Performance & Hotrod Business May '14

Issue link:

Contents of this Issue


Page 67 of 115

66 n Performance & Hotrod Business n May 2014 C arburetors. You've seen a mil- lion of them. You've worked on almost that many. But, like many automotive parts and systems, there are do's and don'ts, and bad habits that have cropped up over the years. We spoke with several carburetor man- ufacturers—and boy did they ever tell us about some things they wish everyone would learn (or unlearn). Read on to find the real, correct information and stop per- petuating bad ideas. Bet you thought about skipping this arti- cle "because you already know." Humor us. Read it. Major manufacturers would not tell us these things if they weren't running into them daily. Make sure you're not doing any of the wrong things described here. And hey, get in on the secret. Learn what's in this article and you can be the guy who knows, instead of the guy doing it wrong. The More Cfm, The Better? There is such a thing as too much. Any given engine can only use a certain maxi- mum amount of CFM. Go above that and you have more than you'll ever need or use. There's simply no point in it. Booth Platt of PROFORM gives us a great analogy: "Ever try drinking water from a fire hose? It's just not the best way to get a drink. The same goes for an engine; give it the right amount of air and fuel and it'll perform. The engine is nothing more than an air pump, and an enclosed area for an ex- plosion to take place. The carburetor feeds the engine exactly what it needs: fuel and air. Give it too much air and fuel and it'll get lazy and sluggish (think Thanksgiving), give it too little and it'll starve (think the movie Castaway)." What you need is just the right amount of CFM. The easiest way to find out is to call the experts. That doesn't mean you can't properly choose the correct carb. 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 HOTROD 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 PHBMAYp58-91.indd 66 4/2/14 1:55 PM

Articles in this issue

view archives of THE SHOP - Performance & Hotrod Business May '14