Gear

DIY Electric Porsche Gives Us EV Envy

I’m gathering the last of the parts needed to finish converting my ’67 Beetle to electric power. I attended a meeting of the local EV interest group last week to ask some questions, get some recommendations and see what my fellow converters have done with their cars. You’d think this would have given me all […]

diy_porsche_911_01

I'm gathering the last of the parts needed to finish converting my '67 Beetle to electric power. I attended a meeting of the local EV interest group last week to ask some questions, get some recommendations and see what my fellow converters have done with their cars. You'd think this would have given me all kinds of motivation to finish my car, but I left feeling a bit depressed.

I've been happily engaged in building my electric Beetle for several months now, knowing all along that my puny budget might be half of what most people say is needed for a bare-bones conversion. Doing more with less, finding creative ways to solve problems, and getting a car rolling without spending a bundle has been exciting. So as I left the meeting, I struggled to put my finger on what was bothering me.

Then it hit me: This meeting of local converters damaged my confidence in pulling it off.

Don't get me wrong -- everyone at the Utah EV Interest Group has been universally helpful and full of information and suggestions. These guys are passionate about electric vehicles, and they really know what they're doing. Many of them have backgrounds in auto repair or restoration and don't cut corners. So although they offer excellent advice, heeding it often requires money I don't have. I've started to wonder if I'm cutting too many corners.

That got me daydreaming about what I'd build if I had the money and could convert any car with any parts. I've always been partial to the Porsche 911. I'd put a really big motor in it and get a motor controller that could send it plenty of power. Of course, some would consider doing that to so great a car utter blasphemy or worse. Oh well.

Turns out I met someone at the meeting who is doing just that, and only a few miles from my house. After meeting Mark Clifford and arranging to check out each other's projects, seeing his beautiful 911 didn't do my depression any good. Neither did his experienced assessment of the rusting floor pans in my Beetle. But that's another story.

diy_electric_porsche_911_07Here's what can be done if you have a substantially bigger budget than I do: Start with a $9,000 1981 Porsche 911 Targa in great condition with just over 100,000 on the odometer and a recent paint job. Remove and sell the engine for $3,000, redo the dash and seats for $1,800, then install a NetGain Warp 11 motor. It's one of the biggest DC motors available for conversions, and it runs $3,000. Add one of the best motor controllers around, a Cafe Zilla Z1K ($2,000, used). Clifford is running a baker's dozen 12-volt lead-acid AGM batteries (156-volt system) that a friend donated, but he hopes to swap them for lithium-ion down the line. He's got another $5,000 in an upgraded clutch, motor adapter and miscellaneous parts.

Altogether, he's into the project for $17,800 so far. That includes $11,800 in conversions, parts and components.

So what will the car do when it's finished? There are all kinds of problems comparing EV performance to fossil-fuel performance. Ratings for electric motors are usually given for constant operation, not peak performance. No one seems to want to provide "max" statistics for a motor, which is somewhat understandable given the many different combinations of voltage and current at which it might be run. Still, EVSource has data showing the motor Clifford is using can produce an astounding 460 foot-pounds of torque at 1,400 amps. The Zilla Z1K will produce up to 1,000 amps of peak motor current.

Time for a quick back-of-the-envelope estimate.

The Zilla's 1,000 amps at 156 volts generates 156 kilowatts. We should be careful about equating electrical power to mechanical power, but they will be in the same ballpark. So 156 kilowatts at 85 percent efficiency (average rated efficiency for the motor, although the motor is more efficient at lower loads) comes out to around 178 horsepower. Not bad at all, and with all that torque instantly available, it will be plenty snappy off the line. No one has done exact calculations, but with lithium-ion batteries the car could be capable of 60 to 100 miles of range. Li-ion isn't in Clifford's budget yet, but the lead-acid batteries provide around 30 miles depending on conditions.

Why would someone do this to a Porsche? Clifford says he's always been interested in Porsches and going fast, and he has some experience restoring and working on other cars. Ironically, he says doing an EV conversion gave him an excuse to buy the 911. Turns out the green aspects of EVs initially appealed more to his wife, making the prospect of buying the car for a conversion an easier sell.

I still drool over Clifford's Porsche, but my depression has passed. My conversion will be much more basic, but I'll be reaching significant performance of a much different sort: overall cost. My total expenses? Around $3,500.

Coming up: I've installed a temporary solution to my rusted-out floor pans, and I hope to get the car around the block within the next couple weeks. Stay tuned.

Got any advice, suggestions or ideas for Matthew? Share 'em with us! Follow his progress on Twitter @Wired_EV and here at Wired.com every week. He’s also launched a blog, evBeetle.com, to cover the conversion in microscopic detail.

Photos: Mark Clifford

UPDATE , 1 p.m. Eastern Jan. 26 : David Dymaxion is an electrical engineer and member of the Utah EV interest Group who is building his own electric Porsche 911. He sent a friendly email saying our back-of-the-envelope math is a bit optimistic because it doesn't account for battery sag. His math works out like this:

On the horsepower calculation: Don't forget battery sag. Let's say Mark upgrades to Optima batteries with 0.003 Ohm (3 milliOhms) of internal resistance for each battery:

Vbatt = 12 V - IR = 12 V - (1,000 A) x (0.003 Ohm) = 9 V

So with 1000 A, each battery sags to about 9V: 13 batteries x 9V = 117 kW = 157 hp (no losses)

But you'll have some losses (around 10+% electric plus about 15% drivetrain): 157 hp x 0.75 = 117 hp

While that seems low, don't forget the torque curve is much fatter on an electric -- Mark's car will have about twice the torque of the gas engine he replaced, at low rpm. So in first gear he'll accelerate like a car with about twice that horsepower.

Also, Mark could get a controller that outputs 2,000 Amps. The batteries would then sag to 6 V each, but his power would go to about 156 hp after losses. If he goes to lithium, with less sag, the number would go higher, plus the car would weigh much less.

So there you have it, from an electrical engineer. Thanks, David!

See Also:

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Mark Clifford's electric Porsche uses 13 lead-acid batteries and a Netgain Warp 11 motor. Some back-of-the-envelope math suggests the car is putting down 178 horsepower and around 400 pound-feet of torque.

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More batteries. You need a lot of them when you're using lead-acid. Clifford hopes to upgrade to lithium-ion at some point. That would give him a range of 60 to 100 miles.

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Top-shelf components include the Netgain motor and Cafe Zilla motor controller. My shoestring budget means I can only dream of that kind of EV-porn.