Thread: Quadrajet

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I have a '68 GMC pick up with a stock 327, topped with its orignal quadrajet. I am building it to be a daily driver so I want as much gas mileage as possible, not just performance.
I have had a lot of conflicting advice about what carboration would be best for this truck. I have had three versions of the quadrajet recommended. I am not sure how they differ from my stock carb or if it would be worth changing. Any insights?

Well I run a little of everything on my rides a Holley on two of them, a quadrajet on my 71 Chevy truck an Edelbrock on my Scout and fuel injection on my Ford. I think other than fuel injection which would probably do best for milage, the next best carb would be the q jet and I would use the one that is on it. You can always change jet sizes and metering rods until you get it just like you want it. Mine gets great milage compared to my others. As long as I keep my foot out of it that's hard to do sometimes.

the 795cfm unit is the best choice...these started in 75 and went into the 80's ....they are the pre smog/electric carbs....they can be ID'd by the big choke housing....some are electric chokes and hook up with just one wire.... and can be easily modified...

Qjets don't work like other carbs, so you're ok running a 750 or 795 cfm carb even on a small engine. You will get the best milage with the Qjet

a Q-JET will work just fine and a 750 cfm Q-JEt is the same thing as a 750 cfm HOLLY. the HOLLY is the odd ball. the Q-JET, the old AFB, and the EDELBROCK works the same way. they use metering rods and jets and HOLLY use metering blocks and jets.

[QUOTE]Originally posted by 68Gem
[B]I have a '68 GMC pick up with a stock 327, topped with its orignal quadrajet. I am building it to be a daily driver so I want as much gas mileage as possible.

What is going on here? I just went outside to see if it was a full moon or something?
This must just be my night to disagree.The man just said he has a bone stock 327 and most of you are suggesting carbs. big enough for a 454.
A 1965 Corvette 327 -350 hp had 11 to 1 compression, dome pistons,461 heads, 202/160 valves,3in.center dump manifolds,and a high performance hydraulic cam and alumn.intake
Guess what??? It had a holley carb.--less than 600cfm. if my memory serves me correctly it was 565cfm.
68 Gem. if you want a little more performance put a mild cam in it comporable to the original 327-350hp cam or certainly no bigger than a 268 comp. cam. use your same carb or no bigger than a 600 cfm.
Summit and Jegs both sell matched sets of Cam, lifters,intake(with or without carb) They are priced pretty reasonable.
Remember with a stock 327 You don't want to go too big. In Your case bigger is NOT better.

[QUOTE]Originally posted by riverhorse59
[B]

Originally posted by 68Gem
I have a '68 GMC pick up with a stock 327, topped with its orignal quadrajet. I am building it to be a daily driver so I want as much gas mileage as possible.

What is going on here? I just went outside to see if it was a full moon or something?
This must just be my night to disagree.The man just said he has a bone stock 327 and most of you are suggesting carbs. big enough for a 454.
A 1965 Corvette 327 -350 hp had 11 to 1 compression, dome pistons,461 heads, 202/160 valves,3in.center dump manifolds,and a high performance hydraulic cam and alumn.intake
Guess what??? It had a holley carb.--less than 600cfm. if my memory serves me correctly it was 565cfm.
68 Gem. if you want a little more performance put a mild cam in it comporable to the original 327-350hp cam or certainly no bigger than a 268 comp. cam. use your same carb or no bigger than a 600 cfm.
Summit and Jegs both sell matched sets of Cam, lifters,intake(with or without carb) They are priced pretty reasonable.
Remember with a stock 327 You don't want to go too big. In Your case bigger is NOT better.

i wont suggesting a 750 carb, i was just correcting where the guy said a Q-jet was dif. than other carbs. i'd put the stock Q-JET carb, on it.

even the very smallest of Q-jets were 750cfm and these came on everyday 327's...the gen II 795cfm unit is better unit...Either unit will have better off idle throttle responce due to the smaller primaries.....

I have said this before....You really canot over carb and engine, The engine will only take what it needs, due to the vacuum pulling thru the carb...derminded by its displacement...

The engine does not know what carb it has on it nor does it care...It does care if it is set right for it to use!

Originally posted by dr_bowtie
even the very smallest of Q-jets were 750cfm and these came on everyday 327's...the gen II 795cfm unit is better unit...Either unit will have better off idle throttle responce due to the smaller primaries.....

I have said this before....You really canot over carb and engine, The engine will only take what it needs, due to the vacuum pulling thru the carb...derminded by its displacement...

The engine does not know what carb it has on it nor does it care...It does care if it is set right for it to use!

the smaller Q-jets was around 560 or 580, i don't know exactly what it was but it wont 750. and I know what you have been saying about what carb to run and there might be some people here that will believe you and bolt on a carb that's to big for them. if what you're saying is true you wouldn't need but one carb. a predator is a verbal Ventura and will do what you say, but that's the only one. I cant imagine where you come up with this idea, because its not true and never has been. the formula for finding the right carb.for a is engine CFM = (engine CID x max rpm x volumetric efficiency)/3456 with the volumetric efficiency being about 85% with stock stuff and will go up some with a dif heads and cam, still under 100 %. anything bigger than this formula says is to big. this is for all around driving. at the strip at WOT all the time you could use something a little bigger, but not much. take a 400 CID x 6000 RPM x 100% volumetric efficiency / 3456= would need less than a 700 cfm carb and this is a formula that been used by just about everyone. if you're running as good as you say you are bolting just any carb on you motors then I hate to think how fast you could go with the right carb.

sure that equation is a ballpark guess.....if your thinking inside the box....step outside...

yes that equation is the general.....for people who just want to bolt on and go....

vacuum secondaries are all varaible....the air being drawn in pulls the door open..only as much as it needs......

now.....lets jump outside the box.....lets say on a normal 350 the engine is only going to move a certain amount of air thru the cylinder according to bore and stroke.... as you know as the piston moves from TDC to BDC and the volume of air it draws in isn't goin to change.....now that same amount of air flows thru the carb...same amount regardless of cfm.....If you get diiferent size carbs to flow the correct air/fuel ratio for that amount of air...any carb will work....."If you only listen to what your taught you'll never be smarter than your teacher" me 89'

I do not mean to bust anyones chops.... you will how ever notice that a 795cfm Q-jet will not flow any more air or fuel than a 600cfm Holley or Edelbrock because all 3 have vacuum secondaries and the Q-jet having the smallest primaries will give him the best results all across the board....

What makes the later 2 a better option is when more idle fuel is needed...as the Q-jet has smaller idle passages and needs modifacation to work with larger cams (230+ dur@.050)....

what I am not saying is you will have the same throttle responce from a 850cfm Holley double pumper as you would from a 500cfm vac sec carb.....out of the box......

To the newbies....Carb tunning/modding is and art....better left to the pro's....

By all means do not go buy a 850 Holley to put on a motor where a 2 barrel once was or even a stock 4 barrel.....these larger carbs or more race oriented than stock calibrated.....

basic rule of thumb.....

small blocks big blocks

4500rpm 500cfm 600cfm
5500rpm 650cfm 750cfm
6500rpm 750cfm 850cfm
7500rpm 750-850cfm 850-950cfm
8500rpm 850-950cfm 950-1050cfm

This is just the basic......

the biggest problem here is were talking about carbs in general and you keep coming up with a Q-JET. a holly and a Q-JET is two dif carb all together. with a holly if it says 600 CFM then that's what it means, the front venture and rear venture stays the same, you can re jet it up or down a few # and it will work. On a Q-JET the primary venture stays the same, but on the rear there is some verbal venture built into it. the small Q-JET will flow from low 500 cfm up to 750 cfm if you adj. the secondary butterfly properly and change the jets and the metering rods. they came from the factory set up for a stock what every motor it came on. there was about 50 dif. part # for Q-JET because every motor had dif jets and metering rod in them. if you have a Q-JET that came off of a 327, and put it on a 400 motor you would have to loosen up the secondary butterflies and put bigger jets and smaller metering rods for it to be 100%. it would take about 3 dif. hollies to cover that ground. it still come down to you cant just bolt any carb on a motor and its gonna run, its got to be close on the right size venture and jet size.

Oh well I may as well jump into this discussion with a few ideas of my own about carburetion engine size combinations.

I read from the above posts that 1 carburetor can cover a broad range of engine sizes. That is true up to a point with vacume secondaries. The primaries are directly hooked to the accellerator pedal and regulate engine speed and load. The secondaries are sensitive to engine load and vacume only and will only open as much to flow enough air fuel mix as the engine demands. Okay everybody knows this. What I want to add is the term Air velocity through the venturie. We are taught that air flowing through a tube that crimps down accelerates though the smaller opening. This increase in velocity also creates a pressure drop at this point. Now it stands to reason if we increase the velocity by using a smaller venturi, the preasure drop is more pronounced . You can only get away with this up to a point, because you cause a flow restriction. Now this is why different size carburetors have different size venturi and throttle bore combinations. Because they are desined to give optimum best flow at certain airflow velocities. Notice that I said airflow velocity and not engine size or speed. The engine size and speed determine the airflow velocity through the carb. You get tochoose where you want the carb to run most efficiently in the engine power range by choosing different carbs springs and jet size.
I like to think as vacume secondary carbs as a pair of elastic waistband pants one lengh size will fit a number of different abdominal shapes but will get the job done without the expense of going to a custom made pair.

I think you are wrong on the Q-jet theory.....I do not want to get into a heated debate hear but......

A vacuum secondary carb is a vacuum secondary carb...they all work on the same principal weather it is a Q-jet or a Holley or Edelbrock or Carter or Weber or Thermoquad....

If you put ANY carb on a smaller motor then put it on a bigger motor you will have to rejet any of them....

Southerner...pretty much nailed it....

Kinda was....68gem was asking about the best carb for gas mileage...he has a Q-jet now and was told of 3 various Q-jets to use....of those 3 I suggested the right one.... and I am almost positive even the first Q-jets were 750cfm atleast the what my GM Q-jet book tells me! I have over 250 Q-jet on the self so maybe I'll measure some early models tommorro.....

Just for reference....the 795cfm Q-jet was on every GM 260,262,301,305,307 with a 4 barrel....and these are smaller than, wait...ohh, the 327...hum

Originally posted by southerner

Oh well I may as well jump into this discussion with a few ideas of my own about carburetion engine size combinations.

I read from the above posts that 1 carburetor can cover a broad range of engine sizes. That is true up to a point with vacume secondaries. The primaries are directly hooked to the accellerator pedal and regulate engine speed and load. The secondaries are sensitive to engine load and vacume only and will only open as much to flow enough air fuel mix as the engine demands. Okay everybody knows this. What I want to add is the term Air velocity through the venturie. We are taught that air flowing through a tube that crimps down accelerates though the smaller opening. This increase in velocity also creates a pressure drop at this point. Now it stands to reason if we increase the velocity by using a smaller venturi, the preasure drop is more pronounced . You can only get away with this up to a point, because you cause a flow restriction. Now this is why different size carburetors have different size venturi and throttle bore combinations. Because they are desined to give optimum best flow at certain airflow velocities. Notice that I said airflow velocity and not engine size or speed. The engine size and speed determine the airflow velocity through the carb. You get tochoose where you want the carb to run most efficiently in the engine power range by choosing different carbs springs and jet size.
I like to think as vacume secondary carbs as a pair of elastic waistband pants one lengh size will fit a number of different abdominal shapes but will get the job done without the expense of going to a custom made pair.

all thats true southerner the vac. work up to a point and its a small point. how many motors can you put a 600 cfm carb on and run good? any 1 given holly carb want cover but a limited # of cu. in. it would have to have some lead way, do you would be putting a dif. carb. for every motor. this statement is not true. "I have said this before....You really canot over carb and engine, The engine will only take what it needs, due to the vacumn pulling thru the carb...derminded by its displacement" how many motors can you put a 1100 cfm carb on a live? one fit all dont work on carb. except for a predator.