Carb size calculators

[ChargerST]

I cam across two different calculators that should help to determine the carb size for your engine:

http://www.holleycarbs.com.au/chooseCarb.htm

http://www.wallaceracing.com/intakecfm.php

Are these calculators accurate as everybody seems to run carbs that should be too big according to the calculators. Most people seem to run 750cfm carbs for a 440 - the holley calculator recommended a 650  instead (HP engine, 6000 rpm max).

On my current engine I have a 625 Carter and wanted to swap it out for a QFT 735 VS but the calculator tells me that carb is too big?

[Challenger340]

yes,
they are accurate according to theoretical parameters to maintain "best" stocchiometric fuel mixtures across the widest bands, and a balance of conditions....
that said,
Altitude, or better "DA"(Density Altitude) of the inlet air, being one of the factors not listed, and one of the primary reasons Guys who DYNO will cheat elevation, by applyíng larger Venturi "Holes", to get more airflow with less pressure, hence larger Carbs,
called "VE" or Volumetric Efficiency of the particular Engine application, which is the "actual" airflow the Engine demands from HeadFlow & Camshaft.

....Unless actual "VE" is measured on an Engine DYNO for a particular Engine, and DA, Carb Size can never be written in stone....hence most guys default to slightly larger carbs, especially at any elevation from sealevel with Cam/Headwork.

IMO

[Chryco Psycho]

The Other point is How accurate is the flow # for the carb
at any rate closer to 2x the CI is a closer start point than the formulas are , kepping in mind from 71 on the 340 & 360 4bbl engines used an 850 TQ

[frederick]

Holley are careful; a bigger carb is more difficult to tune.
Everything is much more critical when you install a bigger carburetor.

Let me try to explain.

The flows Holley list are at 1.5"Hg for 4 barrels an 3"Hg for 2 barrels.
Take a 600DP carb for example, this means that you get a 1.5"Hg pressure difference across the carb at 600CFM.
If you install a bigger carb the pressure difference will be less and horsepower will increase, provided you can jet it correctly.

There is a limit to this; the bigger the carb the less airspeed through it and the less vacuum the circuits will be seeing, making them less active.
In other words, if you go bigger on the carb it will be more difficult for the carb to pull the fuel needed.

[BSB67]

I cam across two different calculators that should help to determine the carb size for your engine:

http://www.holleycarbs.com.au/chooseCarb.htm

http://www.wallaceracing.com/intakecfm.php

Are these calculators accurate as everybody seems to run carbs that should be too big according to the calculators. Most people seem to run 750cfm carbs for a 440 - the holley calculator recommended a 650  instead (HP engine, 6000 rpm max).

On my current engine I have a 625 Carter and wanted to swap it out for a QFT 735 VS but the calculator tells me that carb is too big?

These calculators give way different results....and doesnt that answer you question?
Generally, I have found for maximum track performance the Holley calculator is way off (too small).  There are other variables.