Fuel Injection
The engine intake was built using a Porsche 914 intake manifold. This manifold was used with Bosch electronic fuel injection so there is an injector port at each intake valve. Now that I have installed the turbo the location under the cowl that housed the carburetor when the engine was normally aspired, is no longer available. In looking at what space is available to place the carburetor I made the decision to use a Bosch CIS fuel injection from a late 1970's VW. There are three reasons that I made this change. I have always considered fuel injection to be a better solution than a carburetor. That is why the engine was provisioned with a fuel injection manifold. There is more room on top of the engine for the fuel system components to be installed. The third reason was that the top side of the engine is in cool air rather than hot air which is better for the fuel system. I located a complete Bosch CIS Fuel Injection system from a 1979 VW Rabbit. When I removed it from the car the fuel system still had pressure so all of the components were sealed from the atmosphere. The control pressure regulator installed in this particular vehicle was not the altitude compensating variety so I located one of those on a 1981 VW Golf. The entire system was had for $72.00. I cleaned everything and taken the system completely apart. The total system weight is 8.6 pounds compared to 2.25 to the Zenith carb and Facet fuel pump previously installed.
The throttle body supplied on a 1979 VW Rabbit has a small primary and large secondary to help improve the fuel economy in the auto installation. In my case I don't need that feature so I will substitute a single throat throttle body from a 1976 911S Porsche using Bosch CIS Fuel Injection. The 911S has a 2.7 liter engine and my airplane is a 2.6 liter engine so there should be no problem with the change. The area of the replacement throttle body is 140% larger so there should not be as much flow restriction using this unit.
VW Rabbit Throttle Body area calculation
1.71875” Secondary Diameter
2cm Primary Diameter
Total Throat Area = ((2/2.54)^2)*3.141/4 + (1.7875^2)*3.141/4 = 2.9959 sq-in
Single Throat Diameter = (2.9959*4/3.141)^.5 = 1.9533” = 49.6128 mm
Equivalent single throat throttle body = 50mm
Porsche 911S throttle body
Diameter = 2 5/16” = 58.74mm
Area = 3.14*((2 5/16)^2)/4 = 4.1992 sq-in
911/Rabbit area ratio = 4.1992/2.9959 = 1.40 = 140%
On my supplier page there is a link to Miller Fuel Injection. This is where I found a source of information and components to design and build the fuel injection system. The following photos of a 914 Porsche installation were found from links on their web site.
Here are two schematics from Charles Probst's book "Bosch Fuel Injection & Engine Management".
The Bosch K-Jetronic system is a continuous injection system (CIS) that is mechanically controlled. An air sensor in the intake path senses the amount of air that is being supplied and mechanically raises and lowers the control rod in the fuel control to meter the correct amount of fuel. There is a control pressure regulator that provides enrichment for engine warm-up. The control pressure regulator also senses engine manifold pressure to provide a richer mixture for high output operation.
This fuel injection system will eliminate the need for a mixture control as mixture is regulated automatically. In addition, there is no need for carb heat as the system does not have a venturi that might form ice in adverse conditions. This will be helpful to reduce the pilot workload. In addition the elimination of the carb heat and mixture controls will offset some of the weight increase of the fuel injection system. The net increase should be between five and ten pounds.
The following picture shows the planned layout of the fuel system components.
Here is a photo of the altitude compensating control pressure regulator which is the same as the one on my 1976 911S Porsche. The second photo is of the airflow sensor and fuel metering unit. The airflow sensor in this installation is an updraft unit.
The following information was taken from www.boschusa.com as a guideline to what part numbers were stock on the year model VW Rabbit that my CIS fuel injection system came from.
