Phoenix Turbine Builders Club

Fiero HEV Conversion Project

Building the transmission adapter plate

September 2007

Last month we pulled the engine and trans and are ready now to begin building up the key components to go back into the vehicle. 

The transmission adapter plate is the most critical part of the rebuild. Since we are re-using the four-speed manual transmission that came with the Fiero, we have to go from the traction motor output to the transmission input shaft. We are also re-using the clutch and pressure plate in order to better match the traction motor torque to the vehicle load under varying conditions -- such as climbing hills.

The first step is to match the operating speed of the traction motor to the operating speed of the transmission input shaft. The easiest way to figure this out is to work backward. 

Let's say that our target for top vehicle speed is around 80 mph. That's 7,040 feet per minute. With a 24-inch tire, we are looking at 7040/6.28)=1121 rpm at the transmission output shafts. (Note: 6.28 is 2Br ) Multiply 1121 by the differential ratio of 3.32 and we get 3718.4. Multiply this figure by the fourth gear ratio of 0.73 and we get 2714 rpm on the transmission input shaft.

The J&H type 29 motor/generator we are using for our traction motor has a best operating range of 3,000 to 8,000 rpm. If we figure on hitting 80 mpg at about 7,000 rpm motor speed, then 2714/7000= 0.3877 or 2.56:1 reduction.

That means we will need a speed reducer between the motor shaft and the transmission shaft.

There are several ways of reducing the speed -- gears, hyvo chain, or cogged belts. For our first conversion we have decided to go with a cogged belt and pulley system to avoid building a fluid-filled box for either gears or chains.

As we design our adapter plate, we will also have to keep in mind a belt linked speed reduction system. We also wanted to design an adapter plate that could cradle the generator in the same plane and give us a more compact means of fitting the motor, generator and drive engine back into the engine compartment -- and leave space for the APU, batteries and power controller.

In the Fiero compartment space, that's a tall order. If we can do it in this vehicle, you can do it in any vehicle!

After carefully measuring, and re-measuring, the compartment clearances with the transmission temporarily set back onto its mounts in the car, we began to layout the adapter plate on a sheet of drafting paper -- about 24 inches by 36 inches. Using the transmission shaft as the central measuring point, we must accurately measure from the center of the shaft, to each mounting/bolt hole around the transmission perimeter. We also have to measure between the holes. After accurately placing the holes, we can then draw outer case lines.

The speed reducer may be designed into the adapter plate, or it can be designed as a separate unit. We elected to design both the speed reducer and the generator mount into one unit with the adapter plate in order to reduce overall weight and complexity. This design may require some rework for the flywheel setback value of the GM engine -- more on this later in the series.

We'll continue with building the adapter system in next month's article.