Carb size 600 or 750 CFM?

zeta

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Jul 9, 2008
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I'm running a Chev 350 with a 650 cfm 4 bbl Holley automotive carb in my 19 ft Zeta runabout. I'm looking to install a marine carb. The boat's way too fast right now. I'd be happy to trade away some top end speed to save fuel.

Looking at the 600 cfm Edelbrock 1409. The parts guy says that I could use the Edelbrock 750 cfm, maintain all that top end potential WITHOUT losing fuel efficiency, torque or throttle response at the low end.

True or False?
 

Bifflefan

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May 27, 2009
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Re: Carb size 600 or 750 CFM?

I'm running a Chev 350 with a 650 cfm 4 bbl Holley automotive carb in my 19 ft Zeta runabout. I'm looking to install a marine carb. The boat's way too fast right now. I'd be happy to trade away some top end speed to save fuel.

Looking at the 600 cfm Edelbrock 1409. The parts guy says that I could use the Edelbrock 750 cfm, maintain all that top end potential WITHOUT losing fuel efficiency, torque or throttle response at the low end.

True or False?

the bigger one will cause a loss at the bottom end. With out going into all the details, it just will. You will also loose some performance going from a Holly to a Elelbrock anyway but you should gain a little fuel economy, but a good Holley will do good also. Why not go with the Marine Holley? They have them ranging from 450 to 800 or better in 4 bbls.
 

zbnutcase

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Re: Carb size 600 or 750 CFM?

Use the 600cfm Edelbrock. And get rid of the auto carb NOW! The life you save may be your own! 'nutcase
 

FreeBeeTony

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Re: Carb size 600 or 750 CFM?

I am running the 1409 w/o any issues........ran good right out of the box.
 

cr2k

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Re: Carb size 600 or 750 CFM?

Since Holleys are internally vented over the throat there is not much diff between auto and marine other than jetting and power valves and linkage.

But if you go anywhere where they inspect you will need a marine carb.
I would not use any other auto carb on a boat anyway. Besides not running right they are dangerous. Marine carbs are internally vented so as not to add fuel vapors to the bildge.
 

dan t.

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Re: Carb size 600 or 750 CFM?

600 cfm is more than you need, a chev 350 flows about 600 cfm at 6000 rpm, dont think you twist it quite that high, a smaller carb will give better throttle responce and better bottom end torque
 

intrepidvoyager

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Mar 24, 2009
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Re: Carb size 600 or 750 CFM?

Engine Size (c.i.d.) X Maximum rpm/3,456=cfm@100 percent Volumetric Efficiency (VE)

so ...... 350 c.i.d. X 5000 rpm= 1,750,000 / 3,456= 506 cfm

you will not get close to 100 % VE without a ported big valve race engine so anything over 400 cfm is plenty.

My buddy who builds such engines gets around 90 % but you can figure on 80 % or about 400 cfm ...but thats at 5000 RPM .

try to get a carb that has a vac fitting so you can easily tune the carb by manifold vacuum

like was said before the smaller the carb the better fuel economy.

if you want even better performance and fuel economy instal an MDS ignition.
 

mkast

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Re: Carb size 600 or 750 CFM?

Since Holleys are internally vented over the throat there is not much diff between auto and marine other than jetting and power valves and linkage.

Marine carburetors have certain modifications that make them uniquely adaptable and legal for marine usage. "Legal" meaning that it has the ability to pass the U.S. Coast Guard test for marine carburetors. The main qualification that a marine carburetor must meet (as set down by the U.S. Coast Guard) is that if the carburetor should "flood" only .5cc of fuel is allowed to escape in a period of 30 seconds. There's also a "backfire" test which confirms the ability of carburetors/flame arrestor combination to contain the backfire. For this reason a gasket is not used between the carburetor air horn flange and the flame arrestor. It's possible that this gasket could become saturated with fuel (if the carburetor should flood) and become a potential fire source as a result.

The primary areas of a carburetor that require some sort of modification to meet these requirements include the fuel bowl vent tubes and throttle shafts. The vent tubes of a marine carburetor are bent inwards so that the tube looks like an inverted "J". These tubes oftentimes are referred to as "J" tubes, as a result. The reason for bending the tubes inward is that if flooding should occur, the fuel that would normally come out of the fuel bowl vent tube is rerouted back into the carburetor.

Throttle shafts also get special machining attention. Shafts are "grooved" and "slabbed" to prevent fuel from exiting out the throttle shaft if flooding should occur. When a carburetor floods fuel will end up "puddling" on the throttle plates. A non-marine carburetor will allow this fuel to seep out of the throttle shaft ends and onto the manifold. This is not allowed on a marine carburetor because normally the engine is situated in an enclosed bilge where potentially deadly gasoline fuel fumes can accumulate. Throttle shafts that are "grooved" and "slabbed" channel the flooded fuel safely down into the intake manifold. With no raw fuel allowed to puddle on the manifold outside the carburetor, there is no chance of deadly fuel fumes to accumulate in the bilge and no chance of explosion or fire.
 

Maclin

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May 27, 2007
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Re: Carb size 600 or 750 CFM?

Very nice post mkast. Shows that when you do the research it becomes very apparent why Marine rated components cost more, it is not just a price gouging.

I believe also the fast idle mechanical circuit is "disabled" as well, I know it is on my Holley.
 

SuperNova

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Re: Carb size 600 or 750 CFM?

Don't forget the extra fitting for the mechanical fuel pump vent hose. Even though it looks like it is, it isn't just a vacuum fitting. If you put vacuum to the fuel pump vent, the fuel pump will stop working. So this fitting has to dump into the carb in a way that doesn't source vacuum.
 

zbnutcase

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Sep 19, 2009
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Re: Carb size 600 or 750 CFM?

600 cfm is more than you need, a chev 350 flows about 600 cfm at 6000 rpm, dont think you twist it quite that high, a smaller carb will give better throttle responce and better bottom end torque
But the Edelbrock carbs have an air valve secondary which only allows as much air in as the engine can use. 'nutcase
 
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