BMW Gets Final Tuning
We had our supercharged BMW 318is at Balanced Performance right before our event at the Mitty. Our new fuel pump, pressure regulator and 30 lb.-hour injectors cured the lean conditions that we were seeing at higher RPM. The final score was 196 horsepower and 162 lb.-ft of torque, after some work with the fuel pressure. We settled on 32 psi of fuel pressure, as this gave the best power and idle.
The car is running great now and since the bidding did not meet our reasonable reserve at the Auction at the Mitty, we've decided to hold onto the car for a while and enjoy it.
Look for our final installment of this project car in the August 2007 issue of Grassroots Motorsports
Supercharged BMW Tuning
Last June, we supercharged our 1991 318is and by almost every measure, the project was a success. We upped our power all the way from 127 at the rear wheels to a healthy 187. The secret was the Downing Atlanta Eaton supercharger kit for later four-cylinder BMWs that we adapted to our earlier car. The only real problem was that we had a lean condition under load.
The stock injectors on our 318is flow 19 lbs. of fuel per hour. Based upon our experimentation with fuel pressure, we knew that this wasn't going to cut it with the addition of seven pounds of boost.
We sourced a matched and blueprinted set of injectors for this project from Marren Fuel Injection. Tim Marren suggested a set of 30 lbs./hour injectors, which set us back $39 a piece, including new seals. Injectors are pretty easy to replace on this BMW, just unclip each injector from its electrical harness, unbolt the fuel rail and detach each injector from the rail itself.
On the dyno, we noticed that the car was still going lean under high boost conditions. With a cranked up fuel pressure regulator, new injectors and a computer trying to dump a lot of fuel into the engine, the problem clearly wasn't with pressure or triggering the injectors, but the volume of fuel delivery.
We added an additional pump from Marren that is capable of delivering an additional 255 liters per hour.
Since fuel pumps like to push fuel rather than pull it, we installed the cylindrical pump in the same position as the factory fuel filter. The fuel filter was relocated to the engine bay and we took the opportunity to replace all of the rubber lines with new Goodyear tubing rated for fuel injection use.
We are planning on auctioning the car at The Classic Motorsports Mitty Harper Auction. Come bid on this cool little car!