Thread: Quadrajet

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Difference in altitudes...and apts there used on....

Originally posted by dr_bowtie
Difference in altitudes...and apts there used on....

those # are dif. Q-JET venture sizes, dont have anything to do with altitudes.

I just had a bright idea, should we just talk about 1 make of carb so we dont get confused with different operating systems. I would nomkinate a holley I know this thread is on quadrajets. But the holley bhy it,s modular design makes it infinitely tunable and over the years lots of ideas have evolved on this particular carburetor.

Personally, i would have to lean towards rebuilding the factory Q-jet. Being a 327 that places it as a 60s motor. Should be safe to say its a 700 or 750. As such, that would be the range to look for. "If it aint broke, dont fix it" But as for the holley vs rochester primary debate, I'll have to agree with the Q-jet guys. due to the smaller primaries, the motor is essentially sipping through a straw untill you activate the secondaries. bad for power, good for mileage. However, being a mere 327, there should be enough flow between the small primaries and larger secondaries under heavy throttle for a little fun. Just remember, if you are going to rebuild your first Q-jet A)take a picture when you open it up B) never throw Anything away and C)compare every piece with that your replacing. due to the long run of Q-jets and emissions legislations during the later years, many small differences were made in the quadrajet line. As such, be sure that you have the correct rebuild kit. Nothing quite like placing the wrong float inside a non-emissions carb.

Dont get me wrong the Quadrajet is a good carburetor, it has had changes over the years but it is still physically the same carburetor, and they are a simple carb once you figure all the subsystems out.

Now having said that, look at what has happened to the Holley. For example look at the different booster venturies, There is straight, downleg and annular discharge, they all make differences to the airflow and fuel atomisatuin characteristics. The carburetor body is the same like main venturi sizes but it is the little differences that stack up.

Originally posted by southerner

Dont get me wrong the Quadrajet is a good carburetor, it has had changes over the years but it is still physically the same carburetor, and they are a simple carb once you figure all the subsystems out.

Now having said that, look at what has happened to the Holley. For example look at the different booster venturies, There is straight, downleg and annular discharge, they all make differences to the airflow and fuel atomisatuin characteristics. The carburetor body is the same like main venturi sizes but it is the little differences that stack up.

southerner, every thing you have explained in your last post is still a way to get max. out of a motor, not a way so you can put that carb. on a lot of dif. motors. if you have a motor that needs 850 cfm to make it perform its max. then you can't put a 600 cfm carb. on it and expect it to perform to its max. the Ventura is not gonna change. you could put it on a motor that called for a 500 cfm carb and make it work about 1/2 way, but that's all it will do. i dont know where you and dr_bowtie have come up with the idea that a holly carb. is a 1 fit all. im not talking about 68Gem, he needs a 600 or less, but im talking about some statements that has been made about carb. as a whole. there is a certain cfm needed for every dif. motor and you take the closet thing holly makes and bolt it on and re jet it to fine tune it. the venturi size is for WOT and you fine turn it below WOT with jets, power valves,and vac. secondarys to perform at lower speeds. the Q-JET gives you a little more room to play with, because it has an adjustabul secondery venturi with a lot more choices of jets and metering rods. you can fine tune a Q-JET a lot more than a holly.

Listen here '68gem, There is a lot of good carb theory goin' on here, and you should by all means absorb it and learn it. I don't know the extent of your expertise, that's why I say this. Most of what's bein' said here is true. I don't know about the only 2 sizes of Q-jet thing. I could've sworn my SBC 400's was what they considered 835 CFM, according to the research and books that I've read, but that's another post. If I were you I would keep the stocker and rebuild it. You're probably not going to get better than that for your mileage. I'm also gonna get a liitle off your original question. I'm guessing this is on your '68 3/4 ton truck? You've got a lot to think of beside carburation for your mileage on that tank. A 3/4 ton truck is goin' to be built heavier, stronger, and geared differently. My first guess would be that (if it's runnin' right) it could pull a stump out of the ground, or climb a tree. You shouldn't expect too much from what you're drivin' on the mileage side. That truck is probably geared way down from the factory. You can get better mileage, but don't expect much. I've seen some that do all right, but I wouldn't expect much more than 12 MPG out of a 3/4 ton truck, especially one of that year.

Like I said listen to what is being said here, there is some good theory bein' explained to you. Some good formulas that you can use for future reference as far as carburation goes, but look at the whole picture, not kust one aspect. Mileage is a combination of many things, including carbs, gears, cams, exhaust, etc..

Originally posted by lt1s10
southerner, every thing you have explained in your last post is still a way to get max. out of a motor, not a way so you can put that carb. on a lot of dif. motors. if you have a motor that needs 850 cfm to make it perform its max. then you can't put a 600 cfm carb. on it and expect it to perform to its max. the Ventura is not gonna change. you could put it on a motor that called for a 500 cfm carb and make it work about 1/2 way, but that's all it will do. i dont know where you and dr_bowtie have come up with the idea that a holly carb. is a 1 fit all. im not talking about 68Gem, he needs a 600 or less, but im talking about some statements that has been made about carb. as a whole. there is a certain cfm needed for every dif. motor and you take the closet thing holly makes and bolt it on and re jet it to fine tune it. the venturi size is for WOT and you fine turn it below WOT with jets, power valves,and vac. secondarys to perform at lower speeds. the Q-JET gives you a little more room to play with, because it has an adjustabul secondery venturi with a lot more choices of jets and metering rods. you can fine tune a Q-JET a lot more than a holly.

True . I can recall a company by the name of Ford ( think) put some holleys on it's smaller engines and they were rated at 585 CFM's

Lets just get this straight once and for all....
The Q-jet does not have a variable secondary venturi! period
TheQ-jet has vacuum secondaries just like a Holley or Edelbrock....

The secondary air plate is adjustable to fit a variety of engines according to air flow......This is adjustable for the purpose to get the most potentail out the particular engine at task.....

some engines can handle the secondaries quicker than others while others would just bog out...hence the adjustable feature...

these carbs are more adjustable than any other carb made and were made to fit a very wide variety of autos.....Holleys are by far more popular by the general concensus....I have R&R'd Q-jets for 20+ years and base my theory on my experience.....

IT is common to see a Holley on a 9 second ride but when you see a Q-jet on one you know the person who prep'd it must know what they are doing? dare to be different.....

Vacuum Secondaries
Vacuum secondaries allow a person to run a carb that most people believe might be too big for a particular engine. By playing with various secondary springs, you can tailor the secondary opening point and rate of your carb, and can even make them to where they don't open all the way. This allows you to use a carb that might be too big for your application. The diaphragm housing with the removable top is highly recommended. Trying to align the screw holes while keeping the spring compressed is a quick way to pinch off and tear the diaphragm. One or two torn diaphragms is enough to pay for the quick change kit. I know this from experience.
On some Holley's, there is a steel check ball in the passage that actuates the diaphragm. Leave it there. If you take it out, the secondaries are likely to flop open very quickly and give you a bog.
If your vehicle is heavy, like a motorhome or 4x4, you should always use vacuum secondaries, because you need to be able to keep the secondaries closed during cruise. Since these vehicles are so heavy and often use high rpm rearend gears, your carburetor opening is often so large that many double pumpers will open the secondaries during cruise, causing horrible gas mileage. With vacuum secondaries, you can change the secondary spring to keep the secondaries closed for better gas mileage, yet still have them open during passing and hillclimbing.


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sure you can do this with a holly and a q-jet, but i dont belive you gonna get the maxiam power out of your motor. i thought the idea was to have a carb. that would get the most out of your motor.

as far as a q-jet having a vacumn secondary, it dont have to be i've taken the vacumn diaphragm housing off of every one i've ever owned and threw them away. by adj. the spring on the top butterflys to open when i wanted them to and it is a type of a veribal venturi by the spring being adj. on the big buttersflys to let it open late or quick according to how much air the motor can pull through the carb. to me thats the way the predator works and that is a veribal venturi.

Thanks to everyone for their replies. It has been many years since I have modified an engine and your replies have been very helpful.
At this point I am still infaturated with my new truck. It is in excellent condition for its age and I'm having a blast working on it and driving it. However, I got a dose of reality today.
I put in a new battery and things began to smoke. It seems some one replaced the battery cables, using black for positve and red for negative.