Q Jet Carburetor Air Flow

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F-BIRD'88
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Q Jet Carburetor Air Flow

Post by F-BIRD'88 »

What is the proportional air flow percentage split of air flow thru the primary and secondary sides
of a Q Jet Carburetor ? EG: 60/40.....70/30..... ?
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Re: Q Jet Carburetor Air Flow

Post by mag2555 »

I have found that a lot can depend on the air cleaner base that is used with them!
The 800 cfm Carb with one instead of two primary boosters flows 25 % of the total thru the primary's.
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Re: Q Jet Carburetor Air Flow

Post by F-BIRD'88 »

So you'd say then that the secondary side flows 600cfm @1.5" When full open WOT?
The secondary side is essentially the same on all the Q jet carbs, so they should all flow the same on the secondaries side of the carbs (600cfm @1.5") ??

Or....
(what I am really after is the air flow of the secondaries of a Qjet carb.)

I am not really interested in the 800cfm Qjet. I am more interested in the earlier (pre 1975) simpler
4MV (725-750cfm) and the common 795cfm QJets.
Want to know the secondary sides air flow @1.5" of these.
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Re: Q Jet Carburetor Air Flow

Post by mag2555 »

Why do you need to nail down the secondary numbers so tight?
You posted asking for percentages and now want air flow numbers , which I may have I just need to find some notes tonight that I hopefully still have.

I know I have moded the primary's and pick up 30 cfm in 1.5 hours.
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Re: Q Jet Carburetor Air Flow

Post by F-BIRD'88 »

I do no need to mod the carb for more air flow. The primary side does not matter.
I want to know how much the secondaries flow.
The scheme is to use 2 Qjets mounted in line but disable and block the primary side of the front carb.
It will just be a "secondary carb" The front primaries of the front carb are not exposed to the plenum.
A six venturi setup. Central small primaries. (front of the rear carb) 4 Secondaries.
The primary throttles blades of the front ("secondary Qjet") are removed but shafts and all linkage in tact.
No air nor fuel flow in the front primaries. primary jets blocked.
Motor idles and drives part throttle on the primaries of the rear Qjet.
When at WOT all six barrels open.
Should be about 1250-1350 cfm @WOT 1.5" (725-750 +480-600ish)

2 carbs mounted in line on a carb adapter..front rear bias of the carbs over the plenum adjusted as required
for best flow pattern. Again the primaries of the front carb are not exposed to the plenum and do not function.

How much does the secondary side of a Qjet flow?
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Re: Q Jet Carburetor Air Flow

Post by 427dart »

F-BIRD'88 wrote:I do no need to mod the carb for more air flow. The primary side does not matter.
I want to know how much the secondaries flow.
The scheme is to use 2 Qjets mounted in line but disable and block the primary side of the front carb.
It will just be a "secondary carb" The front primaries of the front carb are not exposed to the plenum.
A six venturi setup. Central small primaries. (front of the rear carb) 4 Secondaries.
The primary throttles blades of the front ("secondary Qjet") are removed but shafts and all linkage in tact.
No air nor fuel flow in the front primaries. primary jets blocked.
Motor idles and drives part throttle on the primaries of the rear Qjet.
When at WOT all six barrels open.
Should be about 1250-1350 cfm @WOT 1.5" (725-750 +480-600ish)

2 carbs mounted in line on a carb adapter..front rear bias of the carbs over the plenum adjusted as required
for best flow pattern. Again the primaries of the front carb are not exposed to the plenum and do not function.

How much does the secondary side of a Qjet flow?
Very interesting setup like having a Dominator with a small 2bbl in the middle for cruising. Now you would have to have the secondary air doors set rather light to get full air flow from a pair of those secondaries
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Re: Q Jet Carburetor Air Flow

Post by F-BIRD'88 »

Higher air flow capacity Higher horsepower capacity.
Double the fuel capacity. 60%-70% more air flow than a single carb.
Can work on a Tunnel ram...Can work on a cross ram.
Can work on a single four bbl intake both single plane and dual plane.
Can work on a modified "dual Quad' intake.
All metering tunning adjustments are easily done from the top.
(90% of that will be swapping secondary rods and hangers and air doors adj.

Good fuel economy when cruising and idling with the small central located primaries.
Easy serviced with ready available parts and rebuild kits.
Adjustable air flow amount and rate to WOT.
Potential for use on supercharged motors both as blow thru (in a air box) and draw thru (roots)

Potential for "dual fuel" Gasoline-primaries methanol mix or ethanol mix secondaries.

Poor Man's "Six Shooter" induction.
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Re: Q Jet Carburetor Air Flow

Post by F-BIRD'88 »

There is no rule saying you could not use a Eddy AFB in the rear "primary carb" position
and a Qjet on the front "secondary carb" position, If thats what you got on hand.
Still easy to do.

No Rules. "Six Shooter"
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Re: Q Jet Carburetor Air Flow

Post by F-BIRD'88 »

Yes the secondary air door opening rate on a Qjet is fully and easily adjusted with a screw and lock nut.
(Lil flat screwdriver and allen key)
No issue making the air door spring tension light. The little vacuum break diaphram on the side of a Qjet
creates a slight delay of the air door opening timing as you go WOT to avoid a bog. It is also adjustable (bleed/time delay) rate. This is retained and used on both carbs, as OEM intended.

Do not strip off all the carb throttle/choke/fast idle set/release linkages. These all tie into the secondary air door control
as the OEM designed it.
All the OEM linkages of both carbs remain functional. Even the choke of the rear "primary carb" is functional.
(can be manual w/choke cable or automatic)
Both carbs are joined with a simple "dual Quad" linkage. Rear carb uses a stock GM throttle cable.
The only real mod to the front "secondary" carb is removing (grind off) the bottoms of the primary throttle plates, Throttle shafts and all linkage is used.
Blocking the primary jets and removing the accelerator pump arm link to disable it.

The Chevy style QJEt with the 90deg right angled fuel inlet fitting would be the most desirable
choice. I like the older simple pre 1975 4mv chevy Qjets. This allows simple inline mounting the 2 carbs as close together as possible.
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Re: Q Jet Carburetor Air Flow

Post by 1972ho »

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Re: Q Jet Carburetor Air Flow

Post by mag2555 »

A single Q-jets fuel bowl is good up to about 600 hp on a stationary dyno , so two will easily let you top that hp number,But do not forget that the full open position of the secondary throttle blades in these Carbs are set up for use on spread Bore duel plane Plenum.

Your also gonna want the type of Q-jet model with the small square notch in the leading edge of the secondary air flap.

Of course if you get 2 Preditor Carbs you will save your self a bunch of time if that matters and headaches very likely too!
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Re: Q Jet Carburetor Air Flow

Post by F-BIRD'88 »

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?
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Re: Q Jet Carburetor Air Flow

Post by mag2555 »

I did not have time last night to dig out my notes on these, but how about you look at it from a purely HP potential point of view?
These Carbs can support 600 hp, if we back out 25% for what the primary side supports out of that 600 hp then each Q-jet scondary on its own can support 450 hp.
Is this not what matters in the end?
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Re: Q Jet Carburetor Air Flow

Post by F-BIRD'88 »

Yes that is another good way of evaluating the potential.
Where it matters... How much horsepower can it support.

One of the +'s is it can be dialed in for a lot of applications
up to that point.

And relative low cost too. A alternative to a dominator carb
on a dominator intake. Street!strip brackets marine.
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Re: Q Jet Carburetor Air Flow

Post by 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.
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