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Old Dec 24th, 08, 11:08 PM
rj67bu rj67bu is offline
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Default piecing carbs together

I want to up grade my carb but instead of buying a new one can i put a different main body on my current carb which is a 750 dp mechanical secondaries? or do i put a differant base plate on the current main body? i'm thinking if the main body is bigger and the base plate stays the same more air will enter but should be faster with the smaller base plate, is this right and will it work
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Old Dec 24th, 08, 11:34 PM
jays64II jays64II is offline
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Default Re: piecing carbs together

It's actually the bigger base plate and same main body. That is how Holley came up with the 780 double pumper. It has an 850 base plate and a 750 main body.
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Old Dec 24th, 08, 11:55 PM
rj67bu rj67bu is offline
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Default Re: piecing carbs together

thanks Jay, Do you think if i kept the 750 main body and put a 950 base plate this would work? trying to get to 850 or close
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Old Dec 24th, 08, 11:57 PM
Big Dave Big Dave is offline
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Default Re: piecing carbs together

The venturies determine the flow rate of the carb and that is the main body. You can buy a billet main body from Proform or Holley if you wanted to remove the choke horn. Otherwise check the boards for carbs for sale. I buy Holleys all the time for under $50.00, usually under $20. Cheaper than buying service parts from Holley. There are basically two base plate sizes (throttle) one for the 600 to 700 and one from 750 to 850. There are actually more but they all interchange more or less seamlessly (though the three barrel 950 comes to mind as an exception, and the 660 850 center squirt is another). The vacuum base plate differs in the linkage from the manual double pumper attached to the throttle shafts. Boosters also make for different main bodies (an annular discharge booster has a larger throttle bore (venturi) than the same size (cfm rating) as the down leg or straight booster.

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Old Dec 25th, 08, 12:46 AM
rj67bu rj67bu is offline
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Default Re: piecing carbs together

Thanks for the info Dave i was thinking of trying something differant. Seems if i can get more fuel and speed up the air intake might work good for the track and the street. But then im sitting here after eating all the holiday snacks and there's still more, my sugar saturated brain is running over time
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Old Dec 25th, 08, 07:40 AM
Big Dave Big Dave is offline
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Default Re: piecing carbs together

You could bolt on a 1250 cfm Dominator which will deliver as much air as any small block I have ever seen could use, or a pair of smaller 1050 cfm 4150 Holley carbs for a combined cfm of 2100 cfm if the engine could use it.

Trouble is it can not use that much. It only pumps so much volume. Think of the engine as a magical self activating air compressor. Just like Sears we have good, better, and our best. All three air compressors do the same thing, suck in air and compress it. The difference is the cfm rating on the compressor ranging from 1.54 cfm for their smallest home use up to 17.6 cfm for their biggest commercial unit. It is the size of the pump (and the motor to drive it) that separates the "quality" of the rating system.

The displacement of your motor is the combined swept volume of the motor. It's volumetric efficiency is how much air it actually pumps. Most street cars are around 80%, with a really good race car motor hitting 104% to 105% of it's displacement (it actually moves more air than will fit inside the engine) at the RPM it reaches peak torque (and only at that one point in the RPM band). So no matter how large a carburetor you bolt on top of the car it won't move any more air than that.

Now we do not burn air in an internal combustion motor; we burn expensive imported oil we buy from a foreign country that doesn't like us. Air is free for the taking. Gasoline fluctuates in cost depending upon how greedy those who control the flow feel they can get us to pay for it. (demand drives supply, as they never make more gas than the public will buy, but they can expand supply to meet any demand). That said, why not just set a five gallon can on top of the car and pour the gas in and make lots of power?

Reason (this will shock you) gasoline doesn't burn. Yup, if you dropped a lit match into a 42 gallon drum of Rocket 104 octane racing gas it would go out. Gasoline will not burn, it has to be mixed with oxygen (air) in a very narrow ratio of fuel to air. too rich and it will not burn; too lean and it still will not burn. So we have a "Goldilocks" ratio of air to gasoline that we strive to hit to get our motor to run on; and it is 14.7 parts of air for every part of gas we burn. It is the carburetors job to mix the just exact amount of gasoline to the amount of air it measured going through it.

As I have shown the air pump the carburetor is sitting on top of only draws so much air depending upon how big the motor is. So bolting on a bigger carb has only one bad effect. It messes up the measurement of the air; which causes the amount of gas being delivered to be "off". If the ratio is off it won't burn so our magic air pump stops turning.

It isn't a bigger carb you want it is a bigger motor. In the world of automobiles bigger is better when it comes to displacement. That is why my motor displaces 582 cubic inches, or as much cast iron as can be successfully hogged out of the inside of a standard deck height BBC. If I want more motor, I have to go to a bigger block.

Big Dave
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