Biff was telling his friend about the dead spot in the carburetor on his engine (a 283 CID with a 1050 dominator carburetor adapted to a dual plane intake, 305 cam, points distributor and cast iron exhaust manifolds).  DUH--the most likely dead spot that Biff has is where he wears his hat!

    Selecting a carburetor can be a headache for the beginner and sometimes troubling for the professional.   Selecting the "perfect" carburetor can be practically impossible.  Whether you are replacing an existing carburetor or building a completely new engine, there are some things you need to consider.

    The first consideration you should make in choosing a carburetor is your engine's needs.  By this, I mean you should decide what size carburetor your engine requires as well as what type of carburetor will work the best.  The engine components used in your engine should all be compatible with each other to achieve maximum performance.  The carburetor, as well the intake manifold, cylinder heads, camshaft and even the compression ratio, should be closely matched.  All of these engine components will affect the carburetor selection because they will in turn affect the carburetor's ability to read vacuum signals.  Carburetors are rated by cubic feet or airflow per minute (CFM).  The cubic inch of your engine is a good starting point for your selection.  A simple formula to assist you in this, based on cubic inch and maximum RPM, is listed below.

        Engine size (cid) x max. rpm/3456 =cfm

        @100% VE (volume efficiency)

        Example: 350 cid x 6,000 rpm =                 210,000/3456 = 608

 A carburetor rated approximately 608 CFM would be required for this engine displacement at this rpm @100% VE.  Very few engines are 100% volumetric efficient, therefore this percentage should be calculated into the formula.  To put engines into categories, most stock/street engines are 80%-85% efficient, high performance engines are 85%-90%, and race engines are 90%-95%.  Some race engines can be more, usually by force (supercharging), or by utilizing very efficient intake, exhaust, camshaft and cylinder heads.  If we factor in the efficiency percentage on this same engine (608 cfm x 0.85=516.8) this engine needs a carburetor 516.8 CFM, a 600 CFM will work well. Take into consideration, engine displacement vs. rpm are not the only things you should consider in choosing a carburetor.  The type of intake manifold used, compression ratio, timing and camshaft are just a few of the others.  All the engine's components can affect the performance of your carburetor.

       I have mentioned volumetric efficiency, I suppose I should attempt to explain it.  Volumetric efficiency is a ratio of a cylinder's actual air capacity compared to what theoretically would be in that cylinder if it were open to atmospheric pressure and the piston at BDC.  Simply put, how much air (ability to breathe) does your engine take in at maximum rpm.  This means that if you could measure the volume of air to the physical volume at 6,000 rpm the cylinder would only be 80%-85% full, on a typical stock engine.  In the formula used, the maximum rpm is 6,000.  Your engine's rpm and efficiency may be more or less depending upon the modifications you have made.  You can categorize your engine from the ones listed above to assist in formulating a carburetor size.

     I have mentioned volumetric efficiency, I suppose I should attempt to explain it.  Volumetric efficiency is a ratio of a cylinder's actual air capacity compared to what theoretically would be in that cylinder if it were open to atmospheric pressure and the piston at BDC.  Simply put, how much air (ability to breathe) does your engine take in at maximum rpm.  This means that if you could measure the volume of air to the physical volume at 6,000 rpm the cylinder would only be 80%-85% full, on a typical stock engine.  In the formula used, the maximum rpm is 6,000.  Your engine's rpm and efficiency may be more or less depending upon the modifications you have made.  You can categorize your engine from the ones listed above to assist in formulating a carburetor size.

    After you have decided what size carburetor your engine requires, you should determine what type of carburetor is best suited for your application.  Vacuum Secondaries: A carburetor with vacuum secondaries (the back barrels) and a single accelerator pump is usually best suited for engines built for low-end torque (trucks, tow vehicles), heavy vehicles, automatic transmission and high gearing (numerical low numbers).  Vacuum assisted secondary carburetors rely on the drop in engine vacuum (under a load, hard acceleration) to open the secondaries and add additional air and fuel to the engine as needed.  This is probably the best choice for fuel economy as well.  Mechanical Secondaries: Another  type of carburetor for your consideration is one with mechanical secondaries, usually with two accelerator pumps (double pumper).  The secondaries (back barrels) on this type of carburetor are controlled by the throttle linkage on the carburetor.  In stock form they usually begin to open when the primary throttle plates have rotated approximately 40 degrees of their opening. This will probably be at the top of the list if you are building a high performance or all out race engine.  The reasoning for this is because the engine components used in these engines, such as camshaft and intake manifold,  cylinder heads and the compression ratio would affect the engine vacuum tremendously.  Furthermore, this type of carburetor has so many options available to give you the performance you want.  This type of carburetor is also suited for light weight street vehicles, low gearing (high numerically) and manual transmission or high stall converter.

    There are a couple of things you should not forget in selecting your carburetor.  A carburetor is a pressure differential air/ fuel metering device.  It is sensitive to all things that affect the pressure above and below it. Above it, things such as the disrupted air flow possibly caused by an open air scoop or something as simple as the air cleaner.  Below it, it could be the cam, intake or just a carburetor spacer.  I mention these things simply because the carburetor that might work great on your neighbor's engine might not on yours unless specific changes to the carburetor were made to compensate for the differences in the engines.  And in choosing your carburetor, if in doubt about the size, bigger is not always better.

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