After spending an afternoon driving the hybrid Vonnen 911, I wanted to dive into the numbers. Instrumented testing would have been fun, but on a car like this, I really wanted to see the dyno numbers. Luckily, the Vonnen guys did, too, and so they invited me to join them for a day at the dyno shop.
To get you up to speed: The Vonnen system sticks a tiny electric motor in place of the flywheel, turning a stock, naturally-aspirated Porsche into a performance hybrid. The kit is intended to work on any 911, Boxster or Cayman built from 2012 to 2o16; Vonnen's prototype is a 991.1-generation 911 Carrera 2 with the PDK transmission.
For the sake of science, they brought along a fully stock 991.1 Carrera 2 PDK. We first ran that car for a baseline, to find out if, and by how much, the Vonnen Shadow Drive (VSD) affects power transmitted to the wheels when the hybrid system is turned off.
What we didn’t realize, initially, is that 991s are a pain to dyno. When both cars, each rated 350 hp at the engine, put less than 230 hp to the wheels, we realized something was wrong. Turns out the 991.1 has a "Rolling Test Mode" that needs to be activated using Porsche's diagnostic testing computer. Luckily, the Vonnen guys had one with them, and in the proper mode, we gained a lot of power.
The baseline pulls for both cars were still pretty weak, with the best being the Vonnen at 259 hp to the wheels. That implies that 35 percent of the engine’s output is being eaten up by losses in the transmission. Not only is that double the loss we’d expect to see, but also mathematically unlikely, since it would result in a top speed of just 170 mph, well short of the car’s claimed 178-mph max. Porsche doesn’t typically overstate its cars’ performance potential, so there was likely some sort of software limitation happening, either because the car knew it was on a dyno, or because it was running on California’s crappy 91-octane fuel. (Porsches are tuned to run on 93-octane unleaded fuel, which we cannot get in California.)
Another note: the difference between the two cars’ output was less than the variation between runs in each, so it appears the Vonnen VSD motor adds a negligible amount of friction to the system. In fact, the VSD car tended to make more power down low, but slightly less up top. We have no way of knowing if this was from inertial and friction differences or just due to normal variation between two cars. For that reason, I used the Vonnen-equipped car’s torque curve for the baseline numbers.
With those caveats out of the way, let’s look at the results.
Figure 1 - DynoJet Horsepower
The VSD has three modes-Street, Sport and Overboost. VSD Sport and Street have roughly the same overall output, so for ease of display, I’ve eliminated one of the curves from the charts. Figure 1 shows horsepower output, at the wheels, for the Vonnen car. Orange is wheel-horsepower with the system switched off, blue is Street mode, and red is Overboost.
Peak numbers jump from 259 wheel-horsepower from the engine alone to 352 and 362 wheel-horsepower with the system active. The peaks should ideally be the same for both VSD-on modes, but the 10 whp difference is within the noise-we saw that much variation between runs, due to heat-soak of both the engine and the electric motor and battery.
What’s interesting is how close the Street and Overboost modes appear to be, especially because the difference on the road feels enormous. The torque curves show that difference much more clearly.
Figure 2 – Dyno Torque
Figure 2 shows total torque measured at the rear wheels. Here you can see the massive difference between the two VSD modes. At 2500 rpm, the electric motor adds 63 lb-ft in Street Mode (blue curve); Overboost adds 110 lb-ft (red curve.)
All that midrange torque in Overboost makes a huge difference-at 2500 rpm, total system output jumps from 188 lb-ft to 297 lb-ft, an increase of 58 percent. All that torque also contributes to the electric motor heating up so quickly in Overboost, as we experienced on our test drive.
Torque output on a naturally aspirated engine is primarily a function of displacement, so a 58 percent boost in torque feels like a 58 percent boost in displacement. We’d expect to see this kind of gain by replacing the 3.4-liter engine with an engine of more than 5.0 liters of displacement.
What’s most interesting here is that the torque curves are so similarly shaped, meaning that the electric motor is adding a relatively constant amount of torque. As a result, the acceleration you'd feel in the Vonnen 911 is similar in quality to a regular 911, just in greater quantity.
Figure 3 – Total Torque Gains
Graphed differently, Figure 3 shows the total amount of torque added by the Vonnen VSD electric motor. This is calculated by simply subtracting the engine’s baseline output from the runs in Street (blue) and Overboost (green) modes.
What you see is that Overboost peaks at about 112 lb-ft at the wheels, and then gradually falls to about 55 lb-ft as it approaches maximum speed. If we assume the same 35-percent drivetrain loss that we mentioned earlier, we get to the 150 lb-ft output claimed by Vonnen.
In the less-aggressive Street mode, the Vonnen electric motor adds a constant 65 lb-ft at the wheels, or roughly 90 lb-ft at the crank, until very high revs, where its assistance grows to a peak of 85 lb-ft wheel, 115 hp crank, at 7000 rpm.
Figure 4 - Total HP Gains
For Figure 4, I’ve charted the actual wheel-horsepower gains from the electric motor. If we adjust by the same 35-percent factor, we can see the system reaching Vonnen’s 150-hp claimed output in Street mode. I’m not entirely comfortable using this correction factor, but it’s likely not a coincidence that adding 35 percent brings both the gas engine and the electric motor up to their rated outputs. With or without any correction, what we’re seeing is a huge power gain from the electric motor.
Note that the two curves should be closer at very high engine speeds, but by that time in the Overboost Mode test, the motor and battery were likely nearing their thermal limits.
If I had to quote an "at the crank" output number, I’m not really comfortable using the 35-percent loss equation. I’m convinced that the engine was instructed by the computer not to produce its full power during our dyno testing. Instead, I’d adjust the gas engine’s baseline output so that it reflects what we typically see on a DynoJet (about 17-percent loss relative to quoted SAE horsepower and torque), and then add the raw horsepower contributed by the electric motor plus that same 17 percent. Rounding numbers slightly, I get peaks of 460 hp and 375 lb-ft in Street Mode, with Overboost pushing that torque number to 415 lb-ft.
So how fast is it?
This is my favorite part. I input all of the torque curves into a spreadsheet and calculated theoretical acceleration numbers for a base 911 Carrera PDK, then one with the Vonnen VSD system installed but turned off, then VSD in Street and Overboost modes.
Note that these calculations use the raw dyno output numbers and were not corrected for drivetrain loss.
You’ll see that the acceleration times are slower than what you've seen published, and that’s for two reasons. Firstly, because of the lower-than-expected engine output previously mentioned, and secondly because this calculation doesn’t take into consideration a clutch-drop launch like PDK’s Launch Control provides-this is merely a “street-start,” flooring the throttle from a stop.
First, notice the difference between the standard 911 and VSD (Off) numbers. This small degradation in performance is due solely to the 170-lb net weight gain from the Vonnen system.
The 0-60 numbers tell a very clear story, dropping from 5.0 to 4.0 seconds in Street mode and down to 3.6 in Overboost. The same effect is clear for the 0-100 and 1/4-mile acceleration times.
However, the most dramatic improvement is during in-gear passing, where the VSD’s low-end torque transforms the car. Going from 50 to 70 mph in 5th gear drops from 6.4 seconds to just 3.9 in Overboost. That means, in Overboost, the Vonnen car accelerates almost as hard in 5thgear as the base car does in 3rd (it would do 50-70 in 3.5 seconds.)
Basically, the numbers back up what your butt feels when you’re driving the VSD-equipped 911: it’s transformatively quick. The hybrid assist does the closest approximation of swapping in a bigger engine. In Street mode, it averages a 36 percent increase in torque from just off idle to redline, magically and silently turning the 3.4-liter into a virtual 4.6-liter. And in Overboost, it averages a 48-percent boost in torque, faithfully replicating what a 5.0-liter version of this flat-six might feel like.
Or in other words, drive the Vonnen in Overboost mode and you’ll experience the world’s first 5.0-liter, naturally aspirated 991.
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