Just what gives the '71 and up QJs their 800 CFM rating vs the 750 rating of the 1970 version? I think I recall from years ago that it has to do with the opening of the air valve and that there is a tab on the air valve that can be filed down to make the valve open further to make the carb into an 800? Am I remembering correctly?
I would think if you could make a 750 into an 800 then everyone would do it. Throat size cannot be changed to flow more CFM I wouldn’t think.
This has been covered many times here. The primary side of the Q-jet has a triple venturi, the last one being the outer wall of the primary. The 800 is larger by 4/32" or 1/8". Here is the 1972 Q-jet specs for example, Look at the 455 Q-jets, under large venturi, 1 7/32" vs. 1 3/32". That larger outer wall has a small bump to clear an internal passageway, The larger Q-jet is jetted richer because it is bigger. Starting in 1971, all Q-jets fitted to the big block were the larger 800 CFM.
Thanks for bringing me up to date, Larry. I read the Quadrajet bible many years ago but the brain has lost some over the years.
There is some truth to the air flap mechanical stop, but it's not the defining characteristic of the 800 vs 750. Rather, the mechanical stop on the end of the air flap shaft differed between application, say 455 vs 350 vs 231. Etc So the 800cfm designation, or 750 cfm even, is a representation of the maximum potential flow of the carb, which.can be limited by the air flap mechanical stop. If I put a 231 v6 air horn on an 800cfm BBB455 main body, it wont flow 800cfm easily. Just an example
I think before I did that to a correct/date coded 1970 GS455 carb, I'd pick up another carb and build it up. Interesting write-up though.
The factory actually made Quadrajets that were missing the outer booster rings but used the smaller castings. They were produced in 1971 and used on Pontiac engines that year. They flow 827cfm. For quite a few years they were highly sought after and folks running in several "stock" racing classes were using them. At some point they did a rule change and folks were banned from using them unless they were on the original application, so the popularity declined considerably. As good as they work for racing the single booster models aren't any better anyplace than the later large CFM units using the outer booster rings. I've back to back drag strip tested them and find no advantage anyplace using one as they run almost the exact same MPH and ET as the larger units. My engine is pretty stout, makes over 550hp/600tq, so for sure at lesser power levels looking for more airflow isn't going to bring much to the table. It's interesting to know that starting in 1971, the same year Pontiac used the 455 "HO" carburetors w/o the outer booster rings is that Buick 455's used the first of the larger castings. The 1971-74 Buick 455 carburetors are excellent units despite being produced when emission regulations were getting tighter every year. The factory had to lean them up quite a bit to get those big 455's thru emission testing but they were efficient enough for the task. Starting in 1975 the newer models still got the big primaries and all front inlet Q-jets, Buick, Oldsmobile and Pontiac used them until 1981 when the CCC carburetors showed up. Another interesting fact is that to this day folks refer to Q-jets as either "750" or "800" cfm when many had pretty long stops on the secondary air door to limit cfm. According to Edelbrock their "famous" 1910 model flows "850" cfm. It was modeled after the later Chevrolet truck carburetors part numbers 17080212 and 17080213. Edelbrock gave them a very short stop for the secondary flaps allowing them to open much further, but otherwise the rest of the carburetor is no different than a plain old 1980 Chevy truck carb. If you have my book Chapter 7 describes them in greater detail. So if one looks at a 1971-74 Buick 455 Q-jet closely they share the same primary bore size as the Edelbrock 1910. The big Buick carburetors also have a short stop for the secondary air-flap shaft. So what is keeping the big Buick carburetors from flowing "850" cfm?........Cliff
All very interesting. So how does removing the outer booster ring effect atomization/ economy running around on the primary's on the street with a street/strip big block application?
You will loose some efficiency and have to recalibrate to get the fuel flow to the boosters back. What you will gain is some noticeable heavy throttle improvements w/o the seondaries in. It will act much like a Holley carb, heavy fuel consumption included. I reserve that modification for "max-effort" stuff. Even my own carb has the outer rings and my engine is up near 600hp. The larger castings with the booster rings removed and airflap stop ground correctly flow 897cfm, more than most of these engines will ever need......
