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Re: CFM for QJ on 1984 Trans AM (export only edition)
Only 2 cfm versions of a qjet I've ever heard of were 750 and 800. The key to having a large cfm number like that to something so low on power was the secondarys only open enough per the engine demands (if calibrated correctly). The primaries being small and the secondaries being "variable" make the "large" cfm work.
Re: CFM for QJ on 1984 Trans AM (export only edition)
The later Q-Jets are typically the larger primary venturi 850 cfm model and they have a tab on the air door that restricts its opening down to about 650 cfm. Edelbrock used the same casting for their Performer RPM 850cfm Q-Jets. Just have to adjust the air door opening correctly to get ~850 cfm on them not that a 305 needs it or could ever use more than the ~650 stock.
Re: CFM for QJ on 1984 Trans AM (export only edition)
The early pre 1975 4MV style QJet was about 750 cfm wide open. The post 1975 style QJET is 795 cfm. the Buicks some Pontiac big blocks had a 850 cfm QJet Like the Edelbrock 850 QJET.
The difference is the size of the primary venturi and its booster venturi.
All can run on small cid /Power engines as the secondary air door opening rate is adjustable to match the WOT demand of the engine its on.
Re: CFM for QJ on 1984 Trans AM (export only edition)
Originally Posted by F-BIRD'88
The early pre 1975 4MV style QJet was about 750 cfm wide open. The post 1975 style QJET is 795 cfm. the Buicks some Pontiac big blocks had a 850 cfm QJet Like the Edelbrock 850 QJET.
The difference is the size of the primary venturi and its booster venturi.
All can run on small cid /Power engines as the secondary air door opening rate is adjustable to match the WOT demand of the engine its on.
The large venturi post 75+ style is 850 cfm. Some of the 80s small blocks also had the 850 cfm version, FWIW. Just ask Cliff Ruggle, he will tell you the same thing.
Re: CFM for QJ on 1984 Trans AM (export only edition)
The key to having a large cfm number like that to something so low on power was the secondarys only open enough per the engine demands (if calibrated correctly).
Last edited by mydanielcreg_3; Dec 20, 2022 at 09:48 AM.
Re: CFM for QJ on 1984 Trans AM (export only edition)
Originally Posted by mydanielcreg_3
The key to having a large cfm number like that to something so low on power was the secondarys only open enough per the engine demands (if calibrated correctly).
GM had a physical limiting tab on the air doors. Some like the 307 Olds only opened to about 30 degress and maybe 500 cfm. The 305/350 truck carbs were limited to around 45-50 degrees and about 650 cfm. Both of those applications above used the 850 cfm castings. Have to grind the tab for the opening distance I posted a picture of to get to 850 cfm. Also have to make the throttle blades fully open.
