Q&A / Tech

Ask Away: How Electronic Fuel Injection (EFI) Affects Cam Idle (Hint: You May Not Like It)

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.

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  1. Good article

  2. Jeff has impressed me with his knowledge and understanding of the old school systems and the newer EFI system. Being and old time drag racer, and Bonneville racer of late I appreciate his insight and application to help all of us achieve our goals.

  3. Hang Ten says:

    Right on! Bringing old school to modern times, making it easy to understand.

  4. We put the FAST 2.0 on a radical 351 stroker, wow!
    Made it start easily and idle without fouling plugs!
    We made the customer very happy, a lot lighter in the wallet but happy….Bill@xkautosports.com

  5. Jesse Atencio says:

    WOW! Great Article. Improved my understanding of how the MAF and MAP work on EFI controlled systems. Now I’m going to see if Jeff Smith has a book or two with his collection of articles and other tidbits that I will find interesting and Handy!

  6. Can you explain what you meant by the following statement?

    “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”

    My definition of a desirable idle is a stable and smooth. Or in this article, is the desirable idle meant to be lumpy?

    I have a XE268H cam in my MOPAR smallblock and I wouldn’t mind if my idle was smooth with a bit of lump to it. What I really want is better MPG’s.

    • That statement was a bit misleading – and I’m supposed to be a professional writer! What I meant was that EFI will make a lumpy or unstable idle become more stable – or smooth. Some people may think that is not attractive. In your case, it will smooth the idle – which is what you want – and probably improve the fuel mileage by maintaining a leaner air-fuel mixture compared to a carburetor. The richer the air-fuel ratio is with your carburetor – the better the fuel mileage improvements you will see with EFI. Also – don’t be afraid to try running a lean 13.9 or 14.0:1 air-fuel mixture at part throttle. The engine should run fine and that’s where you will see mileage improvements.

      • I’ve got a 406 sbc with a comp xe284 cam AFR 195 heads, Edlebrock air gap dual plane intake, and I’ve installed Easy EFI 2.0 It seems my experience is vastly different than yours as my car fouls plugs in a few minutes of idling when cold. It’s very disappointing because the system went on simply and seems to be a high quality product with many features along with great throttle response under light cruise, and normal driving. When warm the idle is smoother than the quick fuel carb ( worked well but I had same experience as your buddy). However currently I can’t drive the car till it reaches 180deg. Leaving me wondering if I’ll have to reinstall the carb. Is there a system which is proven tolerant of the overlap of this cam, and able to idle when cold? By the way my idle vacuum was 9-10″ in park with the carb.

        • I would look at your AFR Target should be around 12.5 cold 13.6 to 14.0 at normal run Check to see that the coolant temp is accurate then check the base timing you may need to add more timing to create a smoother transition between Idle and running. Also be sure the Distributor was installed correctly

  7. Patrick cassano says:

    Great article just installed the Fitech on my 496 can’t wait for those cold start mornings

  8. Good article most people have no idea why big cams idle bad and no brakes on power brakes

    • that’s the same with my big cam and no brakes with automatics never fixed at the time desired to go manual next time what was and how do the problem to fix I
      was told then before i do again

  9. Great article. Im new to efi. Had sniper tbi installed and was easy to tune. Took time to see what really needed to be done but I found the lumpy idle I had with carb but way more stable with decent vac. Cam is 244@50 112 lea 600 lift. I had pertronix hei module which I found was holding me back. Swapped module for msd hei heat with rev limiter and can idle it down to 500. I leave it at 650. Sniper seems very user friendly and has helped me to monitor engine and help keep it together. Also slower iac speed helps stabilze idle with bigger cams.

  10. Frank Hhoeton says:

    O have a 89chevy truck and I installed comp cam thumper the motor doesn’t want to idle I changed the iac and it helped but not enough .I was thinking of changing the map sensor and putting one on from a c60big truck2ton wood that work because the vacume rateing is different

  11. Frank Horton says:

    Also I am using the stock tbi on the 89chev

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