One of the more common mistakes that folks new to EFI and tuning tend to make is to overlook the simple basics. This is most noticeable when the pleas for help originate from a frustration that results from obtaining data and results that don't match expectations or calibration changes. When this happens, you can bet that there's probably a mechanical or electrical fault. The beauty of EFI is that it is entirely based in logic. 1's and 0's in binary...black and white, no gray areas. However, if the EFI system is forced to operate under circumstances that are far from ideal, one can expect there to be issues along the way when trying to tune the EFI system. Here we offer an example of just such an instance and show you how to remedy the situation. In this particular case, we're dealing with a brand new race car build, a Trans Am TA2 car. This car is powered by a built LS3 as is legal in its race class. More and more of these types of race cars are popping up, and increasingly more of them are equipped with EFI as opposed to carburetors. After all, if you want to win, EFI is the way to go. The few who insist on sticking with carbs cite tunability and idle control as the reason they won't convert, but to this we say "pish-posh". In the case of the TA2 you see here, there were a few issues that needed sorting before tuning, and one of those issues pertains to the integrity and reliability of the MAF sensor.
Now that this mechanical issue is resolved, we can mount the new intake tube and MAF sensor back into the race car and prepare it for tuning...
Bad MAF tube causing turbulence and erratic sensor signal.
The Manifold Absolute Pressure sensor (MAP) and Mass Air Flow sensor (MAF) together are responsible for allowing the ECU to correctly measure, determine and model the airflow into the engine. To ensure a proper signal from both sensors, the proper reference supply voltage must be present at the sensor as well as the proper electrical ground. In the case of the MAP sensor, the ECM supplies a 5-Volt reference signal to the sensor as well as a sensor ground (must be below 20 milli-Volt drop with key-on, engine off). The MAF sensor receives a 12-Volt supply and the ground is also tied to the ECM (must also be below 20 milli-Volt drop with key-on, engine off, as must all ECM ground signals). If the sensors are testing properly as far as the electrics go, there may be a mechanical issue. For example, does the MAP sensor have a proper manifold pressure signal? You'd be surprised at how many people install a new intake manifold but forget to drill out the hole for the MAP sensor. And as for the MAF sensor, does it have a proper, laminar air flow to sample? This is an important aspect of MAF operation.
To deliver a good and reliable signal, the MAF sensor must be placed in a section of the air intake that provides a laminar flow as opposed to a choked or turbulent flow. If the MAF sensor is exposed to turbulence, it will read erratically at different air speeds and flow rates and this can make plotting the MAF transfer curve a nightmare. Take a look at the photo below. This is how our TA2 car arrived. Notice that the MAF sensor is located mere inches away from the throttle body and thus the throttle plate, and that the intake itself is composed of 6 different pieces: The initial carbon tube, then a silicone coupler, a short-length aluminum tube, then another silicone coupler, the the MAF sensor/sample tube assembly followed by yet another silicone connector that is finally fitted to the throttle body. What's the air supposed to do here other than tumble and swirl? Bad idea, poor execution. Definitely due to a time constraint or deadline, to be sure. But it simply won't do.
You can see the difference in the tubes internally in the two photos below. To the left is the crazy concoction, and to the right is our smooth new intake that is sure to provide a nice, stable, reliable signal.
Proper MAF tube allowing smooth, laminar air flow.
The solution begins with fabricating a proper intake tube with the MAF sensor placed in a logical area. To achieve laminar flow, a good rule of thumb is to have a length of tube that is 5 times the diameter both in front of and behind the MAF sensor. This isn't always easy in a car as space may be at a premium, and the real estate situation is worse in small cars with tight engine bays, but you have to provide the longest section possible to ensure a non-turbulent, laminar flow. Here we clearly had the space and we fabricated a new intake as shown below. Big difference!
After installing the new intake tube and MAF assembly, we double-check the fit of the intake to be sure there are no leaks whatsoever as well as to ensure that engine movement and vehicle vibration will not affect the fit, mounting and sealing of the entire air intake assembly. Once we're certain of this, the car is started and the integrity of the MAF and MAP signal are checked to ensure proper operation. Satisfied with the results, the car is now ready to be put on the dyno and tuned. By dialing in the MAF transfer curve properly and accurately, the EFI system will be capable of incredibly precise fuel, ignition and idle control. Prepare the champaign! We smell a winner! MET