Tuning Bosch Electronic Fuel Injection System

I am a member of the Chicago SAAB Club. I’d like to share some information about tuning the Bosch electronic fuel injection system with your members.

In trying to locate a fuel pressure test gauge for my ’74 99LE and 99EMS SAABs, I found that no one in the Houston, Texas area stocked such gauges for automotive purposes that read over ten pounds per square inch (psi). One supplier offered to order one for $31 which read 0-30psi.

Another firm suggested that I build one myself using a precision, mechanical Bowdon tube gauge plus appropriate fittings. I did, and it works perfectly with a range of ON60psi. Any of your members who would like one should send me $15 and I’ll ship one to them with appropriate fittings, postpaid.

I set my fuel pressure regulator at about 32 psi after talking with a mechanic friend who specializes in Volvo and Mercedes. His shop routinely sets Volvos at 32 and 35 psi for four- and six-cylinder cars, respectively. I have noted a significant increase in performance in all gears, especially in fourth. Others who have tried this claim increased mileage, although it is too early for me to determine this firsthand.

The Bosch technical instruction booklet (#VDT-UBP 75111 B), Electronically Controlled Fuel Injection, Manifold Pressure Sensitive (EFI-D) and Air Flow Sensitive (EFI-L) states that the EFI-D in SAAB 99s through ’74 and other cars such as Volvo, BMW, Mercedes, Porsche, Renault, and VW fuel pressure regulators can be adjusted to 2-2.2 bar. The EFI-L used in the Datsun 280z, BMW 630i, etc. can be adjusted to either 2.5 or 3 bar. “Bar” is the international measurement unit for pressure.

For practical purposes:
1 bar = 14.5 psi
2 bar = 29 psi
2.2 bar = 31.9 psi
2.5 bar = 36.25 psi
3 bar = 43.5 psi

Author: Bob Moon


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