1983 Porsche 911 SC Targa

Friday, July 6, 2012

Mounting the Electric Motor in the Car

There are four stock mounting locations on the car for mounting the motor and transmission. The mounting points utilize a hard rubber to isolate the car from motor vibration. They are engineered to take the twisting load of a motor under acceleration, so I would like to reuse the mounts for the electric conversion. I'm also reusing the stock cross bar to connect to the rear motor mounts.

I needed a way to locate how the rear cross bar is positioned relative to the electric motor. The geometry is important to ensure suspension travel and drive loads are the same as with the gas engine. Taking measurements on the gas motor proved to be difficult, so I made a jig that connects to the transmission mounting flange on the gas engine block, wraps around the engine, and bolts to the cross bar.

The jig is then mounted to the transmission flange on the electric motor adaptor plate. The cross bar is held in the correct position and I can connect the electric motor to the cross bar with a metal bracket.

To start on the bracket, I made a disk out of 1/4 inch (6 mm) steel plate to bolt to the end of the motor with 4 bolts on a 6 inch (150 mm) diameter, 90 degrees apart. I have never had good luck with precision drilling. I always end up drilling the clearance holes larger to account for placement error. This compromises the strength of the bracket because some bolts will be loaded more than others. I did some research and was able to place the holes very precisely using this technique: lay out the holes using a digital caliper. The first hole is marked by measuring off 3 inches from the center. The next hole is marked by scribing a line 3 inches from the center mark in the general location of where the second hole should be placed, and another line 4.243 inches from the first hole. The intersection of the two scribe lines marks the center of the second hole. All four holes are marked and double checked. Next, a center punch is used to make a small indentation into the metal surface at the center of each hole location. The first punch mark is made very lightly, and if the location is off a bit, a second strike, with the punch angled toward the correct location, can make a deeper pit closer to the correct location. A drill bit tends to “walk” when starting a hole on a flat surface. The punched dimples provide a little well that holds the drill tip in the correct position while the hole is drilled. The result was amazing. No need to enlarge the clearance holes. 

I used 2 inch (50 mm) square tubing to span the gap to the cross bar. Some metal tabs were added to bolt the assembly to the cross bar. For now the parts are just tack welded, in case any adjustments are needed later.

In the lower left corner is an additional cross member that will be welded in between the two "L" brackets, once I test the fit, and fully welds all of the joints.

The motor and transmission were installed in the car.  After checking the alignment, I will remove the parts, weld them up fully, and paint them.

Here is a video of the wheels spinning for the first time under electric power. The motor is connected to a 12 volt battery with jumper cables.


  1. Warm Greetings!

    Today, I visit your website and after reading your blog i realize that it is very informative. I'm highly impressed to see the comprehensive resources being offered by your site.

    Thanks and Regards

    electric motors

  2. ravi, thank you for your kind words. I am enjoying the documentation process almost as much as the build process.

  3. Hmmm, "ravi" left the exact same post on my blog. Interesting...

    Nice work Joey. I'll tell you, I would have given my eye-teeth to have been able to weld while building the car. Instead I drilled lots of holes and used lots of bolts and nuts. Ultimately this will mean a cleaner, and likely stronger build for you.

    So, have you given more thought as to which batteries you'll be using? Those new CA cells from CALB are quite enticing even if they are a bit more expensive. Still, you could build a 20kWh pack for less than $9K. Of course, I'm using Jack's price to calculate that and that does include the big heavy straps they're making and the Nordlock washers. Those items really are a value add.

    BTW, I saw your reply to the comment I left on Jack's blog. I wanted to thank you, but didn't feel that was the right forum. So I'll say it here. Thanks! And I'll follow that up with a question. Any chance you're going to be bringing the car to this year's EVCCON? I realize that might be a bit ambitious, so perhaps EVCCON 2013? I really am eager to see your car once you get it on the road.

  4. Hi Tim, This project was a good excuse to get a welder. Tack welds are no problem. I still need a little more practice keeping my beads straight.

    Yes, after seeing the last post over on http://blog.evtv.me/ I now want CA Calb cells. I'm thinking either 60 cells of the 180 Ah size, or 86 of the 100 Ah size. I wanted to get the range to abaout 100 miles. With the CA cells I'm leaning toward the 180 Ah size. The price is something like 10% more, but you get 50% more current output and up to 40% more lifecycle. It's well worth the extra cost. I was planning to use Jack's braided cell jumpers and nordlock washers anyway.

    EVCCON 2012 won't happen. I just don't have the hours available to put into the car to get done that fast. 2013 is possible. I have family in that part of the country, and it would be cool to let them see the car too.

    It was your blog that convinced me to go with a smaller car instead of a lead battery based pick-up. It got me excited to start this project that had been swirling around in the back of my mind for a long time.

  5. You had mentioned on one other occasion that you were inspired by my blog to start this conversion. I have to tell you, that is a merit badge I intend to wear with pride. A few people have told me that they were inspired to start their conversion after seeing mine. It's actually a quite humbling experience. It sounds strange, but the fact is this movement is way bigger than you and me, and it makes you realize that even the small part you play can have substantial impact.

    I totally agree with you on the CA cells. I think they are worth the difference in price. If I was starting another conversion tomorrow, they'd be on my shopping list. Furthermore I think a 100 mile range car is absolutely doable for you. 911's have so much room for batteries. I think you're going to be so pleased with the finish product.