04/13/99 Updated 05/31/01, 12/20/99, 02/19/03, 07/10/03
This article details the basics of tuning a turbocharged engine running on pump fuel. You will be programming 2 different parameters here, RPM IGNITION and IGN RET/MAN PRESS. The RPM parameter has a timing slot every 250 rpm on most systems. The timing at idle is usually in the 5 to 15 degree range on most engines, 10 being a good starting point. From this point, most engines will run well if you add around 3 degrees per location, with full advance being acheived at 2500 to 3000 rpm. With SDS and most engines, you should avoid more than 30-35 degrees total timing at rpms below 2500. This is counterproductive to power, longevity can cause high current draw on F type coil packs leading to blown fuses and possible failure.
Most engines like between 28 and 38 degrees of advance for maximum power. Most small bore engines with good plug locations, are happy with 30 to 35 degrees. Once you get to around 3000 rpm, the same advance value can be entered the rest of the way up the chart.
With low octane pump fuel, your engine will likely detonate a lot with these basic settings so you will have to enter retard values in the IGN RET/MAN PRESS windows. These tell the ECU to retard ignition timing at a certain boost or manifold pressure. By entering a 2 at 4psi, you would get 2 degrees of retard at 4 pounds of boost. Most engines will require increasing amounts of ignition retard as the boost pressure increases. The amount depends on many factors including the particular engine, compression ratio and the fuel being used
To get reasonable fuel economy, most engines advance the timing past the point where maximum, wide open throttle power is obtained. Since most street engines are cruised in the 2750 to 3750 rpm range, extra timing can be added here if this is a concern. You might add something like 4 to 5 degrees.
Most engines encounter the worst detonation in the rpm range where maximum torque is produced. Most import street turbos find this in the 3750 to 5000 rpm range, although this can vary widely depending on turbo matching. Since this range is mostly above where the engine is normally cruised, timing can be pulled back here without impacting economy. In fact, timing will usually have to be retarded below what is required for maximum power at high rpm in this rpm range. This retard will not be required on high octane fuel usually. It should be noted that most performance turbos make very little boost below 3000 rpm so detonation is usually not a great concern below this rpm.
As you can see, the engine requires different amounts of timing under different conditions. How do we mesh all of this together? Below is a sample ignition map for a 2400cc turbo. Torque peak is in the 3750 to 4750 rpm range so we pull back the RPM timing values in this area. Highway cruising is in the 2750 to 3500 rpm range so we add a few more degrees here. Timing above the max torque range can be re-advanced a bit as peak cylinder pressures fall off. Boost retard starts very low on this engine because it has a high compression ratio. Remember: total timing is the result of the RPM timing minus the boost retard value under a given condition.
|IGN RET/Man PRESS||Value|
Most engines are not very happy with less than 20 degrees of total timing at high rpm under boost. If you have to pull back timing this far, you should either reduce boost, the compression ratio or get some higher octane fuel. This map is only an example, it will not work well on every engine but it can show you the trends of something that worked well.
An Easier Way to Program Advance on EM-1, 2, 3 E and F systems
For those people wishing to program spark advance at cruising manifold pressures, we have found a method which is easier to understand:
If the engine pulls say, 18-20 inches of vacuum at idle and you want some advance in the cruising range at 10-16 inches, you can enter negative values in the MAP/RETARD slots at those manifold pressures. 0 is no change. 1 would RETARD the spark 1 degree. 255 would ADVANCE the spark 1 degree.
So to get 5 degrees of advance at 16 inches, call up the IGN RETARD/MAN PRESS window at -16 inches, enter 251. You can calculate this by using the formula 256 - Y = X where Y = the number of degrees of advance that you want and X = the value to enter. For example you want 7 degrees of advance; 256 - 7 = 249. Enter 249 at the manifold pressure vacuum range that you want 7 degrees advance at.
You will be able to see the advance happening in Gauge 2 mode in the IGN slot as you ease out of the throttle and MAP drops into these vacuum ranges. On some engines this may increase fuel economy slightly. It may show more advantage on other engines which have the RPM ignition timing retarded around torque peak rpm to avoid detonation, allowing advance at part throttle and removing advance under boost and in the peak torque range where detonation is most prevalent.
02/19/03 EM-4 MAP Advance Programming
Programming both MAP retard and advance on the EM-4 can now be done directly in crankshaft degrees through the programmer. Advance is programmed in using the -1 and -10 buttons at each desired MAP range. Retard is entered using the +1 and +10 buttons.