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If you look at the adapter that mates to the rubber hose connected to the manifold, it may only have a very small hole allowing the vacuum to enter the gauge. When you view a manifold vacuum gauge while the engine is running, more often than not, the gauge is rather steady, producing a somewhat false impression of what’s really going on inside the intake manifold. With the sensors delivering a combination of manifold pressure and rpm, it’s not difficult to create a fuel curve to feed the engine. This sensor indicates the amount of load the engine is operating under. With EFI, we still rely on manifold vacuum, but for most aftermarket systems that do not use a mass air flow sensor (MAF), they rely heavily on a manifold absolute pressure (MAP) sensor to monitor manifold vacuum. Now let’s add a typical EFI system-we’ll lump both the TBI-style systems and the multi-point systemstogether because from a generic standpoint, they operate the same way using the same sensors. Holley’s solution for this was to create carburetors with four idle mixture ports in the carb as opposed to just two on the primary side only. In a single plane intake manifold, not all cylinders will receive the same air-fuel ratio as their neighbor-which makes the idle even more unstable. Because we have a lower intake manifold vacuum which will pull less fuel out of a carburetor, the common ploy is to richen the idle circuit to compensate. This reduces the intake manifold vacuum, which is the pressure differential that helps move fuel out of the carburetor. In addition to the exhaust gas, there is also positive pressure pushing into the intake. This exhaust gas mixes with the fresh air and fuel in the intake manifold and causes the engine idle to become unstable-that lumpy sound that race engines have and street guys want to emulate. This gas can easily work its way up into the intake manifold.
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When this occurs, there is usually a small amount of exhaust gas pressure remaining in the cylinder when the intake valve opens. Long duration cam (and specialty cams like the Comp Cams Thumpr) increase this amount of overlap. This occurs when the exhaust valve is just closing and the intake valve is just opening. This is where both the intake and exhaust valves are open at the same time. All camshaftsfeature something called overlap. First, let’s look at why this rumble occurs with a carburetor and then we’ll get into the EFI differences. Calmer idle characteristics occur for a variety of reasons. Jeff Smith: This is a really good question and one that some guys may want to include in the decision-making process when considering upgrading to electronic fuel injection.