and then look around at 
throttle bodies on engines of comparable displacement and power 
production and pick one that fits.

Unfortunately, I've just been going through both of Philip Smith's tuning 
books looking for that handy plug-in formula, and it just isn't there.  
Much of this can be calculated _if_ you have accurate pressure data; no 
wonder access to the flow bench is so valued....   
> Hopefully one of you will have an answer to this.Don't know if it did, but at least it tells you were right not to size 
according to the simple formula. <g>
Cheers.