Large CFM carb - small CFM carb

[Ted Borck <tborck@domain.elided>]

> Date: Mon, 21 Jun 1999 10:02:54 -0500
> From: jbade@domain.elided
> Subject: RE: ihc-digest V6 #912
> 
> >Anyone know of a good source for a new 2bbl Holly with side hung floats?
> >All I've ever found is the 500 CFM center hung universal 2bbl.  To me that
> >seems to be too much carb for a 304 and probably too much for a 345.
> >
> >Dave Read

<snip>

> Dave,
> 
> The 500 should be fine for either motor.  It's probably a little more than
> you need but not by any large means.


Food for thought

A friend of mine in Georgia - who is a very knowledgable mechanic -
has a pet theory about carb size to engine size.

He explained the following theory while we were discussing possible 
engines swaps in my Scout II.

He maintains that you get better gas mileage using a carb with a
*larger* CFM flow capacity, at less than WFO, than with a *lower* CFM
carb, e.g. using a 650 CFM 4 barrel rather than a 500 CFM 4 barrel
carb.

His theory is that with the *larger* carb venturi(s), the Bernoulli
effect (faster the air flow - the lower the air pressure in the
venturi(s) to suck out the gasoline) is lessened (bigger pipe for same
amount of air flow, air flows at lower speed) and less gasoline is
pulled into the engine.

With the *smaller* carb, given the same engine displacement, same RPM,
the air flow through the smaller venturi(s) is faster, thus greater
Bernoulli effect, higher vacuum pulling more gasoline out through the
main jets.

He belives that small CFM's (smaller pipes) higher vacuum means that
the engine runs too rich at part throttle, and thus uses more gasoline
than is necessary for proper combustion.

His theory is that once you've jetted the larger carb for proper
mixture - at part throttle less gas is used by the large carb than by
the small carb.

Naturally, this should not be construed to mean that a 650 spread bore
Holly should be mounted on your 5 HP Briggs & Stratton.

I don't remember the size carbs he mounts, and I don't always agree
with this person, but knowing he gets 20+ MPH out of large block caddy
engines (500+ ci, mostly stock - maybe a different camshaft) running
at part throttle (70 MPH) makes it hard to argue with him.

So if this theory holds true, the 500 CFM 2 barrel (comparing *large*
2 barrel to *small* 2 barrel) should improve your mileage.

Ted Borck	tborck@domain.elided