Q Jet Carburetor Air Flow

[F-BIRD'88]

Yes the Street Demon carb is a interesting
carb for 2x4 applications. Yup the price is right too.

[pamotorman]

always used the GM "windowed" needle seat in a Q jet to make sure you get full fuel flow. you can make one by drilling a .125 dia hole thru both sides just above the seat if you do not have a GM windowed needle seat available so the fuel does not have to go up past the needle to get into the float bowl

[F-BIRD'88]

And the new Street Demons also have an adjustable flapper door, and would have 4 large fuels bowls, and 4 of the .097 or larger jets, and plastic mid sections, if you choose, and they can be had awfully cheap.

A new single carb body "Six Shooter" version 6 venturi of the Street Demon with 2 mirror image outer google valve secondaries and central primaries
would rock.. Adapable to existing 4 bbl intakes.

Only slightly longer than a 4 bbl carb, same width and linkage.
90+% common parts. 60% more cfm flow.
A very symetric metering flow pattern.

Not a big deal to mod the manifold tooling to make new
specific intake manifolds for this. Both Holley and Edelbrock are missing the mark on this logical progression.
Nice on single carb street blowers. And high power larger cid NA v8's.

[MadBill]

Exna n the predators.. Any one who has played with a qjet knows that adjusting the angle of the throttles @wot is easy.
Not a issue. Yes you can employ setting the wot angle of the wot throttle blades (other than straight up open) to create flow pattern direction if needed.

One can also set, limit the wide open max angle of the air door(s) to create direction and ensure metering velocity @wot
on engines that are say, not as powerfull or where car acceleration rate is not that great.
Thats the beauti of the qjets. Full adjustability to match the application. You can make it as agressive or as tame as needed to work best.
fuel needle seats are available in .125", .135" and .149"..
Various floats etc.
One could even have computer controled primary metering using a 80's "feedback" E- qjet in the primary position (rear carb).. While the 4 BIG power secondaries are all "analog"
No PhD required...

Really I am just looking for a simple answer.
How much do the secondaries flow @wot?

As you say, the max air door opening angle can be varied and for O.E.M. QJs it was very model-specific* (I've seen as little as 45°), so there is no single 'right' answer. *During dyno development, GM used stepper motor-controlled air valves to optimize rate and max angle. Even some heavy-breathing Big Blocks made more power at less than 90° max due to distribution effects.

[Tuner]

As you say, the max air door opening angle can be varied and for O.E.M. QJs it was very model-specific* (I've seen as little as 45°), so there is no single 'right' answer. *During dyno development, GM used stepper motor-controlled air valves to optimize rate and max angle. Even some heavy-breathing Big Blocks made more power at less than 90° max due to distribution effects.

One for the metering rods and one for the air door.

[F-BIRD'88]

Interesting setup.

[vortecpro]

I can tell you this: our stock eliminator Q-Jets flow a total of 890 CFM with 660-680 flowing through the secondaries, our Q-Jets are the bigger CFM Buick versions.

[Fireonthemountain]

And the new Street Demons also have an adjustable flapper door, and would have 4 large fuels bowls, and 4 of the .097 or larger jets, and plastic mid sections, if you choose, and they can be had awfully cheap.

A new single carb body "Six Shooter" version 6 venturi of the Street Demon with 2 mirror image outer google valve secondaries and central primaries
would rock.. Adapable to existing 4 bbl intakes.

Only slightly longer than a 4 bbl carb, same width and linkage.
90+% common parts. 60% more cfm flow.
A very symetric metering flow pattern.

Not a big deal to mod the manifold tooling to make new
specific intake manifolds for this. Both Holley and Edelbrock are missing the mark on this logical progression.
Nice on single carb street blowers. And high power larger cid NA v8's.

One more thing on the Demon possible advantage, if you want a "six shooter" and that is the secondaries are just one big oval, so in theory besides the 3 barrels of the single carb , you could add 3 more big secondary halves of it with the Demons, to get to a six barrel limit. That would be a ton of air flow...

[Tuner]

I can tell you this: our stock eliminator Q-Jets flow a total of 890 CFM with 660-680 flowing through the secondaries, our Q-Jets are the bigger CFM Buick versions.

There is about 50 CFM difference in the large and small QJet primary flow. It has been often repeated the QJet is 750 CFM (LOL) and that is so if you measure it without an air filter. Just the entry radius your curved index fingers provide if you lay them on the air cleaner gasket flange above the secondary bores will pick up 30+ CFM. In the big fuss about fancy "improved" fuel inlet fittings the air inlet fitting is overlooked. QJets (and Holleys, for that matter) don't like a velocity stack type thing where the air entry is straight down (vertical, parallel with the air cleaner stud) for several inches above the gasket flange, a radius approach to the gasket flange flows more air at the same bench depression.

[MadBill]

.. QJets (and Holleys, for that matter) don't like a velocity stack type thing where the air entry is straight down (vertical, parallel with the air cleaner stud) for several inches above the gasket flange, a radius approach to the gasket flange flows more air at the same bench depression.

That's probably why a lot of engines lose power with a K&N X-Stream filter top...

[Tuner]

That, and those thick lids encroach on the bowl vent openings. Any disruption of flow near the vent, or placing the vent in boundary layer flow, causes erratic bowl pressure.

[pdq67]

I can tell you this: our stock eliminator Q-Jets flow a total of 890 CFM with 660-680 flowing through the secondaries, our Q-Jets are the bigger CFM Buick versions.

You mean the 800 cfm carbs with the small tit in the venturies?

pdq67

[kimosabi]

always used the GM "windowed" needle seat in a Q jet to make sure you get full fuel flow. you can make one by drilling a .125 dia hole thru both sides just above the seat if you do not have a GM windowed needle seat available so the fuel does not have to go up past the needle to get into the float bowl

I noticed they sell the non windowed seat "for no fuel drain back" purposes. I can't remember drain back ever being an issue.

I can say I noticed better response replacing the X-stream filter top with a closed top with the "donut". It got tighter but I think it speeds up the air and directs it better. Using a rounded base entry, not the common angle drop base type.

[Geoff2]

I don't think the window in the seat adds any flow. The restriction to flow is the small 0.125" hole. If you calculate the area between the needle & inside wall of the seat [ whee the fuel needs to flow to get into the bowl ], it will be greater than the area of a 0.125" hole.

Carbs. Can you fit 850 cfm Carter TQs, used on 360-400-440 Mopars? Best production carb made. Or, if the budget allows, a pair of 1000 cfm Comp Series TQs, # CS4847.

[pamotorman]

always used the GM "windowed" needle seat in a Q jet to make sure you get full fuel flow. you can make one by drilling a .125 dia hole thru both sides just above the seat if you do not have a GM windowed needle seat available so the fuel does not have to go up past the needle to get into the float bowl

I noticed they sell the non windowed seat "for no fuel drain back" purposes. I can't remember drain back ever being an issue.

the drain back can be stopped with a check valve fuel filter. when GM went to the fuel pump with a tank return to remove the pressure between the pump and the carb for emission reasons they went to the check valve filter to prevent the float bowl from draining down when the engine was off