Car computer Tuning
By: Benjamin Strader ©2004
1. Tuning EFI Systems on a Chassis Dyno
Sometimes when you are tuning an engine that is already in a vehicle on a chassis dyno, a lot of strange things can happen that you should be aware of.
First of all, it is important to keep a perspective on all the things that are happening. It can be very difficult to keep track of engine temperature, manifold pressure, air/fuel ratios, engine torque and horsepower readings all at the same time. It will take some practice to get comfortable with all of this while the wheels are spinning.
Try to pay attention to things like traction of the tires on the rollers. Many cars that produce big power can easily overpower the amount of tire adhesion to the roller, and this will dramatically affect the amount of power the dyno records. If you aren't applying all the power to the roller, it can't tell how much power you are making. On a dyno chart, this can normally be seen as an abnormal spike in engine speed and a corresponding drop in power and torque readings.
It is also important to remember that if you are recording Air/Fuel ratios with anything but the most high-speed exhaust analyzers that it is very common to get false readings when the engine accelerates too quickly. This happens because the engine speed changes so rapidly that the exhaust meter cannot update its sampling rate quick enough. By the time it processes a sample and displays it, the engine can be well beyond that operating range and be dangerously lean, or grossly over fueled. This makes it nearly impossible to record data and make accurate changes to the calibration.
Soft compound racing slicks and/or too little tire pressure can cause the tire to deform on the roller and actually decrease surface contact! Use a good radial street compound tire and strap the vehicle down tightly against the rollers.
2. Tuning Ignition Timing Tables
Whether you are tuning an engine on an engine dyno or a chassis dyno, you should always make sure that it gets tuned to the proper amount of ignition timing.
The best way to do this is to use a steady state holding pattern on the dyno and hold the engine to a specific RPM. Then load the engine to whatever site you wish to tune and record the instantaneous power readings.
When you make a change to add or subtract ignition timing, you will normally see a corresponding change in power output.
Using an onboard or aftermarket Knock sensor to check for detonation is the easiest way to find the maximum allowable ignition advance. However, if you do not have access to one, here is another way to get pretty close.
Advance the timing until maximum power is reached and begins to fall off when more timing is added. From there, back off the ignition advance one or two degrees and set it there.
Once you have made a few hard pulls on the engine at this setting, shut it off and remove the spark plugs. Inspect them for obvious signs of detonation or erosion. Pay careful attention to the J-shaped ground strap. You will notice that somewhere on the strap it begins to change color.
Ideally, when the proper timing is set, there will be enough heat in the combustion chamber to make the color change at about the center of the strap. If it changes more out towards the end of the strap, then there is not enough heat, and more advance is needed. Conversely, if the color change is near the bottom where the strap joins the plug, then take some ignition advance out in order to start the burn later and transfer more heat out the exhaust!
3. Using Ignition Timing to Stabilize Idle
When tuning a small displacement engine with very large injectors, you may have trouble establishing a good solid idle. This occurs because some ECUs do not have the injector driver strength to open and close these injectors for a short enough period of time and remain consistent enough to control the fuel.
First, always make sure that your ECU is getting full battery voltage, if not more from the alternator. The ECU will have a much harder time staying consistent if the supply voltage is not up to par.
Second, use a little more ignition advance at idle than normal to help the engine produce slightly more torque and keep itself running a little better.
Lastly, try lowering the base fuel pressure just slightly to take the pressure off of the magnet coil inside of it. The less pressure it has to open the valve against, the easier time the injector driver will have when trying to open it. You don't want to go down too low though, or poor fuel atomization can occur and make the poor idle situation even worse!