Is this a good deal? Will it do what I want? Carb/Intake
Looking for less CFM/primary bore for the highway. I'm thinking the 600 CFM four barrel's primaries should be smaller than the two barrel at 375 CFM. Less air, less fuel. Is that not correct?
Popular misnomer.
The engine will draw the volume of air it needs. The density of that air will vary with the throttle opening and air temperature.
Yes the 4 barrel's primary venturi and throttle openings are probably smaller than the 2 barrels. You could measure them to be sure.
But that alone only means that
if everything else is the same, the air velocity in the smaller venturis will be higher.
Thermodynamics says the larger throttle opening will theoretically be more efficient.
In reality whether this results in more efficiency depends much more on other factors. Air/fuel ratio that more closely meets the engine needs and the most even distribution of fuel at part throttle depends on the boosters, the calibrations, the intake.
IMO the biggest factors for street driving will be:
a) properly heated intake with good part throttle fuel distribution.
b) good manifold vacuum
c) timing curves set for maximum fuel efficiency (power) and AFRs to match.
'72 timing curves and fuel mixtures were setup to reduce emmissions and to some extent that compromised engine efficiency.
For better fuel economy, I'd first look at timing. Something along the lines of what was used in '68, along with a slightly richer idle at lower idle speed. Yes slightly richer idle will be more efficient and produce more torque. Talking maybe 13 to 13.5:1 rather than 14.2:1.
Some carburetor designs like the T-quad and Quadrajet take advantage of the very small primaries for slight gain in signal, sometimes improved further with multiple boosters. Doing so often improved fuel distribution at low and part throttle and also let the manufacturers calibrate even closer. That said, not every spreadbore design does a good job. The Holley spreadbores have a reputation for being hit and miss and in any case the common ones didn't really take advantage of the small primaries.
If you want to play the eddy, that's fine. A four barrel will provide noticbley more power at the end of the quarter mile. :)
