Ok peeps, my car is a street rod with a SBC. I have a 750 Holley with a mechanical secondary on right now but for some reason I think it may be to much carb. I was thinking of going with a 600 with a vaccum secondary or a 650. It is not a race car, just a cruiser. Which would give me waht I am looking for?
Which Carb should I buy?
- Thread starter David Melvin
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if it's a stock motor, 600 should be more than enough-mild tune proably 650 would do just fine.
I know there's a formula to figure out which cfm you need for each application-c,i,d,/rpm range etc..., but I don't remember it... I'm sure one of the members would know a bit better than I do... I know how to rebuild them, just not app specifics...
85 brougham, I know you got the mathmaticals for this one!
I know there's a formula to figure out which cfm you need for each application-c,i,d,/rpm range etc..., but I don't remember it... I'm sure one of the members would know a bit better than I do... I know how to rebuild them, just not app specifics...
85 brougham, I know you got the mathmaticals for this one!
d = Displacement in Cubic Inches. (d x 2) - 15% = CFM
EXAMPLE: d = 350 (350 X 2) = 700 (700 - 15%) = 595 CFM
EXAMPLE: d = 350 (350 X 2) = 700 (700 - 15%) = 595 CFM
Well, I did the math and the answer is Quadrajet. It won't ever be too big (you can adjust the size with an allen wrench and a small screw driver), it will idle better and at a lower speed, and it has better throttle response in traffic. Plus, it uses less fuel than a Holley (if properly selected and set up). Want a bolt on? Get one from a late 70's van or truck with a 305 or 350 (preferably a 350). Want more CFM? 30 seconds with a cutoff wheel will take it from 600 to 750-800. Price? Around $30 if you find a good one on a truck in the junk yard. Just check the primary throttle shaft for wear. If it is loose, it will not hold idle speed. It can be fixed with a simple bushing kit if it need to be, so it's not the end of the world. What do I run on my mild 355? You, you guessed it: a Quadrajet off a junkyard 1979 Chevy van with a 350. It was a fresh rebuild and cost me around $30. I didn't even have to take it apart as it ran great out of the box. I did mod the secondary air valve and go to a more aggressive secondary metering rod hanger, but those are external things that are easy to do in a few minutes. Oh, I also converted it from a heated choke to an electric choke while I was in the junkyard. I just swapped the choke coil cover for one in an 80's Buick.
P.S.: I forgot the actual equation, but it usually says something like 450CFM in this instance. The equation is wrong for the real world, so I normally just use a Quadrajet on a street car. After all, if it is too big, you can always add tension to the secondary air valve return spring and it won't open as easily. This effectively makes the carb smaller without physically changing the size of the castings. Remember: The air valve opens with engine vacuum. If it does not need the extra size, it won't suck it open all the way (unless it is not adjusted right). So in effect, the engine limits the size of the carburetor by vacuum signal. It's sort of an old school way of doing what a MAP sensor does today. Since it is load sensing, it can't open too far if there is not a big enough load. This is also the reason you will not see the secondaries open if you free rev the engine with no load. So why do people love to hate these carbs? They are harder to tune than a Holley and most people don't like to learn complicated systems-no matter how good they are! However, if you set it up right, the Quadrajet will become your favorite carb. It is far more adjustable for good part throttle than the Holley is. Interestingly, it is also the last 4 barrel carb design that was sold new on a production car. It is one of the newest carb designs used in stock applications with only the Carter Thermoquad and AVS being newer and actually good. The TQ is fatally flawed, but the AVS is a little better than the Q-jet in some ways. However, on a G body, the Q-Jet is the best choice as it bolts right in place of the electronic Q-jet with no other changes. It takes the stock linkages and even the fuel line.
P.S.: I forgot the actual equation, but it usually says something like 450CFM in this instance. The equation is wrong for the real world, so I normally just use a Quadrajet on a street car. After all, if it is too big, you can always add tension to the secondary air valve return spring and it won't open as easily. This effectively makes the carb smaller without physically changing the size of the castings. Remember: The air valve opens with engine vacuum. If it does not need the extra size, it won't suck it open all the way (unless it is not adjusted right). So in effect, the engine limits the size of the carburetor by vacuum signal. It's sort of an old school way of doing what a MAP sensor does today. Since it is load sensing, it can't open too far if there is not a big enough load. This is also the reason you will not see the secondaries open if you free rev the engine with no load. So why do people love to hate these carbs? They are harder to tune than a Holley and most people don't like to learn complicated systems-no matter how good they are! However, if you set it up right, the Quadrajet will become your favorite carb. It is far more adjustable for good part throttle than the Holley is. Interestingly, it is also the last 4 barrel carb design that was sold new on a production car. It is one of the newest carb designs used in stock applications with only the Carter Thermoquad and AVS being newer and actually good. The TQ is fatally flawed, but the AVS is a little better than the Q-jet in some ways. However, on a G body, the Q-Jet is the best choice as it bolts right in place of the electronic Q-jet with no other changes. It takes the stock linkages and even the fuel line.
85 Cutlass Brougham said:
with the exception of the distributor, if you still have the computer controlled unit installed...
if you are wanting new out of the box, I'd recommend the edelbrock, around 600-650 cfm...just installed a 600 unit on buddies stock 84 caprice, along with a msd streetfire dist, and woke the stock engine up... left a strip about 3' long in front of the house before I could get off the gas, when it's always had a slow pull from a stoplight.....
Well.... mine has a Quadrajet and it will leave a strip 50 feet long...hitting it from a 20mph roll.... damn open diff! :lol:
yours is also a 355, 2dr body.... not a stock 305 4dr weighing in at a couple hundred pounds heavier! :wink:
I was never knocking the q-jet, in fact, I support you on the subject... a very mis-understood carb... thing gets it's bad name from all the bad rebuilds from aftermarket sources. the things get thrown into bins, and parts get put together(not caring if they were the right castings to which they matched up to originally, coming out as garbage... in fact, the carb I ripped off the caprice was a remanufactured unit that never opened the secondaries once....
I was only mentioning the fact that he could no longer buy a "new" rochester carb anymore, and the edelbrock units are based off of the AFB design, and are relativley adjustment free right out of the box...
and if you noticed, I said it was 3 feet before I could get my foot off the accelerator... I hadn't planned on spinning the tire(know what you mean about the "one-legger"), and had just goosed the pedal to see how it'd take off! 😛
seeing jow it's not my car,and only has 56k original miles on it, I didn't want to tear it up too badly :wink:
I was never knocking the q-jet, in fact, I support you on the subject... a very mis-understood carb... thing gets it's bad name from all the bad rebuilds from aftermarket sources. the things get thrown into bins, and parts get put together(not caring if they were the right castings to which they matched up to originally, coming out as garbage... in fact, the carb I ripped off the caprice was a remanufactured unit that never opened the secondaries once....
I was only mentioning the fact that he could no longer buy a "new" rochester carb anymore, and the edelbrock units are based off of the AFB design, and are relativley adjustment free right out of the box...
and if you noticed, I said it was 3 feet before I could get my foot off the accelerator... I hadn't planned on spinning the tire(know what you mean about the "one-legger"), and had just goosed the pedal to see how it'd take off! 😛
seeing jow it's not my car,and only has 56k original miles on it, I didn't want to tear it up too badly :wink:
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