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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 :3dSMILE: 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. :HMMM: :cool:
[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. :confused: :confused:
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]Quote:
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. :confused: :confused:
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. :whacked:
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. :HMMM: **)Quote:
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!
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.
:HMMM:
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.:rolleyes: :D
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
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. :cool:Quote:
Originally posted by southerner
:HMMM:
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.:rolleyes: :D
Difference in altitudes...and apts there used on....
those # are dif. Q-JET venture sizes, dont have anything to do with altitudes. **)Quote:
Originally posted by dr_bowtie
Difference in altitudes...and apts there used on....
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.
:p
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.
Quote:
Originally posted by southerner
:p
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. :cool:
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..
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'sQuote:
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. :cool:
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. :cool:
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. :)