A friend of mine just added an EZ-EFI 2.0 EFI to his big-block Camaro and while it runs great, it has definitely lost much of its old rumpy cam idle characteristics. Is that normal for EFI engines? Why does this happen if the cam has not been changed?



(image courtesy of Dragzine.com)

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. Calmer idle characteristics occur for a variety of reasons. First, let’s look at why this rumble occurs with a carburetor and then we’ll get into the EFI differences. All camshafts feature something called overlap. This is where both the intake and exhaust valves are open at the same time. This occurs when the exhaust valve is just closing and the intake valve is just opening. Long duration cam (and specialty cams like the Comp Cams Thumpr) increase this amount of overlap. When this occurs, there is usually a small amount of exhaust gas pressure remaining in the cylinder when the intake valve opens. This gas can easily work its way up into the intake manifold. 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.

In addition to the exhaust gas, there is also positive pressure pushing into the intake. This reduces the intake manifold vacuum, which is the pressure differential that helps move fuel out of the carburetor. 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. 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. 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.

Now let’s add a typical EFI system—we’ll lump both the TBI-style systems and the multi-point systems together because from a generic standpoint, they operate the same way using the same sensors. 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. This sensor indicates the amount of load the engine is operating under. With the sensors delivering a combination of manifold pressure and rpm, it’s not difficult to create a fuel curve to feed the engine.

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. 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. This means the gauge is heavily damped to give it that smooth reading. The reality is that there are massive pressure waves bouncing around inside the manifold while the engine is running—made worse with a cam with lots of overlap. The carburetor “sees” these pressure changes which is another reason why the idle is unstable. With EFI, the huge swings in manifold pressure (or vacuum, if you will) will also be damped with a MAP sensor to make it easier for the computer to make a decision about how much fuel to deliver to the engine. This helps to calm the idle, making it more stable.

Most late model EFI systems that also control the ignition timing also offer a feature where you can add or subtract ignition timing–let’s say four or five degrees—to help stabilize the idle. This is also used on the new EZ-EFI 2.0 throttle body fuel injection when you enable ignition control. The system looks at how much the idle speed moves around and then you can tell it how much timing authority you want to have over this rolling idle speed. As the engine speed begins to drop, the computer adds a couple of degrees of timing. Then when the speed picks up, it pulls timing back. The computer doesn’t anticipate but rather reacts to the movement of the idle speed. This is also why these EFI systems that take advantage of this must also offer a provision where the timing is locked out at idle so that you can test the accuracy of your installation. Otherwise, checking timing with a light would be very frustrating because the timing would be jumping all over the place.

So taken as a whole, all these EFI functions tend to make the idle more stable, even though it may in fact make the idle less desirable. In that case, many of these control strategies—like the timing control—can be turned off to help you achieve that perfect blend of idle quality and drivability.

Share this Article
Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.