CFM for QJ on 1984 Trans AM (export only edition)
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CFM for QJ on 1984 Trans AM (export only edition)
My Trans AM has QJ & no CCC stuff. Juat a simple QJ like I had in my '72 Buick GS.
My engine is 305 with 150 hp. Would the cfm rating be 650?
My engine is 305 with 150 hp. Would the cfm rating be 650?
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Car: 92 Firebird, 77 Trans Am SE, 86 Z28
Engine: 5.7 HSR, T/A 6.6, empty
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Axle/Gears: 3.08 posi, 3.23 posi, 3.23
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.
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T.L. (07-31-2022)
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Car: 1983 G20 Chevy
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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.
Last edited by Fast355; 07-31-2022 at 02:18 PM.
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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.
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.
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Car: 1983 G20 Chevy
Engine: 305 TPI
Transmission: 4L60
Axle/Gears: 14 bolt with 3.07 gears
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.
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.
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HawkZ28 (08-10-2022)
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Re: CFM for QJ on 1984 Trans AM (export only edition)
Look for a bump/hump in the primary bore. I seem to recall this being in Cliff's book as well.
Post #8 in the thread below has a photo showing the hump I'm talking about:
https://gbodyforum.com/threads/q-jet-rebuild.70582/
Post #8 in the thread below has a photo showing the hump I'm talking about:
https://gbodyforum.com/threads/q-jet-rebuild.70582/
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#8
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; 12-20-2022 at 09:48 AM.
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Car: 1983 G20 Chevy
Engine: 305 TPI
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Axle/Gears: 14 bolt with 3.07 gears
Re: CFM for QJ on 1984 Trans AM (export only edition)
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.
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