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Performance & Hotrod Business May '14

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May 2014 n Performance & Hotrod Business n 67 up) x .85 = 516.8 CFM. So you will need a carburetor that flows around 600 CFM. 3. You would then want to look at the real world application. Are you actually going to turn the engine consistently to 6,000 rpm? And, how fast will it get there? Kise also provides a couple of examples using the same engine: Example 1: A lightweight 2,000-lb. T-bucket with a manual transmission with a 1:1 high gear, 4:56 gears; 27-inch-tall tires, Weiand Team G single plane intake; and a cam rated for 2,500 rpm to 6,000 rpm. This application will allow you to leave the line or stoplight at a higher rpm and accel- erate the vehicle and conversely the engine at a high rate of speed and will main- tain a 4,000 rpm highway cruise speed at 70 mph. As such it would be a candi- date for a mechanical secondary 600. And the combination would be well matched. Example 2: A 3,800-lb. Chevelle with a 700r4 transmission, with a 0.70:1 high gear, stock stall speed converter, 3.08 gears; and 28-inch-tall tires. This application will not allow you to leave the line or stoplight at the required rpm to get into your torque band. Its slow vehicle acceleration and conversely slow engine acceleration would cause the vehicle to labor. In this applica- tion the engine would maintain an 1,800 rpm highway cruise speed at 70 mph—be- low the minimum ef- fective range of the cam- shaft and intake manifold. As such it would not be a candidate for a mechanical secondary 600. This application would require a smaller vacuum secondary carburetor. Because the combination is not well-matched for the real world operating range of this applica- tion, a lower rpm range cam, dual plane Weiand Street Warrior intake, and/or a combination of higher stall speed converter and more gear would be required to make the combination efficient. John Satterfield, dAMBEST Carburetors, adds that you need well-mixed fuel and air Some of the things you should know to help them make the best determination: • How will the vehicle be used? Full race? Street only? Street/strip? Off-road? • Engine Size • Camshaft Duration • Cylinder Head Flow • Transmission – Manual or Auto? If Auto, what size is the stall converter? • How much does the vehicle weigh? Holley's Tom Kise provides some great information and examples on determining the required CFM: 1. Multiply Engine Size (CI) x Maximum RPM / 3456 = CFM @ 100% VE (Example: 350 CI x 6000 RPM = 2,100,000 / 3456 = 607.63CFM) 2. Next factor in the Volumetric Effi- ciency. Let's say this is a moderately high- performance engine application. You will multiply the CFM x VE or 608 (rounded Carburetor Secrets Choose the CorreCt Carburetor and steer Clear of bad ideas and bad habits. B y S . K e l l i e C o l f Any given engine can only use a certain maximum amount of CFM. This 800 CFM Quadrajet from JET Performance is en- gineered for Chev- rolet big blocks, but may be too much for some applica- tions. (Courtesy JET Performance Prod- ucts Inc.) PHBMAYp58-91.indd 67 4/2/14 1:55 PM

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