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Chad, it's really very simple. Let's see if I can explain it. Otto cycle engines (we call them gas engines) are just large vacuum pumps. As the piston moves down in the cylinder, it creates a negative pressure (vacuum) the draws air past the carburetor's venturi. The gas is then drawn out of the float bowl, through the jets, by the vacuum. Therefore, the stronger the vacuum signal at idle, the better the engine's throttle response is. It will also tend to make more torque at low rpm. A vacuum gauge is essential to get this (and base timing) right. The reason it that the throttle is always the greatest restriction in the intake path at idle. The reason that a vacuum gauge won't help at full throttle is that the manifold, heads, valves are usually the greastest restriction at WOT. This is the point that many people don't understand... putting a larger carburetor on any engine, without changing anything else, will almost always result in lower idle vacuum, poor idle quality, lousy throttle response, less low-rpm torque, and poor fuel economy. High speed power might increase slightly (and it might not), assuming the carburetor is jetted correctly. If not jetted correctly, power will be much less and you might get real lucky and burn pistons. This is why an EGA is so helpful. Remember, engine building and tuning is all about matching components. The correct approach is to make modest improvements to displacement, cam lift and duration, valve size, head ports, intake manifold runners, carburetors, and exhaust. Assuming these components are matched, the result will always be a serious torque and horsepower increase. On the contrary, changing any one of them without the others will almost always result in torque and horespower decreases.
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