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  #31 (permalink)  
Old 08-14-2015, 08:09 PM
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Ben?

Quote:
Originally Posted by ben s. View Post
With those gears and that weight I'd consider the 650-680 cfm with a vac sec.
what do gears and weight have to do with the carb?

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  #32 (permalink)  
Old 08-14-2015, 08:14 PM
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Quote:
Originally Posted by vinniekq2 View Post
what do gears and weight have to do with the carb?
NOTHING LOL

Unless your talking about carbs like carbohydrate intake in your diet ??? But gears have nothing to do with diet ???

Jester (CHRIS)
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Old 08-14-2015, 08:41 PM
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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.

Last edited by ben s.; 08-14-2015 at 08:42 PM.
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  #34 (permalink)  
Old 08-14-2015, 10:14 PM
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Quote:
Originally Posted by ben s. View Post
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

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 !!!


"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 donít 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. Thatís 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 engineís 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

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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  #35 (permalink)  
Old 08-14-2015, 10:23 PM
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carb size Vs RPM?

Quote:
Originally Posted by ben s. View Post
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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Old 08-15-2015, 07:32 AM
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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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Old 08-15-2015, 09:05 AM
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Quote:
Originally Posted by ben s. View Post
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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  #38 (permalink)  
Old 08-20-2015, 08:49 PM
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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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Old 08-20-2015, 11:28 PM
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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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  #40 (permalink)  
Old 08-20-2015, 11:48 PM
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carb size

Quote:
Originally Posted by ben s. View Post
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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Old 08-21-2015, 08:52 AM
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When it comes to carburetors there are always going to be some different opinions on what size etc for things just as much as camshaft stuff. There is no perfect carburetor size for anything as with anything outside of a bone stock motor any thing performance there is always pros and cons and I don't got all the horsepower my engines can make with my 650ish range carbs but I get good mileage for what I got and still good street manners and I don't race so less then 10 percent of the time I drive and when getting on it to pass someoneI hardly even break 4000 rpm with 3.73 and 3.42 rear gears on my trucks.

A 750 holley could be tuned to run the same way but I don't care about every last ounce of power at the top and my holley carbs are on the slightly smaller side but still not so small as to choke my engine to death at the top end of things. Going to big or to small can have its issues as well. There have been many articles on carb tuning and what size and even the pros argue about some things. It some what boils down of what your putting on your engine will it give good results and tune well for your intended use and honestly what you will be doing with your entire package from the engine build clear to the rear gears.

I have used a 750 cfm carb before and it was fine but had better days as it was old and worn out. Bottom line if you get your 650 double pumper to run well and tune it from idle to wide open throttle and your happy with the results then that at the end of the day is all that matters. Good luck on your tuning.

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Old 08-21-2015, 12:33 PM
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1967 Rochester Q -jet

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Old 08-21-2015, 12:39 PM
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A Rochester Q-jet has small primaries for part throttle economy and low end torque and huge secondaries with an air valve to control air flow for high RPM horsepower. A 1966-1971 Q-jet is capable of 750 CFM on demand if the opening of the secondary air valve is adjusted correctly. Best of both worlds. I installed a 1967 Rochester Q-jet and intake manifold on my 1962 Chevrolet 327/300 and put the original 650 CFM AFB and cast iron intake manifold on a shelf. That really woke the engine up. I purchased a 1967 Autoline (Canada) rebuilt Rochester Q-jet and I did not have to adjust the idle speed. The 1966-1971 Q-jets are becoming difficult to find and are over priced by most rebuilders when you find one. Stay away from the 1972-up Q-jets.


My only regret is that there is no factory aluminum Q-jet intake manifold for a Chevy engine that does not have EGR. I insist on a oil fill tube so that eliminates after market aluminum manifolds. Those cast iron intake manifold are HEAVY. If you do not care about originalty, get an aftermarket aluminum intake manifold.

Last edited by MouseFink; 08-21-2015 at 12:57 PM.
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