Tuning: Piggy Back Tuning Issues, MAF vs. Fuel Map

Last time we talked about setting up the MAF map and adjusting it so that the MAF sensor plays nice with the AEM FIC. The way the AEM FIC works, and how most piggy backs work, is by fudging the signals to and from the factory computer to make the motor run in the way you want it. The reprocussion of this is that you will pretty much never get an accurate tune (when compared with going full standalone) and a good example of this is fuel adjustment via the MAF and Fuel Map.

So let's have a look as this little dilemma.

The MAF map adjusts the signal taken from the MAF sensor which gets fed into the factory computer, the Fuel Map adjusts the fuel injector 'on time' signal from the factory computer to the injectors. Both these maps adjust how much fuel is delivered, but they function differently.

MAF Map Fuel Delivery

If you increase the values in the MAF map, it tricks the factory computer into thinking that there is more air going into the motor. Accordingly, the factory computer pumps out a longer fuel injector 'on time' (pulse width). You could tune fuel simply by mucking around with the MAF map but you will be limited to injector control of the factory computer. You won't be able to get longer injector 'on times' that the Fuel Map is able to do as the piggy back intercepts this signal. Another major problem is that the factory computer is smart.

The factory computer looks at MAF as well as the oxygen (O2) sensors to correctly adjust fuel in accordance with its factory map (usually hold at stoich and possibly more richer on full throttle). When the factory computer does this, it is known as Closed Loop. Open Loop is when the factory computer doesn't use the O2 sensor and relies on its internal fuel map. If the O2 sensors read the exhaust to be too rich, it will adjust the fuel delivery accordingly. This is known as Fuel Trims. If you have a scan tool you can look at the long term and short term fuel trims, these show how much the factory computer is trimming fuel. Fuel trims can be both postive and negative meaning if the O2 sensor reads the exhaust being too lean, the factory computer can add more fuel to compensate.

The problem with MAF map tuning is that when you adjust the MAF, the factory computer then re-adjusts the fuel from the O2 readings, so you would then need to adjust the MAF again, but gets trimmed out; leading to a vicious cycle. You could have a whack at trying to control the O2 sensor signals using the O2 map, but linking this with MAF as the load will make out to be a very complicated exercise... You adjust the MAF to increase fuel, but then you reduce the O2 sensor to limit fuel trims, but reducing the O2 signal makes the factory computer think its too lean so adds more fuel, which mucks up your original tune.

Enter the Fuel Map.

Fuel Map Fuel Delivery

The fuel map is a whole different kettle of fish. If you increase the values in the fuel map, the signal that directly goes to the injector is modified to be held 'on' longer. The piggy back computer does this by first detecting the original 'on' time signal from the factory computer and extending that signal by delaying the 'off' time. By tuning this way you have a bit more control over the fuel delivery as it is a more direct method.

The trick to this is that the factory computer still knows that you are delivering more fuel. Although the MAF signal hasn't changed, the O2 sensors sense that the exhaust is too rich and negatively applies fuel trims accordingly. Now, utilising the O2 map you can help to compensate this. For areas you have increased fuel, you can go to the same cells in the O2 map and reduce the O2 signal values. There is no vicious cycle in this as the piggy back has almost direct control over the injectors and all you need to do is adjust the O2 signal. The balancing game of fuel map and O2 map is a time consuming one but eventually you can get a stable tune.

Fuel and MAF Tuning

Let's get complicated. If you have adjusted the MAF map to add a MAF clamp and if you were like me, adjust the MAF map once more to account for an increased intake piping diameter; you'll have one complicated MAF map. Add on top of that a fuel and O2 map and suddenly your tune becomes very complicated. As the factory computer is adjusting for the different MAF signal, you are holding injectors on/off for a different time period and at the same time adjusting the O2 signal being read by the factory computer... What the heck is really going on in the motor?

Your best bet is to rely on your external wideband O2 sensor. The wideband air-fuel ratio is king.

You must adjust the MAF, Fuel and O2 sensor readings to satisfy the following conditions:

1. The wideband air-fuel ratio is at the correct level for the MAP condition (e.g. stoich in vacuum, rich on boost).
2. Fuel Trim, specifically Long-Term Fuel Trim (LTFT) is between +/- 5 (or so).

We'll talk about 'tuning' Fuel Trims in a later article but having a small deviation in fuel trim, such as +/- 5, should be sufficient to hold a tune. Having large fuel trims (e.g. +/- 12, 15) means that you are significantly lean/rich at certain places and you can get tuning holes when the factory computer transitions between those large swing areas.

As you can see piggy back tuning is a lot of hard work! Not only there are so many variables to consider, but because of the smart nature of the factory computer, you cannot really have a perfect tune.