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Old 30-07-2009, 10:52 PM   #1
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850cfm carb needed

Hi everyone,

I need a carb for my 502 BBC build. Some people say i need a 750cfm carb, alot say 850 though. I've got a 750 here already, but i haven't got time to take a risk. I still need to discuss the pro's and con's with people.

Has anyone got an 850cfm carb suitable for my engine that i can buy before i order a new one from real steel?

Cheers, James
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Old 31-07-2009, 08:56 AM   #2
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why not work out what your engine needs instead of guess work, you will be suprised ,and it aint 850 but wot the feck do i know
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Old 31-07-2009, 02:04 PM   #3
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top right hand corner is a carb selector
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Old 31-07-2009, 02:56 PM   #4
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Quote:
Originally Posted by larby View Post
Hi everyone,

I need a carb for my 502 BBC build. Some people say i need a 750cfm carb, alot say 850 though. I've got a 750 here already, but i haven't got time to take a risk. I still need to discuss the pro's and con's with people.

Has anyone got an 850cfm carb suitable for my engine that i can buy before i order a new one from real steel?

Cheers, James
Big is not allways best,years ago my brother in law and myself ran a pair of 16v Vauxhall engines in Autograss his had a pair of Webber 40 DCOE and mine had 45's,they produced simmilar outright power but his had the power lower down the rev range,as you said you don't want to rev it hard I would think the 750 a better option but get it jetted correctly,its all down to air speed and fuel vapourisation,Hope this helps.Another grasser stuck a Holley Double pumper on a freshly built Rover V8 never dynoed it or jetted and the motor was overfuelling and knackered after three races,so an expensive lesson learnt
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Old 31-07-2009, 06:54 PM   #5
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Location: north west
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Boat name: Daze of thunder
Boat make: fastest recorded outboard phantom
Engines: which one
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cfm

here you go i spell it out simply
502 divide by 8=62.75 thats one cylinder
every revolution 4 cylinders draw air so multiply by 4
=251 divide by1728 to covert to cubic feet = 0.145254 then multiply by max rpm intended 5000 grand total 726 cfm.
so no point going larger as it can only draw that amount.
that figure is with 100% volumatic efficiency,due to marine cams,intakes,exhausts etc your more likely to have about 85%
now do you still need 850cfm i will sell you one but dont complain when its no better than what you have already,steve
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Old 31-07-2009, 07:26 PM   #6
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Nope, yer quite right Steve, I'm going with your advise and sticking with the 750. I was just worrying yesterday and wanting to cover all angles. Our chat this morning has put my mind at rest now!
850 no longer needed!

Cheers, James
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Old 31-07-2009, 09:20 PM   #7
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Quote:
Originally Posted by steve h View Post
here you go i spell it out simply
502 divide by 8=62.75 thats one cylinder
every revolution 4 cylinders draw air so multiply by 4
=251 divide by1728 to covert to cubic feet = 0.145254 then multiply by max rpm intended 5000 grand total 726 cfm.
Works out the same way as I learned it!

Cubic inches X max rpm/3456
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