Rear Suspension

The rear suspension was riding about 0.75 inches (19 mm) below stock, which implies the back end is 200 lbs over stock. To restore the ride height, and upgrade to a stiffer suspension, I replaced the 24 mm diameter torsion bars with 29 mm bars. This doubles the spring rate of the rear suspension to 250 pounds per inch of travel.

To replace the torsion bars, I followed the procedure on the Pelican Parts website.

The car has to be jacked up, and the rear wheels removed. The lower mount of the shock has to be removed in order to allow the suspension to completely lower removing all preload in the torsion bar.

 The radius arm is removed, exposing the torsion bar seated in the torsion tube.

The rocker panel trim and the round cover in the lower corner of the wheel arch had to be removed so that the torsion bar could be removed.

The new bar is installed and the radius arm is installed.

With the extra mass and stiffer torsion bar setting the correct ride height can be tricky. When the car is in the air and there is no force on the suspension, the wheel height settles at a certain position. And when the car is lowered down, the car will compress the shocks and load the torsion bar. With extra mass, the car will settle at a lower position. With stiffer torsion bars, the car ride height will be higher. By changing the no load, angle of the radius arm, the loaded position (ride height) can be adjusted. The radius arm has a coarse and fine adjustment. For coarse adjustment, you rotate slip the radius arm onto the splines of the torsion bar in a different angles. You are limited to discrete rotational positions based on the pitch of the splines on the torsion bar. The inner rod and outer rods have different spline counts. The inner end of the rod has 40 splines for a pitch of 9 degrees, and the outer spline has 44 teeth for a pitch of 8.2 degrees. By changing the inner rod by one tooth in one direction and the outer rod by one tooth in the other direction, the rotational position of the radius arm can be adjusted in increments of 0.8 degrees, or about a ¼ inch (6 mm). I think that is a simple but clever bit of engineering. For finer adjustment, the two bolts on the spring plate of the radius arm used. The adjustment bolt has an off center collar, and by twisting the bolt, the spring plate moves up or down relative to the radius arm.

Similar bolts with off center collars are used to adjust wheel camber and toe. I don’t want to mess with these adjustments. Be returning the ride height back to the stock position, my wheel alignment should still be fine. Unfortunately, these bolts must be removed in order to access the torsion bar, so with a paint pen I carefully marked an arrow indicating position, so that the alignment can preserved when everything is put back together. It took a couple adjustment iterations to get the ride height correct.

The main goal was to get the ride height back to stock, to prevent tire rub due to the added mass of the conversion. As long as the work was being done, I decided to upgrade to even stiffer spring rate to improve handling. The ride is firm, but not harsh. The roll in cornering was never bad, but now there is even less. There was a slow rolling feel to the car coming out of turns or going over rolling hills after the conversion. All of these issues are solved and I am quite pleased with the results.

Front Suspension

The ride height with the extra weight of the battery pack is lower than factory specification by 1.125 inches. I was surprised that an extra 250 lbs in the front could have such an effect, but a little research showed that the spring rate of the front suspension is 110 pounds per inch. So, the one inch drop is perfectly explained when the extra weight in the front is distributed across the two front wheels. This ride position is unacceptable because the tires are rubbing against the fenders on mildly rough road conditions.

A common racing setup to increase the suspension stiffness is to add coil springs around the housing of the shock or strut. These coilovers are great because they are easily adjustable, but this option is not cheap. In fact it is quite expensive. But they do look nice:

I chose a more economical option. The stock springs on the 911 are not leaf springs or coil springs, but torsion springs. A torsion spring is a bar, gripped at both ends with splines, that is loaded by twisting as the wheels move up and down. The torsion bars are mounted inside the tube of the A arm (see the red arrows in the image below). The spring rate can be increased by using a torsion bar with a larger diameter, and because increasing suspension stiffness is such a popular upgrade, torsion bars are available in various diameters in 1 mm increments.

The stock torsion bars are 19 mm in diameter in front and 24 mm in back. A commonly recommended setup for autocross racing and street use is a 21/28 mm torsion bars. I did some calculations, to take into account the extra mass of my batteries, and determined that the 22/29 mm bars would be good in my car:

The old bars are slightly bent.

 Installation wasn’t too bad. It took me about four hours, following the procedure on the Pelican Parts website.

With the ride height restored to factory specification, I took the car out for a test drive, and I must say the difference is astounding. The biggest issue was tire rub, and the new springs completely solved that problem. But surprisingly, the steering was much lighter, and more like I remember from when the car was still gasoline powered. I think the alignment and steering geometry must have been compromised with the lower position. Further, I can feel that the back end of the car moves around more than the front. There is a rolling feel or slow bounce that is distinctly occurring in the back of the car coming out of turns or cresting rolling hills. I can’t wait to see how the car handles when I get the new rear springs installed.

Brake Booster Pump Installation

The first EV component I purchased was a vacuum brake pump kit (see Brake Kit).  For a year and a half that kit has been sitting in a large, but as of late, dwindling stack of parts awaiting installation. After driving around for a couple weeks with no brake booster assist (essentially manual brakes), I decided to install the brake pump.  The brakes stopped the car just fine, as I have been limiting my top speed to 45 MPH until the motor brushes are seated. But, if ever a situation demanding a quick stop should arise, I would want the brake booster active.

Here is the block diagram of the brake vacuum system.

The most difficult part of the installation is finding the location where everything fits in the available space. The vacuum pump and reservoir are in front of the batteries, near the front bumper.

The relays are mounted next to the 12 volt battery.

This is the fitting that connects to the brake booster at the master cylinder.  The size is 12 mm and the vacuum kit is 3/8 inch.  I was able to use some of the existing metric vacuum hose and attach it to a 1/2 inch to 3/8 inch reducing barbed hose coupler.

The vacuum kit didn't come with enough vacuum hose for my installation, and at the pressure switch you can see that I switched to a blue vacuum hose.

The pump makes a small whirling or hum sound for a second or two as it pulls a vacuum. The pump turns on after every 2 or 3 presses of the brake pedal. Above 15 MPH, road noise drowns out the sound of the pump and I can’t hear the pump at all.  I understand that VBS now makes a piston pump that draws less power, and is even quieter.

12 Volt Power Source

The reason most cars have a large 12 volt battery is because it takes a lot of amps to start an internal combustion engine. Now that the engine has been removed, the next largest load in the system is headlights at just under 200 watts. With every system turned on I measured 600 watts of power draw. The size of the 12 volt battery can be greatly reduced.

Instead of an alternator to charge the 12 volt battery, there is a DC-to-DC converter which takes the 192 volts DC from the main traction pack, and steps the voltage down to 13.2 volts (exact voltage is configurable and depends on the type of 12 volt battery used). At a minimum, the DC-to-DC converter should be sized to handle the average 12 volt load of the car. During peak power draw, the battery will automatically contribute current to handle the demand, and automatically accept current to charge back up when the DC-DC converter has spare capacity. My DC-to-DC converter has enough capacity to handle peak load, but I still wanted to install a battery for redundancy. If the DC-to-DC converter should fail, I want to have enough reserve energy to get the car safely off the road with functional headlights and hazard lights. I’m using a 18 AHr sealed lead acid battery (this is the same type of battery used in computer power backup systems or UPS) and this is enough to run the 12 volt system solo for 20 minutes.

I made up some aluminum brackets to mount the auxiliary battery and DC-to-DC converter.

I also picked up a metal project box at an electronics surplus store and mounted a fuse and filter network to condition the 192 volts going into the DC-to-DC converter. It is reported that voltage ripple from the controller, switching up to 1000 amps on acceleration, can kill the input stage of the DC-to-DC converter.  A diode and inductor in series on the input protects the system. The diode is mounted on a heat sink. It is probably overkill, but for an extra $1 USD at the surplus store, I figured why not.

Porsche with a Cord

Now that the car is drivable, I need to finish the charging system. The charge plug is something I’ve been thinking about from the very beginning. I decided to install the charging port behind the door of the gas filler tube. The car will still be refueled from the same physical location, but the flow of hydrocarbon molecules is replaced with electrons. The plug is a NEMA L6-30P, a twist lock connection rated for 250 volts and 30 amps. The filler on the left (with plastic cap) is for windshield washer fluid.

Impressions of the First Drive

After the last battery cell was connected and the controller was programmed, I could barely contain my excitement as I lowered the car from the four jack stands where the car has been perched for the last year.  My dad was visiting from out of town and I was proud to show him firsthand the project I’ve been talking about for more than a decade.  I backed out of the driveway with anticipation.  The first thing I noticed was the 8 kHz hum of the controller.  The squeal is audible in my video of the first drive and quickly becomes fatiguing.  I put the transmission into 1^st gear and pressed the accelerator and the car moved forward.  The response to the throttle was rather weak, but with the controller set to limit the motor to 100 Volts and 400 amps, there was only 53 horsepower on tap, a mere 25% of what will be available when the motor limits are increased to 170 volts and 1000 amps.  The motor brushes need a chance to seat properly before handling more power.  But I was moving under electric power and the vibration free propulsion of the electric motor was fantastic.  The clutch worked, the transmission was fine, there was no smoke from the batteries, even the homemade tachometer was driving the stock gauge in the dash, and a sense of relief that there were no major problems was replaced with satisfied pride that the car was finally in motion.  A dream was finally becoming reality.

In the front, the car is sitting about an inch lower with the extra weight of the batteries.  The tires rub the fenders when I go over any bumps in the road.  The solution will be to install stiffer torsion springs in the suspension.

The controller has a quiet mode that uses a higher frequency to pulse power to the motor.  Luckily quiet mode is appropriately named, and now the hum of the controller is not detectable.

I noticed that after a few short drives that the motor gets hot – about 130 degrees F (55 deg C).  I think this could be a reliability issue over time, so I will be ordering a blower to force air through the motor and reduce the temperature rise.  

There is still a lot to be done to finish the car, but driving the car for the first time under electric power is a huge thrill and motivator to keep going.

Video: First Test Drive

The video for my first electric test drive:

My dad was in town and helped with the last two days of wiring, so he got the first ride.