This is going to get a bit math intensive but I have included a couple of links to websites that can help with how I got the flow numbers and Holley got their flow numbers.
Post on the PY Board related to flow thru a carb and orifice sizes
Yesterday a very good question was asked about air flow thru a carb vs its advertised number.
And we had a simple discussion and posted a basic formula to get in "the ballpark" (not what "base" the Baseball Runner is actually on during the game, LOL!) That is the point of this discussion, a more Accurate Number.
So, today, I will post up a more accurate formula and explain how it works. Then we will discuss a 750 cfm carb vs a 850 cfm carb vs a 6-pack end carb.
So here is the Formula I use a lot (that came from some posts made by Loren Budzinski (a Pontiac Racer), Myself, and a guy named "Rocco")
Q=0.61*3.14*3.14*2*2/4*[2*32.2*16*62.28/12*0.07484]^1/2 * 60/144
The assumptions in the simple program are:
Gravitational constant = 32.2
Density of air = .07484
Density of water = 62.28 pounds per cubic foot
Orifice diameter is the flow orifice and we want to find out what it flows
Pressure difference (Flow Test Pressure) in inches of water
Discharge coefficient of the orifice: (Sharp edged, nice radius, or perfect radius which = 1.00
Combined coefficients We combine some coefficients because they remain the same in the testing
Orifice area is calculated by the program
Flow is flow cfm thru the orifice at a specified test pressure and the assumed discharge coefficient.
The simple air flow program I used can be downloaded here:
http://www.flowbenchtech.com/forum/
You need to be a member to use most of the spreadsheets created by members of the forum but it is a no cost deal unless you want to send the guy a contribution to help run the flow bench site.
If you go to this link in the "FLOW BENCH FORUM:
http://flowbenchtech.com/forum/viewtopic.php?f=14&t=73
You can download the simple excel spreadsheet and play with the calculations.
It will be labeled Flow Bench Calculations" 40kb so a very small excel file.
All that being said as part of the preliminary work to get a REAL carb flow number.
Assumptions:
a) We have a ASSSUMED 750 cfm 4 barrel Holley Double Pumper Carb with four 1.375" Venturis machined by Holley in the carb and a 1-11/16" Throttle Blade baseplate
750 CFM rated carb at 20.4" of water test pressure on Holley's wet flow stand with Stoddard Solvent running thru the flow passages.
We will assume that the Stoddard Solvent running thru the flow passages is a constant.
4 carb venturis ("Barrels" in Hot Rod Terms) with each one flowing as a rough number (750 divided by 4) or 187.5 cfm at 20.4 " of water.
The Throttle Blades as mentioned are 1-11/16" Throttle Blades
But we are trying to compare our 750 cfm 4 carb venturis carb with a similar 2 carb venturis 2-BBL Holley carb.
So the carb venturi size stays the same, (1.375") the formula for the most part stays the same but the test pressure is higher. 40.8 Inches of water vs 20.4 Inches of water.
So we plug the info (from above) into the spreadsheet we get (using the simple calculator) for the 4 bbl rated carb and we get roughly 186.56 cfm per barrel assuming a .999 discharge co-efficient, 20.4" of water test pressure, a 1.375" perfect radius orifice, and the same test conditions for all of our testing.
We get 746.25 cfm for our 750 cfm rated carb divided by 4 and tested at 20.4" of water test pressure = 186.56 cfm
So that says that if we tested out 2 BBL Holley Carb at 20.4" of water test pressure we would actually be flowing 186.56 times 2 or 373 cfm on our 350 cfm advertised 2 bbl center carburetor. So the actual cfm of the center carb is 373 cfm at the 2 bbl test pressure.
If we plug in the numbers for the end carbs at the same 20.4 test pressure we would get 481 cfm for each end carb or 963 cfm for the end carbs and 373 cfm for the center carb or 1336 cfm for the whole 6-pack system.
Two thinks to note here. The end carbs on a 6 pack system have larger venturis vs the 373 cfm 2 bbl carb (750 cfm 4 bbl carb).
The venturis on the end carbs are 1-9/16" or 1.5625" for the venturi size (Same as a 850 cfm Holley carb venturi) but at the higher test pressure would flow more cfm. So that is how I got the 1300+ cfm (4 bbl rated number) vs the other posters number.
Hope that helps out on the actual calculations.
The 1.414 is a good simple conversion though.
Tom V.