While many drivers would be happy to run a fixed idle speed with their throttle bodies, this wasn't really attractive to me. I intended to keep a full idle control system to have the car as driveable as possible. With a speed-density setup idle control is easy, you simply need a valve controlled by the ECU that lets extra air bypass the throttle plate and therefore alter the pressure in the manifold. If we were to allow this to happen on a throttle based system the ECU would be un-aware of the extra air entering the engine, and be unable to compensate for it. Because of this with a throttle body setup it is common to run a fixed idle speed, and with a competition engine this presents no problem.

On a road car with a power steering, A/C and lots of electrical items we need a way to automatically control the idle speed with varying loads and deal with stop-start traffic.

One of the best solutions is to use the benefits of both speed-density and alpha-n. In fact, this is the method many production bikes and cars fitted with throttle bodies use - a bumbling idle and poor emissions won't be tolerated. There are a number of ways to approach this, with compensation maps etc which can get fairly complicated. As I intend to use the standard ECU if at all possible to run the throttle bodies, there are fewer options available.

Over at PGM-Fi.org a group have written a hack which allows a hybrid approach, similar to the above. The MAP sensor is used to read engine load at the lower loads, when it is reliable, and above a certain MAP/throttle threshold the ECU will switch over to using the TPS alone. In implementation it's a little more complicated than that, but that's basically what happens. Naturally the changeover points can be adjusted and the tables may even be expanded to give an increased resolution. Because we are using the MAP sensor at we can run idle control as before, all thanks to the hard work done over at PGM-Fi.org.

Now we can run idle control, we have to distribute an equal amount of air to each cylinder. Not paying attention to this will generate a whole host of trouble. It is also important to think about where in the intake system we introduce this air, to help distribute it with the fuel and carry it into the chamber - this is especially important if the fuel injector is not in the stock location, firing directly onto the back of the valves.