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Old 01-26-2008, 02:53 PM
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converting carb from 600 to 750 CFM

what would I have to do to convert my 600CFM 1850 holley to 750 CFM?

is it possible to bore out the secondary barrels only like a q-jet?

what size of butterflies and venturi would be needed?

is the casting thick enough to accept a bigger bore?

I'm not exactly after 750 CFM, but would like a bigger flow number without buying a new carb, I have all the machinery to bore the carb, no problem with that.

thanks

Augusto.
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Old 01-26-2008, 03:10 PM
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Changing the 600 carb throttle base plate to one from a similar 650 700 or 750cfm carb (eg 3310 750VS) will increase the flow of your 600 to 650cfm. This is a common swap.
At that point you can buy a Holley or Proform HP style center body and really kick up the flow. You loose the choke thou. Rejetting is required.
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Old 01-26-2008, 04:16 PM
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in this side of the world I will not find easily a bigger baseplate, that's why I ask for the size of the bigger carbs so I can machine the one that I have now.

are you saying that the bodies of the 600 and larger carbs is the same? so I won't have to machine the venturis and barrels? that would save me a lot of work.

do you or someone could measure the diameter of your 750's butterfly bores please?

thanks.

Augusto
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Old 01-26-2008, 06:14 PM
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You cannot just bore out the barrels of a carb. The barrels are a complex venturii shape. you can find the sizes of the minor diameter of the venturii of each model carb and the throttle blade diameter in the holley carb listing on http://www.holley.com/data/TechServi...%20Listing.pdf

a holley 600 like the 1850 has 1-1/4" and 1-5/16" venturii and 1-9/16" throttles.

A Holley 750 has 1-3/8" venturii and 1-11/16" throttles.

Last edited by F-BIRD'88; 01-26-2008 at 06:20 PM.
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Old 01-26-2008, 06:25 PM
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Holley

Holley catalogs have a tech section listing carb numbers, kit numbers, gasket numbers, venturi diameters bore diameters etc. They come out every year, maybe you could get an old one from one of our fellow gear heads.
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Old 01-26-2008, 07:11 PM
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Quote:
Originally Posted by F-BIRD'88
You cannot just bore out the barrels of a carb. The barrels are a complex venturii shape. you can find the sizes of the minor diameter of the venturii of each model carb and the throttle blade diameter in the holley carb listing on http://www.holley.com/data/TechServi...%20Listing.pdf

a holley 600 like the 1850 has 1-1/4" and 1-5/16" venturii and 1-9/16" throttles.

A Holley 750 has 1-3/8" venturii and 1-11/16" throttles.

actually I have machined venturis for many years it's a common method of improving a carb's airflow performance in the karting and MX world, machining a multiple barrel carb is more difficult because a fixture must be fabricated to hold the body in a 4 jaw lathe, and lots of patience to have the bore centered, you must know what your doing though, a wrong venturi shape and you'll loose the vaccum signal and the carb will run lean.

several venturi shapes are kept as close secrets in the bussines, I like the infinite radius venturi which gives a great midrange and top end atomization but have not seen one in any 4 barrel, it would be great.

what concerns me the most is not the venturi area but the throttle blades, I'm not sure if there is enough meat to work out without exposing the idle transfer passages behind the slots, this is the main reason why I'm asking if someone has ever done this before. there is also a chance of breaking into a fuel well or passage in the main body.

Augusto.
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Old 01-27-2008, 06:42 AM
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Augusto I,m not sure but I think holley used the same casting for the throttle plate on all 4150 a 4160 carbs, just used different throttle plates.It makes sense as it is alot cheaper for them to use one casting with just different sized openings.Of course it would probably have to be from the same series carb I.E vac sec to vac sec, double pumper to double pumper.
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Old 01-27-2008, 07:22 AM
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makes sense to me also, I'm gonna probe the discharge holes to see how deep they go.

thanks

Augusto
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