DeTomaso Mailing List: January 2001, Message #5

On Mon, 1 Jan 2001, Mike Dailey wrote:
> So far from the messages I read it looks like there would be an average 2.3
> intake valves open that would be sharing the available and potential 750
> CFM.  750 divided by 2.3 is 326 CFM for each cylinder.  I also got some
> messages that focused on RPM and indicated that a 351C with a 750 CFM carb
> might be able to turn in the 7000 RPM range depending on the cam, head flow,
> etc.
> 
> So if we all agree on this the next question is:
> 
> If each cylinder only uses 326 CFM on a 351C equipped with a 750 CFM carb
> and might turn in the 7000 RPM range, how big would the opening (bore) need
> to be in a single port on a EFI or carb system that has a individual port
> for each cylinder to be equal to the 750 carb on a regular intake manifold
> setup?   We know that the individual port would need to flow about 326 CFM
> and that would be big CFM for a single port system.   The total single port
> system CFM would be 236 CFM times 8 cylinders for 2608 CFM.  That is monster
> CFM number compared to the 750 CFM carb!

Now you know why Webers appear to act so funky.  :-)

The real difference here is that the central throttle body (carb, or just
EFI throttle body) only has to deal with a reasonably steady demand, while
individual throttle bodies have to deal with a demand that's playing a game
of red-light-green-light up to 60 times/sec (at 7200 RPM).  

For the central throttle body sitting on an 8-runner intake manifold/plenum,
the average flow rate is a reasonable metric for the demand placed upon it
because the flow is relatively steady (ignoring tuned intake runners,
assuming an effective plenum, etc).

For the individual throttle body, it's a whole different problem.  There is
no meaningful average flow rate.  It's all about strong negative and
positive pressure pulses from that individual cylinder, as your calculations
have sort of backed you into seeing.  Incidentally, because you will see
backward flow on the down-side of the pulses, the peak flow rate will
actually be even higher(!) than you calulated.

I think you need to actually delve into advanced fizzix in order to be able
to understand the individual throttle body problem. 

For instance, if you view it as a "system", at some point in the system
(perhaps outside the throttle body, in open air) the flow rate comes close
to the smoothed average (94 CFM in your 750 CFM total example).  How the
throttle body manages the transition/transformation from very strong
high-velocity individual intake pulses to an average/smoothed flow rate
(and, in the case of Weber carbs, how it meters and introduces the proper
amount of fuel to the airflow) is probably a large part of the black-magic
secret of Webers.  I suppose you could view it as a flow transformer.

There are definitely economy of scale advantages to a central throttle
body.  :-)

> This is exactly what we were trying to figure out in our OKC discussion.
> How much flow does a Weber carb need to make 7000 RPM?   It looks like the
> Weber bore would need to be very large to equal the good old $350 carb
> system.

It's probably worth pointing out that the bore of a throttle body does not
_necessarily_ reflect it's flow capability.  The velocities at which it
works are very meaningful too.

-Andy
#3822 (with no carbs or throttle bodies at all at the moment)

72 Pantera - Rocky              91 Miata - Steve                96 A4Q - Rudolf
87 E350XL - Andylance           84 RZ350 - Sting