Throttle Body Sizing

[Markus Strobl]

> 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.

[snip]

The formula you quote is correct. It assumes 100% VE. In the real world
it seems most engines prefer slightly larger carburators than the formula
indicates. For instance: a 350 cid V8 w/ 6000rpm redline would need a 610
cfm carb. Yet most built 350s run better with 750 CFM carbs. My guess is this
is the result of how the carbs are rated (ie the pressure drop used to rate
carbs is larger than is optimum for engine power). 

A single 40mm TB sounds alittle restrictive for your application. But you
won't flow over 500cfm, like you calculate. I don't know enough about MGs
to recommend a specific size TB. Just as a comparison, my 5.7l V8 uses
twin 48mm TBs w/ 5800rpm redline.

Markus