Carb size question
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500, 472, 425, 368 Discussion, Carb size question in Cadillac Engine Technical Discussion; I have a 1968 472ci with a bad carb. I want to replace it with an edelbrock with manual choke ...
  1. #1
    voodoo is offline Cadillac Owners Member
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    Carb size question

    I have a 1968 472ci with a bad carb. I want to replace it with an edelbrock with manual choke because I would like to add flame throwers and also change it to a 500 with slight mods someday. What cfm would you guys recommend for what I need ?


    thanks.

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  3. #2
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    MMNineInchNails is offline Cadillac Owners Enthusiast
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    Re: Carb size question

    edelbrock 750 is what i went for a little more performance than stock. If you want a gas hog, go holley 800 or for max gas go for demon 850.

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    lux hauler is offline Cadillac Owners Connoisseur
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    Re: Carb size question

    My engine is a mildly built 500 (.060 over-bore, MTS VT-10 cam, 9:1 compression, Edelbrock intake, mild port work, larger valves, etc.). I was looking at getting a new carb and was thinking of going with a Demon. I emailed Demons techs, gave them my engine specs and asked what they'd recommend. They suggested a Road Demon 725 http://www.barrygrant.com/demon/default.aspx?page=8 . I thought that might be a little on the small side so I emailed them again to make sure. They confirmed that the Road Demon 725 was all that was needed for my combo unless I planned on doing more to the engine.
    I'd email Edelbrock and see what they suggest but going by what Demon told me, 700cfm on a stock 472 should be more than enough.......a 650 might even be big enough.
    I've decided to have The Carburetor Shop http://www.customcarbs.com/ build a Q-Jet (from a '76 DeVille) for my engine. They'll build it specifically for my combo.
    Remember, the bigger carb you go with, the less throttle response you'll have. A bigger carb may do a little better on the top end but that's not where most street cars operate and it's definitely not where these engines like to run. Most of the power in a stock 472/500 is found in the lower RPM's.

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    voodoo is offline Cadillac Owners Member
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    Re: Carb size question

    Thanks you guys. I know to little carb is not good and to much is worse. Im going to go with a 700 cfm .

  6. #5
    Angela Desmond is offline Cadillac Owners Fanatic
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    Re: Carb size question

    A simple math equation for figuring carb size is: RPMx dispalcement divided by3456. Take that and times it by .85 and you'll be pretty close to what you need. The RPM is peak horse power RPM for your engine which should be around 4200 or 4400. (You did say it was stock right?) So, multiply 4400 by 472 and you have 2076800. Now divide that by 3456 and you have 600.92. That number gets multipied by the engines "volumetric Efficiency", which on a street motor with no mods. we can guess at about .85.(A race motor can be much higher, up to 110%.) So, multiply 600.92 x .85 and you get 510. The Q-Jets the engines come with are 800cfm but the way they work, they'll only deliver what the engine calls for, unlike Edelbrocks or Holleys. An Edelbrock in the 600 range will give you gobs of torque off the line but will limit top end, which in your case shouldn't matter because you have no mods to raise the RPMs of the engine.If you decide on a larger carb, I would stick with the Q-Jet because it won't hurt low end torque

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    lux hauler is offline Cadillac Owners Connoisseur
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    Re: Carb size question

    Below is a link to some good carb size info. It's an Olds site but good info just the same.
    Scroll down to where it says "Calculating Flow".
    http://www.442.com/oldsfaq/ofcrb.htm

  8. #7
    Angela Desmond is offline Cadillac Owners Fanatic
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    Re: Carb size question

    That's a great link "LUX"....

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    lux hauler is offline Cadillac Owners Connoisseur
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    Re: Carb size question

    Quote Originally Posted by Angela Desmond
    That's a great link "LUX"....
    The whole FAQ section of that site is good. I've used it for reference quite a few times.

  10. #9
    voodoo is offline Cadillac Owners Member
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    Re: Carb size question

    You guys are the greatest. Thank you. With all of your knowlege and experence and advice I think this old 67 deville convt. will be on the road turning heads again. I did get a little bad news the engine dosent have enough compression to start. I guess the rebuild will set me back a little, but untill then I have the sharpest yard statue in the neighborhood. I still love my cadillacs! Oh buy the way I bought a edelbrock 750 with manual choke. With the cam, heads, and pistons I want , she will need the extra gas. Again thank you.

  11. #10
    Angela Desmond is offline Cadillac Owners Fanatic
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    Re: Carb size question

    I thought this was a basically stock rebuild. Now it's pistons, cams and head changes!Sounds like you want to be a torque junkie like the rest of us. Be careful if you have domes put on pistons for compression.Plugs have LONG reach and some piston makers just happen to forget a notch for the electrodes.

  12. #11
    Imprl59 is offline Cadillac Owners Fanatic
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    Re: Carb size question

    Quote Originally Posted by voodoo
    I did get a little bad news the engine dosent have enough compression to start. .
    How long has it been sitting? If the engine has sat for years you might want to try to get it going and let it run for a while then recheck the compression once things get loosened up again. Put a little oil in each cylinder and see if you can get it to start. I have seen engines come back from the dead once the valve seats get cleaned off and the piston rings get freed up again.

    'Course your rebuild sounds like much more fun!!!

    Steve B.

  13. #12
    voodoo is offline Cadillac Owners Member
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    Re: Carb size question

    Im not sure how long it has been since she ran. the guy I bought the car from bought a cadillac that looked like crap but he said it ran A1. He was installing the new motor when he had a heart attack, he did not finish the install and let the car sit along time. I seen that beautiful machine sitting there and bought it. Now here I am with a project I dont know a whole lot about doing alot of guess work and rolling wheelbarrels of new parts to it. That is what is going on now. I do want to get some enjoyment out of the car a little while before I do the engine rebuild. she need a few other small things I can see, but a shake down would show me where my money would be best invested. Then comes the torque and hp. Thanks alot for all the help and support.

  14. #13
    voodoo is offline Cadillac Owners Member
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    Re: Carb size question

    I put some oil in the spark plug holes and the compression raised. going from front to back on drivers side 112,90,50,100 and from front to back on pass. side 85,80,80,115. Who knows maybe if I can get to run for a while then change the oil it might come up a little more. Thanks so much. IT may run again with a little tender loving care. YOU GUYS ARE THE GREATEST. THANKS AGAIN

  15. #14
    Angela Desmond is offline Cadillac Owners Fanatic
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    Re: Carb size question

    Like I said in your other post, your rings are gone to ring heaven.there's no substitute for fixing that problem.

  16. #15
    voodoo is offline Cadillac Owners Member
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    Re: Carb size question

    SHE'S ALIVE!!! I did a few things and it runs. I put desiel fuel in the spark plug holes to help the rings free up and and then did oil the same way, that seated the rings. still not enough compression. I always heard a weird noise when ever i turned the engine over. sounded like a boat motor. I traced the noise to valve covers, removed them and discovered a stuck valve and a bent push rod, a hammer and a new push rod still not much compression , pulled oil filter and it was dry. primed the oil pump . tund it over untill oil pressure raised enough to work lifters ... DAM THING STARTED AND SCARED THE CRAP OUT OF ME!! I didnt expect that to happen. Thanks so much for all the help. You guys helped bring another one from the dead.

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