Q-jet

[Lyndon S.]

On a Q-jet four barrel carburetor when one is going 55MPH the two primaries or the only ones working? At what speed do all the barrels come into play 56mph and up? Are they all working at 55 mph? and a lower speed than 55 the two primaries working?

[Patrick H.]

Re: Q-jet

Kelly,

The two primaries are "always" working. The only time they're not is occasionally at idle, depending on how your particular engine is set up.

The secondaries tend to come into play about 3000-3500 rpm, but they are dependent more on engine vacuum (airflow and rpm) than miles per hour. There is no "set" miles per hour for anything on your carb.

I know that I can, without a doubt, go well over 55 mph on any of my Q-jet equipped cars using only the primaries.

Patrick

[Clem Z.]

Re: Q-jet

if the secondary air valves are set correctly they do not open till there is enought air flow thru the secondary mechanical butterflys so it depends on the RPMs of the engine.

[Lyndon S.]

[Clem Z.]

Re: Q-jet

the mechanical secondary barrels start to open at about 40% of the primary opening BUT unless the air flow is enough to overcome the secondary air valve no air/fuel flows thru the secondary. if you are worried about fuel mileage disconnect the secondary because you can run full throttle on the primary as the Q jet is calibrated to run at WOT just on the primary with out leaning out the fuel mixture. i have set up Q jets using 2 throttle controls on boat engines to save gas by just running on the primary and using the the second throttle control to open the secondary if they wanted more power

[Patrick H.]

Re: Q-jet

Kelly,

As Clem and I are trying to explain, it's a matter of AIRFLOW, not automobile speed. I'd bet I can run 100 on my primaries, no problem. It would just take me longer to get there.

The secondaries open when there is AIRFLOW DEMAND for them to open. Otherwise, they stay closed. They may open shortly after you leve the line if you're punching it down, or they might never open if you don't "ask" them to do so. The speed of the car has NOTHING (directly) to do with whether or not the secondaries are opening.

"The secondary side of a Quadrajet is vacuum operated meaning the secondary air valve will only open as far as the motor actually requires. The secondary venturi are large enough that overall WOT (Wide Open Throttle) performance is satisfactory since the motor tells the carb what it needs and when it needs it. The secondary air valves are the sprung loaded larger plates on the top of the carb. Tension is adjustable and can be set for the plates to open quicker or slower depending on how it has been set." - http://www.arkansaspontiacs.org/tech...ockyQ-jets.htm

[Lyndon S.]

Re: Q-jet

Ok then if we can run on the primaries at a higher speed then would one get the same MPG? Are would you still get better mileage at a slower cruising speed say 55 and slower?

[Patrick H.]

Re: Q-jet

Your mileage is "always" better at lower speeds.

And if you're worrying about mileage so much in your Corvette.... ?

Patrick

[Jack H.]

Re: Q-jet

(1) Even with a WHIP, you'll not be able to climb above 4000-4200 RPM using only your QJet's primaries...

(2) As the others are saying, it's not a matter of highway speed (that's a function of power train gearing). It's a matter of engine RPM.

(3) Fuel consumption increases as the cube of velocity. Why?

F=MA or M[dV/dt] meaning force is proportional to MxVxV.

The defintion of mechanical work is W= FxD making it porportional to DxMxVxV so work increases with the square of velocity, given a constant mass in motion.

Power is the time rate consumption of work performed or dW/dt.

Substuiting the identities above.

P= d/dt[W] or d/dt[D x M x V x V]. Now, associate the derivative with the distance component in the parenthesis to presume you're crusing at a constant velocity with a constant mass and you get dD/dt which is a physical identity: V

Therefore, P is proportional to M x V x V x V or mass times the cube of velocity. That's why fuel consumption drops DRAMATICALLY with small reductions in the maximum speed limit....