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A 650 carb can run up to 6500 rpm no problem as what matters is the engine its matched too. Depending on how much your engine will flow will determine if you would leave power on the table or not. Since I don't race mine I use the 650ish range rated carbs for both of my builds for better bottom end throttle response and I don't hardly rev it up that much anyways.

A double pumper can be made to get just as good of fuel mileage just like a vacuum secondary carb can if you know yow to advance tuning on the fuel curve.

As far as quality I have never had any problems with any machining wise on my quick fuel carbs or holley but did have a few bad holley carbs out of the box as well as one dud from quick fuel. As with anything a lemon can slip through at times but does not mean there bad. The main part that I like is on the quick fuel you can change everything on them and go back to out of box settings if needed.

The nice thing though with a vacuum secondary is if its too big the engine will only use what it needs as to a double pumper if it ends up to big then it may cause problems but its mostly for booster signal then on wide open throttle. I would say a 750 double pumper would work fine on your build as it can be tuned to run with that amount of power since your most likely 400 hp or a little over but just an estimate.
 

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"The nice thing though with a vacuum secondary is if its too big the engine will only use what it needs as to a double pumper if it ends up to big then it may cause problems but its mostly for booster signal then on wide open throttle. I would say a 750 double pumper would work fine on your build as it can be tuned to run with that amount of power since your most likely 400 hp or a little over but just an estimate.

even a carb that is way too large will meter fuel correctly at less than full air flow. If you put a 950 on a 283 you would have a touhg time getting to the correct tune but it would work. The only time you may run out of air is at WFO near the redline.
 

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My $0.02, the Quick Fuel HR680VS would be plenty. A 750 would be make better all-out numbers but the 680 would be close or maybe the same. For something street driven the smaller carb will probably feel more crisp, if you're really worried about running at 6500 then get something bigger. Really, it depends on what you think you'll actually be doing. I run the the 680 on a bigger engine than yours and I'm happy with it.

The slayer series carbs are nice, you could get the 750 and probably never notice a difference between anything else. The slayers don't have 4 corner idle, the hot rod series does.

Vacuum or mechanical secondaries: you got a light car and decent rear gears go mechanical, heavier car or highway gears then I'd go vacuum. The HR series has a nice vac sec. Screw adjustment. Can be real fast or slow or in between.
 

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Discussion Starter · #24 ·
My $0.02, the Quick Fuel HR680VS would be plenty. A 750 would be make better all-out numbers but the 680 would be close or maybe the same. For something street driven the smaller carb will probably feel more crisp, if you're really worried about running at 6500 then get something bigger. Really, it depends on what you think you'll actually be doing. I run the the 680 on a bigger engine than yours and I'm happy with it.

The slayer series carbs are nice, you could get the 750 and probably never notice a difference between anything else. The slayers don't have 4 corner idle, the hot rod series does.

Vacuum or mechanical secondaries: you got a light car and decent rear gears go mechanical, heavier car or highway gears then I'd go vacuum. The HR series has a nice vac sec. Screw adjustment. Can be real fast or slow or in between.
My Camaro weighs around 3200lbs. It has 3:42 gears in the rear!
 

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Vinniekq2 on the carb being to big part that info is what I have read from some of my holley tuning books but have never tested myself so I can't say its a hundred percent accurate as even tuning books can differ on opinions at times.

I always like to leave a few on the table for power wise on the carb to get the best of low and mid rpm range response more so then wide open throttle as I don't race so as mentioned above I have one of the quick fuel 680 vacuum secondary hot rod series carburetors and its a pretty nice carb but you could use either one and both would still run good.
 

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Discussion Starter · #28 ·
Vinniekq2 on the carb being to big part that info is what I have read from some of my holley tuning books but have never tested myself so I can't say its a hundred percent accurate as even tuning books can differ on opinions at times.

I always like to leave a few on the table for power wise on the carb to get the best of low and mid rpm range response more so then wide open throttle as I don't race so as mentioned above I have one of the quick fuel 680 vacuum secondary hot rod series carburetors and its a pretty nice carb but you could use either one and both would still run good.
My cousin who builds and dynos motors for a living recommend the quick fuel HR-650. He said similar motors with that carb run awesome. And most of the time he didn't even have to change the jets!
 

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My cousin who builds and dynos motors for a living recommend the quick fuel HR-650. He said similar motors with that carb run awesome. And most of the time he didn't even have to change the jets!
All carbs out of the box are set up for SEA LEVEL, and on a grocery getter or family car should need no jet changes unless elevation starts to change in different areas, but as HP and RPM increase jet changes are needed and on race cars a jetting in the mid hot day may run you lean at cool evening or lose you HP ! So recommending a carb because it didn't need jetting or tuning out of the box is a bogus recommendation for a carb:drunk: That same carb would be terrible in Colorado at high elevation!!! And dino tuning can get you close but that's not REAL WORLD !! At the strip or track everything changes in the real world during tuning and testing and jetting becomes more precise and needed !!! I know I run dynos and owned my own, and do real world testing and tuneing too and it always changes on the track!!!

Jester ( Chris)
 

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Short gear with a car that's heavier I would think to feel more responsive with the smaller carb.

Maybe I'm wrong.

Ben, Ben you can get many answers on that depending on what people are looking for in a carb! Here's my and many others take on it:thumbup:

First I'll say that when I dino tune an engine at the flywheel to peak performance with the best carb for engine demand through the power range and give it to the customer I would not tell him to put a smaller carb on if he has a heavy vehicle he is putting it in !!!:nono:


"Racers like to see no higher than 0.5 inch-Hg vacuum on the top end at WOT . But race cars have high-stall converters and steep rear end gears, etc, Racers don’t mind re calibrating the carb on the spot for changing track conditions through the day and evening. They are unconcerned about low-end drive ability. That’s why all-out racers, when not restricted by the rules, run huge carburetors! The bigger the carburetor, the lower the pressure drop across it at any given airflow! The higher the vacuum at WOT the more the carb is restricting the engine and needs a bigger carb!!!" Track tuning with a vacuum gauge in plain site will tell you at WOT if your carbs too small the more restrictive it is the more its restricting air flow and the higher the vacuum reading!

On Grocery getters and street/strip cars. "The limiting factor in carburetor size selection is on the low end of the fuel curve". Will the carbs meter correctly at lower air flows? Will it have good part throttle drive ability in the engine’s normal low operating range? How will it drive in the winter, desert, and high altitude etc,etc,! Engine load versus engine size in cubic inches. Compression ratio, distributor mechanical advance curve, cam duration as it factors into actual running , RPM and torque, compression, trans and converter, and final drive ratio all factor into carb selection. Car wght comes into play when figuring gearing, tires and final ratio in each gear to get the wght moving with the engines torque available at any given RPM! putting a smaller carb on is only beneficial when the carb that's on the engine is too big and not efficient for the use of that vehicle or your looking for low speed drive ability and econemy!

Think about this : If you put an engine out of a light car into a heavy one and that engine is running at peek performance would you go to a smaller carb? Lets say you have a full load on a big 10,000 Lb truck with a creeper gear climbing a steep hill at full throttle would a smaller carb help it pick up speed better or be restrictive ? Heavy cars and trucks do run smaller carbs but for drive ability not speed With an automatic-trans car, too low a torque-converter stall-speed for the application requires a smaller carb, Too large a cam for the application requires a smaller carb, Normally a mild converter, weak rearend gears, heavy vehicle will call for a smaller carb only to retain decent low end performance and econemy. But its common knowledge the smaller carb restricts power mid range to WOT! So in Mamas heavy family vehicle she would be happy driving around with a small carb, But Hubbys heavy 65 Impala 427 would be a slug with a smaller carb because he likes to "PUT HIS FOOT IN IT!!"
Its a choice between low speed drive ability and real power:mwink:

Qjets were designed for heavy or lite vehicles very small primarys for drive ability at low speeds and huge secondarys for passing and power on small blocks and big blocks , heavy or light vehicles, and all Qjets were 750 CFM and up and could flow even more on demand the secondarys only opened the secondary butterfly on the vacuum demand and on small engines never opened all the way, so it was a very versatile low performance street carb !

Jester (Chris)
 

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carb size Vs RPM?

Short gear with a car that's heavier I would think to feel more responsive with the smaller carb.

Maybe I'm wrong.
OK,so my 950 double pumper works great in my car,comes off corners hard, in first gear tries to rip the rubber off the wheels no matter what rpm I stab it.

When I drive that same car on the freeway and Im in 5th gear @ 2050 rpm doing 60 mph,is my carb now too big when I decide to pass and accelerate only to 80 mph?
 

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Look you can put a goddamn dominator on it for all I care. The vehicle weight and rear gear paints the rest of the picture of the car as a system doesn't it? Vinnie, jester, I'm not going off weight and gear alone. There's a street car, ~400hp, 5 speed, heavier, short gear. I'd run the vacuum secondary and keep the carb smaller to get the decent mileage and such that the OP wants. You can take a comment out of context and jump all over it or you could back up a few pages and look at what I posted previously. For the info available it suggests a carb in the 600's with a vacuum secondary... and that's my opinion. I'm glad your 950 works vinnie, but are you suggesting he try one because it works for you? There are those that over carb an engine for a street car and I think that with a 650 or 680 you wouldn't. Would a 750 "be over carbed and doggy"? Maybe, maybe not. I just feel the 600's work better for this.
 

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Look you can put a goddamn dominator on it for all I care. The vehicle weight and rear gear paints the rest of the picture of the car as a system doesn't it? Vinnie, jester, I'm not going off weight and gear alone. There's a street car, ~400hp, 5 speed, heavier, short gear. I'd run the vacuum secondary and keep the carb smaller to get the decent mileage and such that the OP wants. You can take a comment out of context and jump all over it or you could back up a few pages and look at what I posted previously. For the info available it suggests a carb in the 600's with a vacuum secondary... and that's my opinion. I'm glad your 950 works vinnie, but are you suggesting he try one because it works for you? There are those that over carb an engine for a street car and I think that with a 650 or 680 you wouldn't. Would a 750 "be over carbed and doggy"? Maybe, maybe not. I just feel the 600's work better for this.
Ben,
I have no problem with anything you suggest for a carb size! It was the WGHT and gearing comment that I have a different opinion with, and I answered your question about it, (I took it as a question ?)

"Short gear with a car that's heavier I would think to feel more responsive with the smaller carb."...."Maybe I'm wrong."

I tried to be respectful in my post I even started out with:
" Here's my and many others take on it"

THERE ARE OTHER OPINIONS!
The "Ben ,Ben" was a typing error I tried to correct but was out of time to delete it! It sounds like I set you on an anger trip aimed twords Vinnie and I??

I do not think I even suggested a cfm carb rating in this whole thread yet or did I suggest any particular carb brand yet!!!

The original thread starter said:
" It's a built 350 with Vortec Heads and a LT4 Hot Cam." So he has a perfomance engine in my book !!

If you use the old formula for CFM (equation), what does it recommend for a 350 engine turning 6,000 rpm at 100 percent VE = about 600 cfm
No one not even the manufacturers put little carbs like that on any 350 performance engine!! The CFM formula is old and bogus!! In the real world, everyone WHITE HAIRED from age knows these engines make more power with larger carbs.
Carb flow ratings cfm, are taken at a theoretical vacuum drop decided on many many years back: it is 3.0 inches Hg for two-barrels and 1.5 inches Hg for four-barrels and at about 80 % efficiency at WOT!! But in the real world todays carbs at WOT that are the right CFM for an engine do not see those high vacuum reading at WOT and are way more efficient !! And if you keep your foot out of em are pretty economical ! and a 500 cfm or 900 cfm carb on the same engine uses the same amount of air and fuel to go the same speed the only difference would be how far the throttle plates open and atomization and how much throttle the foot gives it to get to that speed!

And it's not ring and pinion that are important to carb choice it's the final drive ratio That can be tailored with tires and trans gearing or overdrives and stall!!

As for Vinnie I read his post and think he was using his car as an example of how the big carb performs very crisp when matched to an engine and still have gobs of power and speed at WOT It did not read as disrespectful to me! and you never answered his question.." is my carb now too big " ? Your Comment to Vinnie "are you suggesting he try one because it works for you?" is silly !! anyone reading his post can see he was not suggesting anything of the kind!!!


Jester (Chris)
 

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Discussion Starter · #38 ·
I did finally buy a carb! I bought a Quick Fuel HR-650 with mechanical secondaries. I plan to put on this weekend. Thanks for all the replies and support! I learned a lot!
 

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Love that Qjet, but hard to find one even rebuildable (don't buy one that "needs rebuild". It will be wore out, sloppy, missing stuff)(don't buy "rebuilt" as its likely a back-room hack job). Also note that those old intake manifolds for the Quadrajet are cast for the 90 deg bolts. When did they stop making those? I've done a couple and had to modify the manifolds for the 72 deg center bolts. You can probably get a Qjet manifold for 72 deg, but why? I go with beertracker on 21st century. I am old and retired, with lots of time to work out those little problems; but if you don't have the problem in the first place? Get the new stuff.
 

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carb size

Look you can put a goddamn dominator on it for all I care. The vehicle weight and rear gear paints the rest of the picture of the car as a system doesn't it? Vinnie, jester, I'm not going off weight and gear alone. There's a street car, ~400hp, 5 speed, heavier, short gear. I'd run the vacuum secondary and keep the carb smaller to get the decent mileage and such that the OP wants. You can take a comment out of context and jump all over it or you could back up a few pages and look at what I posted previously. For the info available it suggests a carb in the 600's with a vacuum secondary... and that's my opinion. I'm glad your 950 works vinnie, but are you suggesting he try one because it works for you? There are those that over carb an engine for a street car and I think that with a 650 or 680 you wouldn't. Would a 750 "be over carbed and doggy"? Maybe, maybe not. I just feel the 600's work better for this.
You should know that a 750 is the wise choice? If you do not then please ask. Ask Chris if you think Im wrong? I use a 750 on a 283,why would I use a 680 on a warm 327 or larger engine. Smaller carbs do not give better mileage than bigger carbs. Weight and gear ratio do not have an affect on carb size. The correct carb is the correct carb,,,PERIOD.

The correct carb for you probably is a small carb,,,
 
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