Driving the 1988 Vonnen Porsche 911 Hybrid: Yes, You Read That Correctly

Motor Trend

6 days ago

No, there's no mistake. You read that correctly. This air-cooled 1988 Porsche 911 is a hybrid. Yes, it's a modern retrofit, and no, it has nothing to do with saving fuel.
Vonnen has retrofitted a 1988 air-cooled Porsche 911 with its Shadow Drive hybrid system, enhancing performance with an electric motor adding 150 hp. The system, costing $69,000, doesn't improve fuel economy but boosts driving dynamics with multiple drive modes for varied driving experiences.
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This is the work of Vonnen, an offshoot of the well-known Porsche suspension engineering shop Elephant Racing. If any of it sounds vaguely familiar, it's because this is far from the first Porsche the company has electrified. You may remember the water-cooled, 991-generation 911 where this system was pioneered a few years back. What in the World is Going on Here?
Now, Vonnen has made good on its promise to adapt its water-cooled hybrid technology, dubbed Shadow Drive, to an older, air-cooled 911. The system's electric motor has been necessarily downsized to fit the smaller bellhousing of the much older transmission, but otherwise the technology is the same. We're only talking about a 3mm difference in the diameter of the axial flux motor, so peak power is still 150 hp like in the water-cooled cars, but peak torque drops to 120 lb-ft (down from 150). As before, the hybrid system fits between the engine and transmission, and if those are stock, it requires only new mounts but not any cutting.
The car's starter, ring gear, and flywheel are also replaced in the process, helping to offset weight gain. As for the inverter, it can be mounted in several spots, though tucking it into the engine bay helps hide the upgrade and maintain the car's stock appearance. A few holes do need to be drilled to mount it and pass the cooling lines and high voltage cabling up to the coolers and battery in the front. As for the coolers, there's the stock oil cooler mounted ahead of the front passenger wheelwell, while a second powertrain cooler is matched to a second oil cooler ahead of the driver-side front wheelwell—each backed up with electric cooling fans.
The battery is a 400-volt lithium-ion job holding just 1 kWh of usable power. It's smaller than Vonnen's earlier efforts thanks to a new, more energy-dense chemistry, which cuts weight by about 20 percent. Mounting it in the frunk requires losing the spare tire and cutting the gas tank down by about a quarter, from 20 gallons to 15. All in, the system adds about 150 pounds to the car's total curb weight.
Because Shadow Drive is self-contained, it's also compatible with other power-adders like turbos or a supercharger. It works with rear- or all-wheel-drive powertrains and manual or automatic transmissions (the car we drove was a manual), and it probably shifts the car's weight balance forward slightly given the battery and extra coolers up front.
Of course, there's a little more to it than simply setting up the motor and battery. The car's nearly 40-year-old electronics couldn't handle all this modern hybridization, so other bolt-on modifications were necessary. Included on the list are a throttle position sensor, brake pressure sensor, clutch switch, crank position sensor (integrated into the motor), and a can bus system to make them all talk to each other. A stronger clutch is also a wise investment.
No need for screens, though. An app on your phone controls the whole thing, allowing you to select the drive mode and monitor vital system statistics like power output and temperature. We recommend investing in a good, strong phone mount so it doesn't fly off the dash every time you drive this car like Vonnen intends. Super Shadow
There are now five Shadow Drive drive modes, one more than when we last tested a Vonnen creation. We started in self-described Off mode to reacquaint ourselves with the nearly 40-year-old Porsche's behavior. It was a stark reminder of how slow old sports cars feel compared to modern ones. By today's standards there's precious little torque, and nothing fun really happens until you're into the top third of the odometer.
Stealth is your default around-town mode. It kicks in when you push the gas pedal 30 percent or more, ramping up electric torque to 80 lb-ft initially, and when you push the pedal past 50 percent, it bumps up further to 91 lb-ft. It's intended for local city and highway driving when you might want to win a short stoplight drag or make a quick overtake. Because of the lower output, it comes on more like a supercharger or a perfectly sized turbo, usually around the time the engine hits 3,000 rpm—a nice little shove in the back that ramps up perfectly in line with the gas engine's output.
If anything, we'd like to see it arrive at a lower threshold. There are times when traffic is accelerating slowly enough that you'll never get to 30 percent throttle before the next light. The extra power is addictive, and we want it immediately. Although it's not intended for performance driving, we actually found Stealth quite useful on the mountain roads above the company's Santa Clara, California, headquarters, too. Sport mode provides more power, but it doesn't kick in until you're at 80 percent throttle. On the tight, technical roads of the Santa Cruz Mountains, you're rarely that close to wide-open throttle between corners, so the earlier activation in Stealth mode can be more helpful with quick, short bursts of moderate acceleration.
But when the road opens, you'll want Sport. Peak torque is actually lower at 80 lb-ft, but it comes on much more quickly when you floor it. Why? To reduce the strain on the battery and cooling system and allow the system to last longer before depleting the former or triggering a cooldown period for the latter. Both recharging and cooling down happen quickly, but losing all your boost even temporarily spoils the fun until it returns. Those bursts of full-throttle acceleration, though, are worth the rarely encountered penalty. The combined power is about as much as you want on roads like these in an old car like this, even one upgraded with parent company Elephant Racing's suspension and big brake kits.
When you want even more power, there's a new semi-secret Sport Plus mode activated by tapping Sport three times. It bumps the initial torque application to 112 lb-ft for 0.5 second when you drop the hammer for that extra kick before ramping back down to 80 lb-ft. It'll deplete the battery and heat up the system quicker, but it's worth it.
Then there's the showoff mode, called Overboost. That's how you get the full 120 lb-ft, and like Sport, it only kicks in above 80 percent throttle. Think of it as your dragstrip mode. It's the most fun in the right environment, but it's also the quickest to run down the battery and reach the cooldown threshold.
Whichever mode you prefer, the Shadow Drive difference is palpable, and once you've tried it, you'll never turn it off. The car just feels too slow without it. What's more, its overall tuning is so good, you'd think it came from the factory this way. There's no light switch moment when the torque comes on, and the tiny amount of regenerative braking it does to recharge the battery is completely imperceptible. Most people you give rides to will just assume the engine is built, not hybridized.
What it won't do is increase your fuel economy. Vonnen's system is a pure performance enhancer. Theoretically, you might not burn fuel quite as quickly driving at wide-open throttle as you would without the electric motor, but it would be hard to measure. There's no engine stop-start system, and the electric motor never drives the car with the engine off. Until you're gunning it, the only sign it's even there is the nearly instantaneous engine startup rather than the usual old car crank over. Big Power Is Never Cheap
Like the water-cooled system, the new air-cooled variant runs a cool $69,000 for the kit, plus installation at Vonnen or an authorized installer. You can save a few bucks ordering the half-power system for $54,000, but at that point you might as well just spend the money for the full monty. If you've got money not just for old Porsches but for modifying them, why go halfway?