Displacement is no longer a suitable metric for engine output. The Porsche 911 you're looking at here has the same old 3.4-liter in its ass—but don’t tell your ass, because it’ll swear there are five liters worth of grunt on tap. The trick? VSD, short for Vonnen Shadow Drive.
Don’t feel badly if you’ve never heard of this—Vonnen is a new offshoot of Elephant Racing, a supplier of aftermarket high-performance Porsche 911 parts. And they’ve managed to do something that Porsche hasn’t yet done, but will soon: electrify a 911.
Some of the specs have changed as the system nears production-readiness, but we covered the basics in detail a while back. Here’s a quick refresher: VSD yanks the factory flywheel and starter out of a normal 991-chassis 911 and replaces them with a sandwich-type electric motor. It then adds an 85-lb battery to the front trunk, and that’s about it.
When we hear the word "hybrid," we think fuel economy. Blame the Prius and its copious MPGs. This hybrid has a different mission: speed. The system’s total weight penalty of around 170 lbs includes separate cooling systems for the electric motor and battery. But the side of the equation is substantial: additional output peaking at roughly 150 hp and 150 lb-ft.
We spent a few hours behind the wheel of Vonnen’s first installation of the VSD system, a 2013 911 Carrera with a PDK automatic transmission. The biggest indication that this isn’t a normal 911 comes when you turn the key. Instead of the chur-chur-chur of a conventional starter motor, the Carrera’s 3.4-liter jumps instantly to life. At idle, there’s transmission gear-chatter like you’d hear in a single-mass-flywheel GT3.
From there on, it’s business as usual. Vonnen uses an Android device mounted to the dashboard to control the system. With the hybrid add-on switched off, you’d have no clue that this 911 has been modified.
Activate Overboost mode, and there’s no question. Squeezing into the throttle at low revs, you’ll hear electric-motor whine overlaid atop the sweet flat-six music. Suddenly, 3.4 liters turns into what feels like 5.0. The difference in thrust is unmistakable. The base 991 never felt particularly alive in the lower half of its rev range, partially because of the transmission’s long gears, but VSD Overboost fixes that, making the Carrera 2 pull like a Turbo would, just without the lag. But, admittedly, only for a few seconds at a time.
We put the Vonnen 911 on a chassis dyno and saw some impressive gains—which you can read about here—but we also encountered some difficulties because the 911 limited its engine output on the dyno. (I’m generously leaving out the obvious cheap-shot joke about VW Group cars recognizing when they’re on a dynamometer.) The lack of direct apples-to-apples numbers makes an exact horsepower and torque figure impossible to calculate. But crunching the data the best we can, it’s safe to assume the engine and motor together are producing peak output on the order of 460 hp and 415 lb-ft in Overboost, dramatic increases over the stock engine’s 350 hp and 287 lb-ft.
Four hundred fifteen pound-feet of torque. To get that kind of twist out of a naturally aspirated flat-six, you’d need something like five liters of displacement.
Overboost is made for very short bursts before the system has to dial back its output due to heat buildup. It’s great for a laugh, but whipping up 150 hp in an inch-thick space inside the transmission’s bellhousing is a recipe for heat. Even with its own dedicated liquid cooling system, Overboost can heat the electric motor from ambient temp to its thermal limit in under 15 seconds. Vonnen says it’s working on revised motor cooling that increases cooling capacity by a third. Even then, Overboost will remain a quick thrill.
VSD’s real magic happens in Sport and Street, the two modes meant for continuous use. Peak output is similar in both modes, with the difference coming in how they ramp up e-boost. Street Mode gives more boost at low revs, but runs out of thermal capacity quicker under heavy use; Sport Mode dials back assistance in the low- and mid-rpm range to help keep the motor cooler. In either case, electric output can still peak at 150 hp, but torque-fill is dialed down to under 100 lb-ft.
Don’t sound the whomp-whomp trombone yet: a 100-lb-ft boost to a 287-lb-ft engine is still enough to completely transform a base 911. It’s not only 2.5 times the difference in peak torque between a base Carrera and an S, but it’s available all the time, from around 1700 rpm onwards.
Again, crunching the dyno numbers shows a peak system output of something like 460 hp and 375 lb-ft. In this mode, think of the Vonnen as acting like a 4.5-liter flat-six.
The power and torque are indeed commendable, but Vonnen’s biggest accomplishment is signified by the "Shadow" in the Vonnen Shadow Drive name: it’s invisible. VSD is so seamlessly integrated that, if you couldn’t hear the electric motor's high-pitched whine, you’d never know it was there. In fact, the Porsche’s engine computer doesn’t even know it’s there—Vonnen’s add-on system receives throttle position, RPM, and PDK clutch-pressure data from the car's OBDII port to do its thing, but does not talk to the factory Porsche control systems. For all the car’s computer knows, it’s a particularly quick, bone-stock 911. Maybe it’s heading down a hill? Maybe it’s filled with helium? It doesn’t ask such questions.
Climbing 2100 feet over a seven-mile stretch of twisty mountain road near the company's Silicon Valley offices, the Vonnen 911 felt just like a 991, only with a ton more torque and power. In Sport mode, the system didn’t hit its thermal or battery-capacity limits, but by the top of the mountain, things were sufficiently preheated that Overboost lasted just a second or three before it started to cut power. A few seconds in time-out brought it back—but it’s safe to say that while Overboost isn’t quite repeatable, Sport mode is up for the rigors of what most people can use on public roads.
VSD performs regen any time the battery isn’t full, and will do it when cruising or idling. It ramps up regen in lockstep with brake-system pressure, and it feels completely natural—with regen active, it simply feels like there's slightly more engine drag off-throttle.
All of which continues to give the impression that you’re driving a conventional 991 with a far larger engine. Except, as Vonnen points out, living with VSD means living without the compromises of a large, angry, cammed-to-filth engine: No terrible fuel economy. No lumpy idle. No reliability issues. I’d put a "5.0" badge on mine, just to piss everyone off.
If this all sounds too good to be true, it’s not. It’s just… too expensive.
See, Vonnen charges $75,000 to install the system. In the case of a base 991.1 Carrera, that means VSD costs as much as the car. Much of that considerable expense is likely due to the battery. On average, lithium-ion car batteries now cost car manufacturers $200 per kWh of capacity. But the VSD's 1.0-kWh battery is worth far more than $200, because it doesn't act like a normal EV battery. At max power output, the robust battery in a Tesla Model S P100D could empty itself in about 15 minutes. Of course, it would overheat long before this.
The VSD’s battery is asked to supply enough peak power that it would fully deplete itself in less than 30 seconds! Or, it would be, if the motor didn’t overheat first. Point is, this battery needs to be extraordinarily power-dense, as compared to the typical EV battery, which is energy-dense. And that, according to Vonnen, costs serious money.
Vonnen insists this Carrera is a proof-of-concept prototype and understands that the market for a $75,000 upgrade to a base 991.1 C2 is likely infinitesimal. Next up? Well, there’s a 991 GT3 sitting in the back of Vonnen’s workshop. You do the math.
Still, the beauty of a GT3 (and indeed any Porsche) isn’t just the capability, it’s that the level of performance is always there. In that sense, VSD's system doesn’t measure up to the robustness of the car it’s installed in. And frankly, a 100-hp boost in a GT3 that's already cranking out 500 hp would feel like an incremental gain.
But on a car whose engine is only cranking out 130 hp, like an early 911, this would be a order-of-magnitude game-changer. And, luckily for Vonnen, not only are those cars worth a fortune, their owners have money to burn. If I owned an early long-hood air-cooled 911, I’d keep the original small-displacement flat-six and throw in a VSD. I’d have a sweet, numbers-matching little car that would blow away big-block monsters with none of the personality change that comes from an engine swap. And if you’ve priced performance builds on big air-cooled Porsche engines lately, $75,000 suddenly doesn’t seem so expensive.
Until then, the 2013 Vonnen Carrera VSD remains a fascinating proof of concept, showing off a truly magnificent engineering project that’s been installed in the wrong car.