Holley/QFT 600/650 carb venturi confusion?

[427dart]

I've noticed this before and while researching the old and new Holley carbs that the quoted venturi sizes of the 600 and 650 carbs seem to vary a lot within same CFM rating.
Example....Holley 1850 series 600 CFM Pri. Venturi 1 1/4 and Sec. Venturi 1 5/16 with 1 9/16 throttle bores
Holley 4150 series 650 CFM Pr. Venturi 1 1/4 and Sec. Venturi 1 5/16 with larger 1 11/16 throttle bores. Makes sense!

QFT Slayer Series 600 CFM Pri. Venturi 1 5/16 and Sec. Venturi 1 5/16 with 1 9/16 throttle bores with straight boosters like 1850 Holley
QFT Street HP Series 650 CFM Pri. Venturi 1 1/4 and Sec. Venturi 1 1/4 with of course larger 1 11/16 throttle bores and down leg boosters. Strange.

QFT Hot Rod Series 600 CFM Pri. Venturi 1 5/16 and Sec. Venturi 1 5/16 with 1 9/16 throttle bores and straight leg boosters. Shouldn't that flow 625?
QFT Hot Rod Series 650 and 680 carbs Pri. Venturi 1 5/16 and Sec. Venturi 1 5/16 with of course larger 1 11/16 throttle bores and down leg booster.
Strange as would it not flow more than the above mentioned QFT Street HP 650?

The question is do my Holley 1850 series carbs with milled air horn and larger 650 type 1 11/16 throttle bores flow 650 CFM?

Why the change up with the QFT carbs in venturi sizes in same 600 and 650 flow rates?

What is the advantage of the down leg booster in the 600 and 650 size carbs over the straight leg booster?

In a dual quad setup would the straight leg or down leg booster be better?

NOTE: All the stated QFT carbs had chokes!

[Walter R. Malik]

The topography of the carb inlet, booster contour, venturi shape, exit angle and throttle shaft size & shape all will have a distinct effect upon actual air flow through that carb; and then most companies simply round-off those numbers to whatever they want anyway.

[Casper393W]

It has been my experience that a machine step downleg booster is the best for most combos. If you have a heated manifold such as a low rise stock type deal the straight leg might break about even with the stepped down leg because the heat will atomize the fuel for you. I have have seen great gains ( 20lbft 15hp of torque gain at the rear wheels) on a 4.6 2v Ford going from a straight leg to a machine stepped down leg in a Street HP 650 Holley DP.

I am a huge fan of Annular boosters but you must becareful because on the wrong type app. You can lose power from having the fuel atomized too much! I only would recommend running these on Air-Gap style manifolds or Tunnel Ram setups.

I'm sure others will chime in on this subject... But this is my take on it...

[427dart]

I must mention my 600 to 650 CFM conversion carbs I built using the 1850 main body do use the QFT race type throttle plates with the thin blades and thinned down throttle shafts. It's just I was noticing how the venture sizes vary from small to large in same flow ratings even when using carb bodies with choke housing.
Would be great to see a flow test or even a dyno shoot out between several variations of the same CFM carbs.