Vonnen Porsche 911 Hybrid Prototype Review

“Aww, where’d my power go?” I asked, sarcastically. I’d found one of the two limits of the Vonnen Shadow Drive hybrid performance booster fitted to this 991.1-generation Porsche 911 Carrera, and the difference was palpable.

This time, I’d just plain run out of juice. The Shadow Drive’s 1.0-kW-hr battery is small both for packaging (it sits on the floor of the frunk) and weight reasons. On the one hand, being small means it recharges quickly when you’re not drawing off it; on the other, it means you can use it all up on a long straightaway.

As Cinderella famously sang (the band, not the movie character), “Don’t know what you got ’til it’s gone.” Unless you’ve just gotten out of a stock 911 with the same engine, you wouldn’t necessarily notice the extra 150 peak horsepower and 150 peak lb-ft of torque Shadow Drive can deliver. In fact, if you’ve driven a 991.2 or 992-model Carrera, you might just confuse it for the turbo torque of those cars. It’s that well integrated. When you run out of battery or reach the system’s thermal limit, though, the loss of electric boost is unmistakable.

“Thermal” is the other limit of Shadow Drive—and one you’re less likely to encounter when the production model reaches the road. A redesigned motor casing will improve cooling efficiency by roughly 30 percent and allow you plenty more hard driving before reaching the safety cutoff.

I ran into each limit once during my test drive on the snaking roads of Southern California’s San Gabriel Mountains. On that beautiful spring day, the production model likely never would’ve found the thermal limit. If you plan to go racing in Arizona in August, though, it’ll still be a factor.

That’s the great thing about Shadow Drive, though. Until you use it up, you don’t feel it working unless you know exactly what to feel for—and that’s by design. Vonnen (founded by the same minds at Elephant Racing, if you’re familiar with the Porsche tuning scene) designed the system purely as a performance enhancer, not a typical broad-range hybrid system. It’s not trying to improve your fuel economy or reduce your emissions. It’s just here to make the car faster.

It does that by sandwiching a 112-kW electric motor between the engine and transmission, replacing the flywheel. The whole unit is small and thin enough to fit in a medium pizza box, and it works with both manual and automatic/dual-clutch transmissions as well as rear- and all-wheel-drive powertrains. It’s controlled by a futuristic-looking inverter mounted on the parcel shelf under the back window and powered by that battery under a false floor in the frunk. Also up front are a pair of radiators, one in each corner mounted in front of the stock radiators. All told, the system adds 210 pounds, with about 40 pounds offset by losing the starter and flywheel and replacing the 12-volt battery with a lightweight variant for a net gain of 170 pounds over stock.

You provide the controller, downloading Vonnen’s app to your smartphone. Simple and straightforward, it gives you graphic readouts on the power and torque being added or regenerated, battery charge, and battery, motor, and inverter temperatures. The charge and temperatures are displayed as bar graphs that shrink as you approach their limits, so it’s easy to see where you’re at with a glance.

Currently, Vonnen offers only four modes: Off, Street, Sport, and Overboost. Off and Overboost are most likely to be used to show off, the former to demonstrate the difference and the latter to make full-throttle, full-power pulls. Regardless of mode, Shadow Drive is tied to the gas pedal, not the engine speed. In Overboost and Sport, Shadow Drive activates at 60 percent throttle, so you really have to be on the power before it kicks in. In Street, it activates at 40 percent throttle to give you a boost when you’re just scooting around town or getting on the freeway. Street and Sport give you an extra 93 lb-ft of torque, whereas Overboost gives you the full 150. Overboost is only meant for quick sprints, as it’ll draw down the battery and heat up the system much more quickly than the other modes.

Because the motor doesn’t kick in until you’re 40 to 60 percent of the way into the throttle, the extra power generally doesn’t come on until the engine has already revved up to around 3,000 rpm, making it feel like a well-tuned turbocharger getting up to boost. That makes it easy to mistake for a system tied to engine speed, but you’ll be disabused of that notion by flooring it in gear rather than downshifting (or letting the transmission downshift itself).

No matter where you hit it, it’s not like hitting the NOS button in a movie. There’s no monster surge of acceleration. Shadow Drive ramps up the power quickly as you go deeper into the pedal, but it’s always smooth, as if the engine has found some extra power it forgot about. Quite honestly, I can’t imagine Porsche tuning its own system any differently when the factory-built 911 hybrid finally becomes a thing.

Likewise, you don’t notice when the battery is recharging. Under light throttle and braking, the motor applies a slight drag to the engine to generate power and recharge the battery. The regen under braking is intentionally mild so as not to upset the braking balance and destabilize the rear end. It doesn’t improve stopping power by any measurable amount.

You also won’t notice it doing any hybridy stuff. As a pure performance booster with a tiny battery, Shadow Drive can’t power the car solely on electric power or switch back and forth between pure electric and hybrid power (though you can shut it off for pure gasoline power). Although there is a hidden plug in the frunk, it’s designed to act like a trickle charger so the battery won’t die if you don’t drive the car for weeks. This is important because the motor replaces the engine starter.

Shadow Drive doesn’t just describe how the motor interacts with the gasoline engine but also how the whole system plays nicely with all the onboard computers. In every case I can think of, adding this kind of power to a car requires an ECU flash, a piggyback ECU, a standalone ECU, or at least rejetting the carb(s). Vonnen doesn’t do any of that. Instead, the system is programmed to operate within the tolerances Porsche’s computers are willing to overlook. That is, the factory computers don’t see anything wrong when the motor is operating despite all the extra total system power being produced, so you don’t get codes or limp modes or stability control hysterics. Shadow Drive just monitors the CAN bus and reacts; it doesn’t try to interfere with or modify the information being transmitted between vehicle sensors and computers.

That assumes, of course, your 911 has a CAN bus. Although the first Shadow Drive system to market will be for modern 911s, Vonnen has versions in the works for air-cooled cars, as well. Cars without things like a throttle position sensor will need them installed, but the system is designed to be as bolt-on as possible. A version for mid-engine Boxsters and Caymans is also being developed. Eventually, Vonnen wants to have systems to fit any rear- or mid-engine Porsche, rear- or all-wheel drive.

Lithium-ion batteries, inverters, cooling systems, and compact, high-torque electric motors don’t come cheap, and with the amount of work needed to install it all, neither does Shadow Drive. At launch, it will cost a cool $75,000 to have Vonnen modify your car, and that’s after you buy the car and ship it to Santa Clara, California, and back. The price includes modifying PDK cars to increase the clutch pressure to handle the extra power and a shorter prop shaft for all-wheel-drive cars necessitated by the slight transmission relocation (roughly 1 inch forward to make space for the motor). Vonnen expects the price to come down with future enhancements and cost savings, but don’t expect it to ever be cheap. After all, this is a performance system for people who can already afford to not only buy but also modify a 911. If you’ve got the cash, though, Shadow Drive delivers better than your typical aftermarket power-adder in every way.

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