It Howls with small throttle bodies

[Todd Knighton]

tom cloud wrote:

> Now I'm confused....  With my limited knowledge and cranial ability,
> I thought that throttle size had little effect on FI.  I can see that
> a larger TB gives less velocity and therefore less momentum and
> mixture of the fuel/air, but can anyone 'splain to this ole boy
> how to determine a TB size (talking normally aspirated).

Tom,
	There was a really good article just recently in a motorcycle magazine
that was specifically about this.  They were talking to Mr X from
Cosworth about port sizing and manifolds, and according to them, the
port sizing was all about a specific port velocity to be obtained at a
specific rpm.  Though we calc'd some of the numbers backwards to a
street oem 911 Carrera engine, and his velocities dictate that Porsche's
ports are about 30% too large.  Maybe their independent runner, short
stacked, 14,000 rpm v8's work differently from 7,000 rpm plenum
chambered flat six's.
	About the most interesting thing in the whole article was in reference
to stacks.  They proved that an intake runner with any more than about
.5 degrees taper, yup that's a 1/2, significantly decreased the flow.  I
guess that doesn't say much for carbs, jeez they go big, then small,
then up and around, then small, then big again.
	Be careful about your throttle body sizes when referencing them,
because it makes a huge difference if you have a single throttle per
cylinder, or two cylinders per throttle, or even 8 cylinders feeding a
plenum with 4 throttle plates, they're very different worlds.

Todd Knighton
Protomotive Engineering