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How to Select the Right Carburetor Size

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How to Select the Right Carburetor Size HOW TO SELECT THE RIGHT CARBURETOR SIZE Volumetric Efficiency Volumetric efficiency should be A highly tuned intake and exhaust sys- Before you can determine the correct computed at the expected operating tem with efficient cylinder head porting carburetor size for your engine, you RPM or your engine application. and a camshaft ground to take full must know its volumetric efficiency. advantage of the engine’s other equip- Volumetric efficiency is an indicator of Use the following examples as a ment can provide such complete cylin- how well an engine can breathe. The guide to estimate the volumetric effi- der filling that a volumetric efficiency of better an engine’s “breathing ability” ciency of your engine. 100%, or slightly higher, is obtained at the higher its volumetric efficiency. It is (A) An ordinary low-performance the speed for which a system is tuned. expressed as the ratio of the actual engine has a volumetric efficiency mass (weight) of air taken into the of about 80% at maximum torque. The graph below can be used to find engine compared to the mass which (B) A high-performance engine has your airflow requirement. It’s based on the engine displacement would theo- a volumetric efficiency of about 85% 100% volumetric efficiency so any indi- retically take in if there were no losses. at maximum torque. cated airflow must be multiplied by the The ratio is expressed as a percent- (C) An all-out racing engine has a volumetric efficiency of your particular age. It is quite low at idle and low volumetric efficiency of about 95% engine. Use a carburetor with an air- speeds and varies with engine speed. at maximum torque. flow rating equal to or slightly smaller than the air requirement of your engine. AIR FLOW vs. ENGINE RPM Let’s take for example, a 300 C.I.D. with Various Displacements V-8 which has a maximum RPM limit CHART BASED ON 100% VOLUMETRIC of 8000 RPM. It’s been determined that EFFICIENCY this particular engine has a volumetric efficiency of 85%. According to our chart the engine’s airflow requirement 1000 is 700 C.F.M. at 100% volumetric effi- CID ciency. At 85%, however, the C.F.M. 450 requirement is 595 C.F.M. This engine CID would, therefore, require a 600 C.F.M. 400 800 carburetor. CID 350 NOTE: Supercharged engines generally require carburetors with 40% to 50% more C.F.M. than normally aspirated 300 CID 600 engines. Holley offers a new line of supercharger carburetors. These carbs 250 CID are identified by this logo: AIRMinute per Feet Cubic – FLOW 200 CID 400 CID 150 CID 100 200 4000 6000 8000 2000 RPM SELECTING A MECHANICAL SECONDARY CARBURETOR For high performance engines a car- If your car has a manual transmission, may be able to use a 700 or 750 CFM buretor with mechanical secondaries use the lowest RPM at which you use unit, especially with a high numerical has an inherent advantage over a car- wide-open throttle. This must be a very gear ratio (low gear ratio). When in buretor with a “controlled” secondary conservative RPM (on the low-RPM side, doubt, select a smaller carburetor size system (air valve or vacuum that is!) and should be found by observ- because it will typically give better diaphragm). This is possible because ing your own driving habits in the vehi- acceleration times – even though power a controlled secondary carburetor, until cle involved. Watch your tachometer! may fall off slightly at top RPM. You it reaches wide open throttle, will not The heavier the vehicle and the lower the can believe that you'll be happier with have as great a pressure drop below numerical axle ratio (higher gear ratio) – the smaller carburetor nearly every the throttle plates as would a mechani- the lower this RPM must be. time! cal secondary unit. The greater the pressure drop below the throttle plates With engines from 300 to 400 CID, the * From “Holley Carburetors & the more dense will be the fuel/air right choice usually works out to be a Manifolds” by Mike Urich and Bill charge to the engine and, hence, the 650 to 700 CFM carburetor. A light car, Fisher more output. such as a Camaro, Mustang or Duster Greater care, however, must be taken in selecting the correct size mechanical Airflow vs. Engine RPM secondary carburetor for an applica- For Model 4150 Double-Pumpers, Model 4165 Spread-Bore, Model 4500 Dominator A B C tion. Double pump, mechanical sec- 600 ondary carburetors initially depend only on the accelerator pumps to pro- 650 vide adequate fuel until enough air 4000 flow can be 3500 established to begin pulling in the 700 main system. The larger the carbure- tor the higher the air flow required to 3000 750 accomplish this. If the carburetor is 200 too large, the pump shot will be con- 2500 sumed before the main system starts. The result is a “bog” or a “sag”. 800 300 2000 The handy chart, at right, will help you Grey line is EXAMPLE described below to determine the correct carburetor size CARBURETOR SIZE – CFM 400 850 for your application. 1500 ENGINE DISPLACEMENT – CID 500 Carburetor Size Selector* INSTRUCTIONS For Model 4150 Double-Pumpers, 1.Select minimum RPM at wide-open throttle in col- Model 4165 Spread-Bore, Model umn A; this will be converter stall speed on cars 4500 Dominator WIDE-OPEN THROTTLE MINIMUM RPM AT with automatic transmissions. Do not over-esti- 1000 mate RPM column A. 1050 More about using the chart – If your 2.Select engine size (cubic inches) in column B. car has an automatic-transmission, 3.Draw line between selected points in columns A & B, extending the line to intersect column C. make sure you know the converter stall 4.Maximum recommended carburetor size is read speed before using the chart. If in from point at which line crosses column C. doubt, use the figure shown for a typi- EXAMPLE: 350 CID engine with 1350 RPM cal Chevrolet converter (1350 RPM). If converter stall speed. (Typical for you are using a modified converter for stock Chevrolet converters). a racing application, make sure the NOTE: Applies only to mechanically operated stall speed is what you think it is. 500 secondaries. 25.
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