When we built the fuel system for our LS-powered Orange Peel Chevelle, we added a 1/2-inch
Performance Gas Cap
Vince Terrana; Bradenton, FL: A couple of years ago I was looking for a motor for my daily driver. A friend had a 350 Chevy ready to be assembled but never did. I bought the basket case and put it together. When first built it had a Magnum 280 cam, a Performer dual-plane, headers, a Holley 650 vacuum-secondary carb, an HEI with an ignition box, stock heads, and a 2,000-rpm stall convertor. I installed the motor in an '83 G-Body Olds wagon. I was not impressed with the performance and realized it had too much cam. First, I put in a set of Rhoads lifters but still was not happy, so I changed to a Comp 268. Still not much change; this car could not spin the tires. The Holley carb was new, but I changed to a Quadrajet. That felt a little better but not much. I played with timing, different plugs, whatever I could think of with no change. I vented the tank by modifying the gas cap by removing the restrictors and drilling a hole in it. I also left the vent line uncapped. I'm running a stock mechanical fuel pump with no return. Recently, after a gas stop, I left my gas cap behind and that created the biggest performance increase ever-it's a different car now. My question is-what happened?
Jeff Smith: Sometimes the simplest solutions are the best. From your description, it sounds like a simple case of insufficient venting. This is not an unusual situation. It is much like trying to pour liquid out of a closed container with no vent. I have one of those 5-gallon plastic fuel cans on which the vent hole has not been opened because when fuel is stored in the can, I don't want it to vent. This is the one I use to store fuel, but every once in a while I try to use it to pour gas into one of the cars. With one of those 1-inch hoses on the end of the jug, it takes forever to pour fuel into the car because there's no vent. Use a jug with a vent and the fuel shoots into the tank in seconds.
We've been on the dyno plenty in the past 30 days. Here's another teaser. T
Your gas tank works the same way. Combine a high-output engine that has greater fuel requirements with a too-small vent and the result is that insufficient fuel will reach the carburetor. Mechanical fuel pumps don't really "pull" fuel toward the carburetor. These pumps operate by creating a low pressure area in the fuel line and tank. Higher atmospheric pressure pushes on the fuel in the tank, moving the fuel toward the engine. The greater the pressure difference between the fuel pump and the atmospheric pressure in the tank, the more fuel moves. With a restricted vent, atmospheric pressure drops inside the tank and to no one's surprise, the fuel moves more slowly. So figure out the best way to increase the vent size on your tank and you will be good to go!
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