The large cfm units typically dubbed "800" cfm will flow 847cfm without any grinding, sanding, or polishing anyplace.
Removing the booster rings on those units takes them up to 897cfm.
We reserve that modification for engines making over 600hp.
I've drag strip tested the smaller castings (750), the 1971 HO castings (827cfm), the later "850" cfm units, and the larger 897cfm carbs at the track on my own car.
Even the 750cfm castings are pretty good at keeping up with my 455 engine over 550hp. I only lost about .03-.04 seconds and less than 2mph.
The HO carburetor (827cfm) and my own 1977 unit (850cfm) ran just about dead even on back to back runs.
The improvement moving up to the larger cfm unit was barely noticable, but it did run a few hundreths quicker and about 1mph faster.
It's also interesting that I have one customer running a legal Super Stock Camaro with a small block running mid 9's around 150mph with a smaller "750" cfm casting.
I would also mention that NHRA outlawed the 1971 HO castings (single booster) models for drag racing in all Classes, unless you are running a 1971 HO Pontiac entry.
Anyhow, I do NOT recomend removing the outer booster rings for most set-ups. Even on big HP large CID engines, the slight improvement in top end power comes at the loss of throttle response and efficiency right off idle. You loose the booster area, and it takes more throttle angle to pull fuel from the jets. This keeps the carb on the transition curcuit much longer, so fuel economy and driveabilty suffer.
The triple booster area provided by the standard Q-jet design is what makes them superior to aftermarket carburetors for "normal" driving. When you remove this feature, they act and drive much like a Holley carburetor, using more fuel and taking more throttle angle for light throttle cruising, etc.
Like a Holley, they can still be tuned to work pretty well, but you will never acheive the same efficiency without the outer booster rings in place.
One advantage to this modification, is that you will have a lot more cfm available for spirited driving on the primary side only, so you can acheive higher vehicle speeds without bringing in the secondaries.
In any case, I'd reserve this modification for BIG cid engines and big power levels, where one wants to continue to use a Q-jet. They still offer some nice advantages for this sort of deal, one is fully adjustable secondary opening rates and tuning full throttle A/F in seconds without taking the carb apart......Cliff
