Import car tuning

by JEFF HARTSELL

In Part 1 of Maximizing your performance through Car Accessories and Performance Parts we discussed the necessity of having accurate air/fuel metering devices and control over the main basis of the fuel system, it's pressure.

The absolute necessity of accurate fuel control remains, but today we are going to look at a different adjustment available to cars using today's OBD-2 EFI systems.

Whether your vehicle has employed a vane type MAF, hot wire type MAF, or MAP sensor, the basic principal remains the same for the computer. Look at a voltage or resistance signal to determine the amount of air in the motor. I will point out the main difference between these different types however. Mass Air Flow (MAF) meters are looking at how much air passes through a cylinder in the intake path. Once this air passes through, the computer calculates how much fuel is needed to offset this. On a manifold absolute pressure (MAP) setup, the car doesn't know the amount of air, but the pressure, which in correlation to the rpm's of the motor, air mass can be determined. Many people will argue for hours over the MAF or MAP debate, but that isn't what this article is for. (Though I do prefer MAP for various reasons)

Using these systems for our benefit is actually very simple, however great care must be made to find an acceptable change that not only does the computer accept, but can work with completely. I say this simply because many people do not look at side-effects of changing your computers view on how much air is in the motor, which can effect shift patterns, fuel or ignition cutouts, long term and short term fuel trim changes, as well ignition advance or retard.

Using both a wide-band air/fuel ratio monitoring device and a OBD-2 datalogging device, we can safely alter these values to the computer without producing harm to the vehicle. I stress the point of using good equipment while doing this, because minor to absolutely catastrophic engine damage can occur if this is done incorrectly.

We will begin with how they work. Quite simply, whether you're using an Apex'i S-afc, Greddy e-manage, TurboXS D-tec or any other air meter piggy back system, they all work the same way. The S-afc's are the most common of the bunch, while I believe the e-manage and D-tec are far better at fine tuning the air flow signal. All car accessories or performance parts in this group work basically the same way. Lower the signal output of the air flow meter, so that the computer sees less air intake, or raise the signal so that it sees more. When it sees less, it reduces injector duty to feed the car less, and more, it raises injector duty to compensate for extra air. Simple enough really.

All accessories listed above also monitor throttle position as well as RPM's, and will let you change the respective air meter value across a correlation table of both. This is how they tune the car. So now, on how to use them.

You will want both your air/fuel monitoring and obd-2 datalogging running constantly while making changes and for a short time afterwards. Your first look is going to be at your air/fuel ratio, which should be right around 14.7:1 for a gasoline powered car. If it is not, then small changes in your idle range should be made to correct this. Before we begin, you should also take a quick look at your fuel trims in the obd-2 software. LTFT and STFT should all be within /- 5% of 0. If they are not, and even if your air/fuel monitoring states it's at 14.7, bring the computer as close to 0 as possible. STFT is an immediate change to the fuel maps of the vehicle according to it's oxygen sensors and other emission devices, where-as LTFT is an off-set that is built up overtime to try and 0 out STFT. Basically what this means, is that your car may seem like it's fine, but your computer is already pulling 30% fuel out by itself with LTFT at -25% and STFT at -5%. 25% is normally their max value, so you have basically eliminated all help from the computer to pull fuel at this point and will eventually throw bank rich codes after some driving. Once they're close to 0%, you've restored your computers ability to remove or add up to 25% through the fuel trims.

After those changes, once again make sure all your metering devices are hooked up, and do some driving. Your first look will always be the air/fuel ratio. At light cruising speeds, you should always be around 14.7:1 air/fuel regardless of vehicle, but on All motor cars at WOT I prefer the low 13 range for safety and 11.4-11.8:1 on turbocharged cars. Review the air/fuel information, and begin making corrections to bring yourself to your optimal settings. Be wary of adding fuel through the piggy-back near the sensors max values, as these will normally induce check engine lights and various other faults such as fuel cuts and ignition cuts. The computer is designed to expect about a 4.7v max signal from the sensor, giving it 5.1 volts makes it think there is a problem. Every cars limit varies, and there's a good chance you'll find it while tuning, and have to back away from it a bit.