Pre-work 2: Performance – Motors, horses and ft.lbs

The goal of the project is to build an all-electric 1959 Cadillac that performs the same as or better than the original car. The original is not a muscle-car, but it had a large V8 engine at 390 cubic inch/6.4 liter and (if you didn’t mind the maelstrom in the tank) it would get the car off the line ok-quick. The Chevrolet Impala that year, the normal-normal car of the time, had 236 ci/3.9 liters, so the Cadillac V8 really did rank up there for performance:

Cadillac DeVille engine specs

Displacement : 6391 cc | 390.0 cu in. | 6.4 L.
Power : 325 HP (239.2 KW) @ 4800 RPM
Torque : 430 Ft-Lbs (583 NM) @ 3100 RPM

Chevrolet Impala engine specs

Displacement : 3859 cc | 235.5 cu in. | 3.9 L.
Power : 135 HP (99.36 KW) @ 4000 RPM
Torque : 305 Ft-Lbs (414 NM) @ 3800 RPM

Design requirement #1 was top speed, and lining up the cruising speed with optimum efficiency.

The engine/motor drives the gear-box, the gear-box goes into the long driveshaft running down the center of the X-chassis which drives the differential, and the differential drives the wheels.  The tire-size of the car is 8.2×15, which means 1 turn of the wheel will get me 8.64 feet. There’s 5,280 feet in a mile, thus you need 610 turns of the wheel to go a mile. 

The original car/engine would hit 120 mph top speed (that’s where the speedometer ends). So, I need the wheels to do 120 miles * 610 turns in 60 minutes, comes out to 1,220 wheel-rotations per minute / rpm.

To calculate the rpm for the motor, I then follow the drive system in reverse. To go 120 mph I need 1,220 wheel-rpm’s coming out of the differential, the differential has a ratio of 2.94:1, so I need 2.94 * 1,220 = 3600 rpm coming out of the transmission / going into the drive-shaft. 

The electric motor I picked is a Siemens Azure AC motor. It’s super-compact, dust-proof, water-cooled and just really nicely made. It will do 10,000 rpm max, and it’s optimum efficiency is around 4,000 rpm. I do not like pushing parts to their limit, I prefer to max out at 85-87%, so that’s where I’m putting my 120 mph “marker”.

87% of max rpm on the electric motor is 8,700 rpm, and I need to turn that into 3,600. That means I have to find a piece between the motor and the drive shaft to take 8,700 motor-rpm’s and turn them into 3,600 drive-shaft rpm’s. That’d be a  to 2.42 : 1 reduction gear.

Finding a compact, serious-duty 2.42:1 gear box and then fitting it to both the Siemens output-shaft and the Caddy drive shaft turned out to be a lot harder than I’d figured. I finally landed on a 1.94 : 1 unit made TorqueBox, and it is a very well-built unit. It should definitely hold up to any torque increases I can come up with. 

Given that I now know my gear-ratio I can calculate out what the actual rpm’s will be for cruising and max speed. The 8,700 motor-rpm’s translate into a max speed of 150 mph and the 4,000 rpm at max efficiency comes in at right around 70 mph. My commute consists of mostly freeway at a posted speed-limit of 70 or 75 mph, so optimum efficiency is right where it should be. Requirement #1 met, and this should work. 

Design requirement #2 was putting “the power” behind that speed set-up 

I’ve tried to calculate this out, and while the way I did this is probably wrong in a million-and-one spots, it should be close enough that I can be confident The Plan will work. 

For my conversion, I have picked the Siemens-made AC induction motor that was designed for the Azure Dynamics project. That project went bankrupt so these motors were for sale at a very reasonable price and they are very elegant motors. They’re sealed/dust-protected, they have built-in water cooling, and they are rated to handle 300V & 248A. That’s roughly 75kW or 88 hp. That’s pretty good for an electric motor, but a lot less than the 325 hp of the original engine. 

with a graph like this   

I really want to meet or exceed original performance, so to match the original horse-power I have taken two Siemens motors and joined them into a Siamese-twin setup. That should produce 175 hp at rated power i.e. continuously and 280+ peak hp’s in a pinch. That gets to within 15% of the original engine, so now we’re getting somewhere. This is what that then looked like in my garage (with the TorqueBox):

Getting to within 15% of original feels right, because I have never had to floor the car with the gas-engine that hard (to 85% of it’s red line) in pretty much any driving condition, and the car has always been very responsive to me pressing the gas-pedal. Still, I’ve tried to make double-sure by comparing the power-graphs of the original gasoline engine and the Siemens motor. The engine, with the optional tri-power carburetors, has a graph like this one:


From that graph and the Siemens graph (a little ways up the page) I have (in Excel) mapped the speed of the car in mph, calculated how many rpm I need to go into the drive-shaft, and from there through the different gearing ratios of the original transmission and the Torquebox. That mapping compares the torque and horse power driving the car at each 1-mph increment. That is highly slanted in favor of the engine/gasoline version in  that it ignores all of the losses on the engine-version (the transmission, the pumps&compressors for AC, steering etc), but if the motor(s) can out-do the engine in this math it should be that much better/nicer in reality. 

The compare of the rpm’s needed from the engine/motor to go through their respective transmissions looks like


You can see the gas drivetrain (in orange) go through its gear-shifts, and the electric motor is just a clean, straight line.

I then mapped those rpm’s to the respective power graphs, one mph at a time, and compared the two. It comes out looking like this:

In these graphs the first little bit (zero to 6 mph) is a little weird because there is no real ICE data and the influence of the torque-converter messes things up, but that piece can be ignored IMO because the ECV performance is so far ahead of the ICE performance.

To me, this shows that while the motors have to work for a living to do what the engine did in the 0 to 35 mph range, once speeds go up there is plenty of margin / I don’t have to run the motors at their peak 248 Amp power, I can run at 30-50% of that. That would mean running at 80-100A/300V so 25-30kW on each of the 2 motors so 50-60 kW total, which lines up with the battery-sizing estimates that I did earlier.

As for the torque, the ECV version is pretty much 1.5 – 2x its ICE predecessor through ~ 85mph, after which it starts to become ever closer. At 120mph it looks like the two are roughly equal, so overtaking should be no better no worse out on the freeway, and in town the car should have plenty pulling-away power. 

Factor #3 was acceleration

Turning those same numbers into “seconds” makes the same point. The original acceleration was rated as 0-60 mph in 10.6 seconds, 0-100 mph in 29.4 seconds, and 0-110 mph in 42.5 seconds. With the increase in weight and the new electrical setup, 0-60 should be around 8 seconds, 0-100 in under 15 seconds, and 0-110 in under 18 seconds. It’s not Plaid-mode or Ludicrous Speed, but it’ll do.

The fixed reduction-gear that replaces the transmission means that there is no power-loss in a torque-converter/clutch, i.e. motor power is pretty much 100% transferred into wheel-power (the original loses 20%+ of its generated power in the transmission, the clutch, the torque-converter etc). This is part of why I think I can improve on the horrible fuel-efficiency from back in the day, why I think there’s additional margin in the performance assumptions etc, and I’ll wait for the first test-drive to confirm all that. It just makes the project that much more likely to hit the deliverable of “meet or exceed original performance”.

The fixed reduction-gear also comes with one cool feature: the car can go backwards as fast as it can go forwards. I’m not sure when this would come in handy, but it would be something. The original car always applied it’s 4:1 reduction while in reverse, so I have electronically limited the car’s reverse speed to 1/4 of  forwards-speed, just to avoid glitches and to make life easier on people who guest-drive the car. This way it’s probably over-engineered, but it made me feel better…

All in all, if I can get the EV version to perform even close to these performance numbers, I’ll claim that this project has met its expectations around better-than-original performance…