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Determining Carburetor Requirements

Discussion in 'T-Bucket Engine and Driveline Articles' started by Mike, Nov 23, 2011.

  1. Mike

    Mike
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    One of the most common mistakes people make when assembling an engine is not properly sizing the carburetor for the intended application. Some people figure if some is good, more has to be better and they dash out to buy the biggest carb they can afford. Only to later grumble about how the engine won't idle well and is lazy under hard acceleration.

    There is a very simple formula for determining what size carburetor you need for your engine. Let's look at how it all comes together.

    For our example, let's use a 350 cubic inch engine, equipped with a mild hydraulic cam, to go into a T-Bucket.

    The first part of the formula is to determine engine displacement in cubic inches. We know we have a 350.

    Now we need to determine what our maximum RPM will be for this engine. Realistically, with the use of a hydraulic cam, maximum RPM is going to be in the 5500 - 6000 RPM range. Let's use 6,000 for our example. We now want to multiply our total displacement by maximum RPM. 350 X 6,000 = 2,100,000

    We now want to divide that number by 3,456. 2,100,000 / 3,456 = 608 (rounded up to a whole number)

    The 608 figure is how large your carburetor should be, assuming 100% volumetric efficiency. Assuming this engine is naturally-aspirated, you're not going to get anywhere close to that kind of VE number. More realistically, your VE numbers are going to fall into the 75% - 80% range. Let's assume 80% for this example. We now want to determine 80% of the total calculated CFM, which was 608. 608 X .80 = 486.40 actual CFM.

    Your 350 will be capable of flowing about 487 CFM of air at 6,000 RPM, assuming 80% volumetric efficiency.

    ENGINE DISPLACEMENT X MAXIMUM RPM / 3,456 = CALCULATED CFM AT 100% VOLUMETRIC EFFICIENCY

    CALCULATED CFM X VOLUMETRIC EFFICIENCY = ACTUAL REQUIRED CFM

    350 X 6,000 / 3,456 = 608

    608 X .80 = 486.40

    I would look at bolting a 600 CFM vacuum secondary carb onto this engine. It will idle really well and have excellent throttle response.

    Already, I can hear the murmurs from the back of the room. "Well, I used an 800 CFM double-pumper on my 350, and it ran just fine!"

    That all sounds real good in the drive-in parking lot, but a 350 cubic inch engine at 80% volumetric efficiency would have to be running at 10,000 RPM before that 800 CFM carb will ever be required. We all know a Dominator is going to attract more gearheads at the local car show than a 450 CFM vacuum secondary carb ever will, right. So now, all you need do is determine if you want your engine to run really well, or to look really well.

    Just to show you how rapidly this number can change, let's assume you have a 350 cubic inch engine that will only manage 75% volumetric efficiency and that you do not intend to ever rev it beyond 4500 RPM.

    350 X 4,500 / 3,456 = 456

    456 X .75 = 342

    In this case, a 350 CFM carb is all that would ever be required. A 570 CFM vacuum secondary carb would work for an application like this. It will provide good idle characteristics and will also have good throttle response, without ever needing to pull the secondaries open, unless you were in an extreme situation.

    When you use this formula, try to be as realistic as possible with the VE number and the maximum RPM number. If 6,000 RPM is going to be the absolute maximum number, don't try fudging the number to 8,000. Doing that is only going to cost in terms of idle quality and in throttle response. As you can see, small changes can change flow requirements by a considerable amount.

    Stick to this simple formula and you'll never find yourself needing to get outside the tuning window of your carb, trying to get it to idle and take throttle.
     
    Snicklefritz and smokeyco75 like this.
  2. smokeyco75

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    I just did this equation for my 454 at 6000rpm assuming 75% VE. I come up with 591. Interesting. Thanks for the info Mike. I was figuring on about 650 cfm but I now see a 600 will work just fine.
     
  3. butch27

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    I know that most of us are over-carbed. I have a 600 on my mild 302" Ford and only because it is CHEAPER than the 500 cfm. I have changed some of the jets to bring it down a bit.
     
    Snicklefritz likes this.
  4. railroad

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    The most used formula is the same as Mike's, but has (x 2) as the last function.
     
    benT likes this.
  5. RPM

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    But, But, But the 302 Chevy in the Z28 from 67-69 had a 780 Holley on it.

    "It featured a 302 cu in (4.9 L) small-block V-8 engine, 3" crankshaft with 4" bore, an aluminum intake manifold, and a 4-barrel vacuum secondary Holley carburetor of 780 cfm. The engine was designed specifically to race in the Trans Am series (which required engines smaller than 305 cu in (5.0 L) and public availability of the car. Advertised power of this engine was listed at 290 hp (216 kW). This is an under-rated figure.[9] Chevrolet wanted to keep the horsepower rating at less than 1 hp per cubic inch, for various reasons (e.g. insurance and racing classes). The factory rating of 290 hp occurred at 5300 rpm, while actual peak for the high-revving 302 was closer to 360 hp (268 kW) (with the single four barrel carb) and 400 hp (298 kW) (with optional dual-four barrel carbs) at 6800-7000 rpm. The Z/28 also came with upgraded suspension, racing stripes on the hood and trunk lid, '302' front fender emblems on the 67 and early 68 cars, and 'Z/28' emblems in late 68 & 69. It was also possible to combine the Z/28 package with the RS package."

    How did it run so good being way over carbed?

    I remember when you could buy these cars used for $1000.
     
    cooter likes this.
  6. jcunning

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    I had a 68 Z/28 and it did indeed run like a scared rabbit. 302, 4 Speed, 4:56 Posi. It was a blast to drive. Unfortunately I sold it to pay for a wedding. I still have her and I've gone through a number of other cars in the last 34 years. It was great combination.
     
  7. cooter

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    DZ302
     
  8. butch27

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    Another thing to consider is the fuel pump pressure. My Edelbrock pump runs from 6.5 to 11 but the 600 carb only runs good at about 5.5 lbs. This IS "rocket science" LOL
     
  9. RPM

    RPM
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    I have a good friend that used to run top fuel. He has a huge big shop in Oak Ridge TN and they build most of the International Space station at his shop. On the crew shirts it used to say " This ain't rocket science, rocket science is easy". His crew chief was Fuzzy Carter after Eddie Hill quit.

    Oak Ridge Tool - Engineering
     
  10. one finger john

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    Nice to hear Mike's voice again.

    John
     
    railroad and RPM like this.
  11. Spanky

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    This is one subject I find creates more arguments! I agree, most street engines are over-carbed. My 350 Chev with a 625 CFM Demon and Vortec heads pulls like a freight train from a stop light right up to about 5500 RPM. All I need, for sure!
     






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