How Many CFM's for a 347

Discussion in 'Fox 5.0 Mustang Tech' started by Wildstringer, Feb 21, 2011.


  1. Wildstringer

    Wildstringer New Member

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    I have a 91 that I am building. For a long time I have loved the thought of fuel injection. I see that alot of Foxes are running carbs though. So I got to thinking...My wife says that usaully means I going to spend money,LOL. Actually the simple clean engine compartment along with the number of items I don't need to buy. I also now can sell my fuel injection set up. I have always wanted a couple of carbs sticking through the hood with a hilborn style scoop on top. So my plan is to go with a tunnel ram intake with dual quads on top. There are still alot of variables that I also need but assuming I go with the biggest cam I can and an unknown cc of aluminum heads what CFM carbs should I go with. at first I thought about dual 600's however i read in another thread here that 750 CFM's seem to work well with a 347. That's what i have by the way. after reading that I thought maybe 450's would be the way to go. Any advise would be appreciated. heres what I can think of that you all may need to know;

    5 speed
    347 short block
    10.5:1 compression pistons
    cam unknown
    heads unknown
    #1
  2. Ty_1314

    Ty_1314 New Member

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    the formula for figuring out VE which will be able to help you figure out what carbs to run is (RPMxDISPLACEMENT/3456)x.85, if youre building a solid street/strip engine then i would guesstimate your VE at around 85-90% so then times your sum by .85 or .9 and that should give you Volumetric Efficiency and tell you how much cfm's you want to flow into your beast (ref. HP Books, Ford Windsor Small-Block Performance by Isaac Martin) < awesome book, highly recommended, hope this helps
    #2
  3. LarsD

    LarsD Founding Member

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    A tunnel ram with dual quads? Yick, tuning one carb is enough hassle as is it is. You sure you want to attempt that on a street car? Or are you going to track it only?
    #3
  4. madmike1157

    madmike1157 the humor is still lost on me SN Certified Technician

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    You should also talk to carb mfgs. They will always have better advice on the proper carb size as they'll have a formula for you to follow as well wildstringer. You will probably find that they'll recommend about 600 CFM total after you plug in the numbers. I'd recommend that you follow it, and only fudge about 10% beyond that. i.e. 660 CFM. Carbs have traditionally been too big by just about everyone that has bought one. Putting two 750's on a tunnel ram w/ a big-assed cam, and big port heads on a street car will be a miserable experience. All of that stuff is intended to make power in a very high RPM powerband, (read 4500-8000). If this is a street car it will be a pig below that, AND even if you wanted to turn a stock block/cast crank 347 7500-8000 RPM it won't go there too many times.
    If you want two carbs, there are several mfg's building a dual quad high rise intake that will work way better than a TR.
    2 450's, or 600's vacuum secondary carbs at the max. A medium sized cam. and 185cc heads and you'll be way happier.
    #4
  5. srtthis

    srtthis the guy doing it does every local racers rear end

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    carbs are a lot easier then people think

    only advantage to having 2 carbs on a motor that small i can see is you can jet each cylinder to have the fuel needed. other then that a single 850 would be more then enough for a mild 347 like that
    #5
  6. 90lxcoupe

    90lxcoupe Mustang Master

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    Pick the heads and cam before decieding what carbs go on it
    #6
  7. madmike1157

    madmike1157 the humor is still lost on me SN Certified Technician

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    seriously, The motor size, RPM band and its' intended usage is gonna be all any carb manufacturer is gonna care about. Do yourself a favor and give these guys a call:
    Quick Fuel Technology
    129 Dishman Lane
    Bowling Green, KY 42101
    Phone: (270) 793-0900
    Fax: (270) 793-0951
    #7
  8. 90lxcoupe

    90lxcoupe Mustang Master

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    How do you figure these things out before choosing a cylinder head or cam
    #8
  9. Wildstringer

    Wildstringer New Member

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    Thanks everyone
    #9
  10. madmike1157

    madmike1157 the humor is still lost on me SN Certified Technician

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    He already knows that it is a 347. He should know whether it is a street car or a race car (or dual purpose). He already knows what trans he has ( mt or at).
    He should know what RPM band he intends to typically be in. ( i.e. street car....typically spends its' life in the 1500-5500 range). Unless and until he intends to go beyond that, Heads and a cam will not affect ANY of those numbers. Obviously, if he goes beyond that RPM, he'll need better components to allow that to happen, but again, he'd know what RPM he wanted to turn the thing to before he even bought a single thing.

    Cam and head choices will affect where power is made w/i that power band, not what CFM a carb has to be.

    straight off the holley web site:
    You need to know the CUBIC INCHES of the motor. You need to know the maximum RPM the engine will be spun. You need the VOLUMETRIC EFFICIENCY PERCENTAGE (VE%) of the engine. The first two items (CUBIC INCHES and RPM) are relatively easy to determine. The engine VE% is another matter. If an engine could use all of the air it could consume, it would have a VE% of 100%. Many performance engines reach this level. Certain race engines can actually exceed this and reach a VE% of over 100% at certain points in their RPM range. Most production engines and most street performance engines have VE levels below 100%. Most stock production and street performance engines will fall around 75%-85% of volumetric efficiency.

    The math formula is: CARB CFM = CU.IN. X RPM divided by 3456 X VE% (already been posted previously)

    that said, if the motor is 347 , he intended to spin it 5500 rpm, and he had a "typical street performance" ve% 85%,... his carb size should be 650 CFM
    #10
  11. LarsD

    LarsD Founding Member

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    I know it isn't that bad, I can read plugs and tune a carb. I've just gotten lazier as I've gotten older and prefer to make adjustments on a laptop as opposed to leaning over a fender and futzing with a carb. :p
    #11
  12. 85_SS_302_Coupe

    85_SS_302_Coupe it sucks (I know) to be on the receiving end

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    Personally, i wouldn't bother with duel carbs or a tunnel ram unless you're planning on running crazy RPM. Get a Victor Jr and a custom carb from one of the carb shops (Pro Systems, Quick Fuel, The Carb Shop etc). My understanding about CFM needs was blown out of the water when i ordered my Pro Systems carb and they built me a 780cfm for my 347.

    In related news, Barry Grant just went out of business....so i wouldn't consider one of their carbs...and i wouldn't anyways because for what i priced them at, i only paid $30 more for my custom carb and it's super nice.
    #12
  13. HotFox

    HotFox Active Member

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    Yes I agree, I have a 750cfm quick fuel modified 800cfm by Williams carb on my 331. It is actually too big and will be changing the venturies to 780 or maybe even 750 to eliminate the off idle lean condition. I think with the better flowing cylinder heads of today that formula is no longer valid. I would have to have a VE of 1.15 or better to run that big of a carb.

    I would not put a dual quad on your 347.
    BTW does anyone even make a dual quad for a Windsor?

    Scott
    #13
  14. madmike1157

    madmike1157 the humor is still lost on me SN Certified Technician

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    #1. is your carb a vacuum or mechanical secondary? because 780 CFM carbs have traditionally ALWAYS been vacuum secondary. (for 85 ss 302)
    #2. Dual 450's or 600's are also "on demand" vacuum secondaries. They will not flow 900 or 1200 cfm unless (which I doubt it ever would) the engine demanded it.
    #3 I'd have two carbs on the 2v engine I'm building in a second if there was an intake that would allow it.
    #4 there is definitely a 2 4 intake for 351 w Edelbrock Performer RPM airgap
    #14
  15. HotFox

    HotFox Active Member

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    mechanical made by Quick Fuel.

    Scott
    #15
  16. 85_SS_302_Coupe

    85_SS_302_Coupe it sucks (I know) to be on the receiving end

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    My 780 is mechanical secondaries...Holley HP main body, flow bench tested.
    #16
  17. travisranger200

    travisranger200 New Member

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    Yeah I'm thinking that the formula listed is outdated as well. I did mine and it said I should run a 740 cfm carb. 7200x418/3456x.85=740.
    #17
  18. 85_SS_302_Coupe

    85_SS_302_Coupe it sucks (I know) to be on the receiving end

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    Well...my guess is that when you start running strokers and big cams (like mine is) then needs change a lot. If we're talking about stock displacement and mild cams, you don't need big carbs. As a few others have mentioned, the heads and cam make all the difference in the world. One 347 might be happy with an out of the box stock 650, while another might run a 1050 Dominator because it has 13:1cr, .700 lift cam with crazy duration, and 10k RPM. There's more to it than total displacement and RPM range.
    #18
  19. madmike1157

    madmike1157 the humor is still lost on me SN Certified Technician

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    OK. I'll agree to the VE thing being a little too ambiguous to truly nail down a carb size due to varying combinations ( net result is recommendations for minimum carb size) but,...seems to me that if you look at the original posters' question, he was trying to determine if he could run dual 4's w/ a big cam on a TR. (I assume on the street, he bowed out at reply "way the hell back")
    He originally was trying to determine how big the carbs should be based on that.

    The TR idea was killed off by every one. The dual 4's, in any other form killed off by another 99%.

    Common concensus: everybody replying to this thread is running 750-800 CFM when running a carb on their 347. (at least from what I can tell) and (other than running a single 750-800cfm carb on his 347, If poor ole dude runs either of the two vacuum carbs recommended for multiple carb street use, his engine will use what it needs, and leave the rest of the available CFM in a partially closed secondary. (probably using between 700-800 CFM in total)

    It will probably NOT make any more power than a single, It will be more of a hassle than a single. It'll certainly cost more than a single, BUT,... it'll be what YOU want.

    I'll bet you guys are thinking...."I'll be glad when this guys' hand heals, maybe then he'll have a purpose other than spouting know it all" :bs:
    #19
  20. 85_SS_302_Coupe

    85_SS_302_Coupe it sucks (I know) to be on the receiving end

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    Well...i'm guilty of giving more advice than is asked for, but i never thought that was a bad thing?
    #20

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