Hmm, my previous response doesn't factor in an aftermarket exhaust. With a
free flow exhaust, I'd increase the cfm to 650, which IIRC has the same
1-3/8" primaries as the 500cfm 2bbl.
Yes, torque spins tires, but since B's don't hook up well, not sure I'd want
much, if any, more. According to a C&D road test, the stocker's peak torque
is 285ftlbs/2200rpm.
Fuel flow (gal/hr) = (cfm*4.38*F/A)/6
where:
F/A = fuel/air ratio, i.e. 12.8:1 = 1/12.8 = 0.078
So for 500cfm/12.8:1 = 28.47gal/hr, 600cfm = 34.16, 650cfm = 37.01, 750cfm =
42.7.
Of course this assumes WOT for an hour, but it still tells the relative
difference in fuel economy if you get into the secondaries very often, so if
it gets 16mpg/500cfm, expect ~13.3mpg/600cfm, ~12.3mpg/650cfm, and
~10.7mpg/750cfm. Otherwise, the difference will be somewhat less as there's
not a big difference in the size of the various spreadbore's primaries in
these cfm's IIRC (can't find my spec shts).
Back when the first gas 'shortage' occurred, we disabled the secondaries of
our 'vettes, musclecars, etc., and as long as we didn't floor them or run
them over ~65mph, we never missed them and increased mileage ~33% IIRC.
GM
----- Original Message -----
> Thanks for all the info so far. This is great stuff and I'm having alot
of
> fun with it. I ran some numbers through my software and this is what I
came
> up with.
>
> I'll speak in terms of torque since that is what makes the tires spin,
yes?
> These figures are on our cars with a stock 351W, no mods other than the
carb,
> exhaust and manifold.
> Also a 5000 rpm redline.
>
> RPM 500cfm 600cfm 750cfm carburetor
> 2500 285 291 294
> 2750 293 300 304
> 3000 297 303 308 peak
> 3250 297 303 308 same
> 3500 289 296 302
> 3750 275 284 290
> 4000 257 267 275
> 4250 230 243 252
> 4500 203 216 225
> The off the line difference I can not determine but I would bet that all
> three senarios will be very close in the quarter. Question is, what is
the
> fuel savings between the 500 and 750 cfm?
>
> Greg Arend
> Advantage Air
> Flagstaff AZ
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