This makes a lot of sense.

http://www.hotrod.com/how-to/engine/...41125_36205777

Pretty much everyone will say they got better WOT performance with a larger carb than the standard formula suggests. One explanation was the formula assumed a single plane intake, and dual plane intakes split the carb in two - but that didn't give you a factor to use to get a better result. I found that using the chart for the max DP size for the engine/lowest-RPM gave you a pretty good idea what size carb to use. I have also arbitrarily used a 25% increase factor to "correct" the formula.

For simplicity, if you use the 0.9 in-HG factor they use in their example (halfway between 0.8 & 1.0), 0.775 is close enough (0.77459666924148337703585307995648, to be "exact"). So, using their example, 559.5 CFM/.775 = 721.9 CFM - both this and the "exact" result round off to 722 CFM.

So, my 25% factor - well, this comes out to 22.5% (and 0.8 in-Hg is 27%, 1.0 is 19%), so that explains why it "worked", even though I didn't have the science to back it up.

 

I'm glad somebody else is out there trying to explain why the typical formulas will almost always direct you to a CFM carb that is too small (unnecessarily).

All CFM calculators should have a big flashing neon sign on them that says "For Mechanical Secondaries Only". It's absolutely impossible to over-carb with vacumm secondaries. I'm not just talking about the QJet, but for Holley-style carbs as well. A Holley 3310 750CFM vac sec carb can EASILY be used on even a mild 305 in front of a stock transmission, despite the fact that it is RADICALLY too big, no matter what carb size calculator you use.

For Double-Pumper carbs, I guess that modified calculator is an improvement.

There's really too much that goes into it to boil it down to a simple mathematical formula. A high-winding 350 making 500HP will want more more CFM than a low-revving big block making the same 500HP, even if everything else is held constant, for example.

Even if you discount everything I said, above, too many people are "afraid" of over-carbing their engine than should be. It's one of those old rules of thumb that's hung around since the days when a 327ci engine was considered "big" and stock cylinder heads flowed 200 CFM at most. Most modern motors can actually benefit from a carb that's considerably bigger than the old formulas suggest.

 

Another tactic I would take is to ignore the VE reduction factor, which would up the CFM rating about the same as the 1.0 in-HG correction in this formula.

I would take one exception to your statement about not being able to overcarb with VS: Throttle response can suffer with an over sized VS carb because the air speed in the primaries is lower than it would be with a smaller venturi carb, which reduces the booster signal. Q-jets don't have that problem with their tiny primaries, of course.