Air:Fuel Rations (Fuel Mapping)

Ethanol requires less air to combust than does gasoline. The stoichiometric air-fuel ratio for gasoline is 14.7:1 by mass. The air-fuel ratio for ethanol [proof?]is 9:1

From moment to moment, the amount of fuel that is injected into the cyliders is determined by the fuel injection engine control unit (ECU). The ECU considers inputs such as the amount of air flowing thought the air mass meter and the percentage of oxygen in the exhaust.

The stock ECU for a 1986 900T is Bosch 0-280-000-537, LH 2.2. It has on board a fuel map that specifies how long the injectors should open for difference conditions.

The stock ECU is programmed to assume that gasoline will be the fuel. In closed loop situations, It adjusts the air-fuel ratios as best it can to achieve the desired oxygen content in the exhaust. It can compensate up to a point for different fuel mixtures, such as higher and lower grades of gasoline, and to gasoline blended with varying quantities of alcohol.

There are limits to the amount of ethanol that may be in the fuel and for which the ECU can compensate. Beyond that the fuel system becomes untuned, and it runs inefficiently if it runs at all.

To use high concentrations of ethanol as fuel, the fuel air map must be modified to reflect the stoichiometric air-fuel ratio of ethanol. If the fuel injetor capacity is increased, or if the fuel rail pressure is increased, these adjustments may also be folded into the new fuel map.

Some fuel remapping thechnques allow the car to be readily switched from one fuel to another, or to handle a continuous spread =of fuel blends, theser are referedd to as FlesFuel options.. Some technoques allow flexFuel or dual fuel, others don't.

Here are some techniques people use modify the fuel maps.

Stock ECU except for a modified ePROM

The air:fuel map is on an ePROM inside the ECU. This can be unsoldered and removed, and a reprogrammed ePROM substituted. is dedicated to reprogramming SAAB ECUs.

As a practical matter, it might make sense to prepare a second ECU, wou you can swap them if you shitch between etahnol and gasoline.

Swapping the ePROM requires that the ECU case be opened, the original ePROM be unsoldered, and that a replacement ePROM beinstalled and soldered in place.

Could an ePROM carrier be soldered ionto the board and then an ePROM chip frinctin mounted into it? Is there clearance?

Split Second injector computer

LandJet: I have a Split Second injector computer that we will program for moonshine after we sort out the petrol maps. Two injectors are set up on the intercooler outlet tube that Brad at George's Imports made for the car.


MegaSquirt (manual ) is an after market fuelectronic fuel injection computer that takes over certain frunctions fom the ECU.

Jerome: ... the Saab computer will not be able to run the car at the proper air fuel ratio. You can do this with a Megasquirt system, and in fact you can run the car as a true flexfuel system because an oem 'fuel type' sensor has been worked into the Megasquirt system as I recall. The sensor figures out which fuel type you have and then Megasquirt runs the proper fuel map. You will need to go to their website to learn more about flexfuel. There you can also probably find definitive information on corrosion issues.

Saab Trionic T5

Jerome: Another alternative is to use the much more modern Saab Trionic system but I don't know if you would be able to run the car as a flexfuel vehicle, you might have to run it 100% alky or 100% gas. The Trionic system has been completely opened up and there is free software to access the chip/maps, search for t5suite on google. My guess is that you can easily convert a stock 2 liter Trionic gasoline map to alcohol without much trouble. If the multiplier to convert a gas map to alky is not on the t5suite website it must be on Meqasquirt or elsewhere on the internet.