1993 Rx7 + Tesla Motor

[rjmcdermott81]

Hey gents - I was hoping to get farther along in my build before posting but I need advice and help from the forum. I realize this isn't a BMW but you guys are the most knowledgeable bunch I know and I'm definitely stretching my technical depth here so jump in if you have advice.

The car - Like many of us...I've always dreamed of owning a 1991-1993 RX-7. To me, this car looks amazing, BUT I've always stayed away from it because of the unreliable rotary engine. And honestly, now that I've driven it I'm not very impressed. Part of that is because I have an automatic and the gearing sucks but in stock form it doesn't really feel that fast. So, I figured, if I'm going to electric swap a car I should start with one that really needs it.

The challenge with this build is getting the rear-mounted drive unit in an independent rear suspension car. There have been some really good builds done on BMWs (e30 and 850i) but I believe the rear trailing arm suspension lends itself to the swap. As you'll see getting this sucker to fit while maintaining the integrity of the rear subframe is my first big hurdle. I think for a fabricator or hotrod shop this may not be too difficult but I'm a novice so it's going to take some thinking and trying.

To catch everyone up here's the plan (things will probably change as I go): 1993 FD RX-7

• Motor: Tesla Model S drive unit (motor, inverter, and single-speed gearbox) rated at 362 hp and 325 ft⋅lb or more
• Batteries: Chevy Volt or Tesla Model S (Tesla if I can fit a whole pack)
• Color: Red, white, or maybe BMW Long Beach blue? more on that later...
• Timing: I want to take this car to Radwood Austin 2021 under it's own power. If I had to it repainted by then that would be amazing but I don't think that's possible.

I tend to get into projects one step at a time. Make commitments and complete small jobs. So first step was...just buy an RX-7. In 2019 here is where the market was:

• Non-running, half stripped cars: 12-15k
• Decent looking, running automatics: 15-19k
• Decent looking, running manuals: 19-22k
• Good looking manuals: 22k+

After finding so many issues with my e30 that had been sitting for years I really thought paying a little bit more to get a running and decent looking car would be worth it. So I searched high and low and finally found a 93 auto in Tallahassee, FL for 17k.

Here are the pics from the ad








I talked to the seller at length and he described it as in good running condition with an older repaint. We agreed on price and I paid using escrow.com (I highly recommend). I decided to fly out there and drive it back because....that is always exciting.

[rjmcdermott81]

When I got there the car looked good for it's age but it had a few noticeable issues: bad oil leak from the engine, some sort of clunking in the rear-right when lifting the throttle, and when I hooked up a boost gauge it was making 6psi of boost when the forums said 10. The seller conveniently wasn't there and I had to deal with him over the phone while trying to decide whether to fly home or just take it. I figured I wouldn't do any better with another car so I bought it. After spending more time with the car I am thoroughly impressed at how original it is. Every covering, shield, piece of plastic is where it should be. The guy said he bought it to do an LS swap but couldn't do it because of how original it was. Well...here I am about to ruin it.

Here's a shot from my 12-hour drive home. Drove it through the night with no issues other than - gas smell and no start unless you twist the key 5-10 times (apparently these are both common "features" of RX-7s).



Here she is safely at home:



Other issues I would later discover: cigarette lighter doesn't have power, slight surface rust on the inside of the rear hatch, and most importantly...the keys don't work in the door locks, or the door locks don't work at all.

[rjmcdermott81]

At this point, I started to plot and plan. I got the tesla drive unit measurements, jacked up the car, and started to measure. I'd never seen the underside of an RX-7 so at this point I was still going on blind faith that I could get something to work.

When you remove the gas tank and cut out the rear spare wheel well there is plenty of clearance in the back:



and plenty of width for the drive unit gearbox to fit if you cut the center of the subframe out:



This gave me enough confidence to buy a drive unit. Which I needed to do because without it, it would be impossible to figure out how to fit it in there. I bought it from a Tesla dismantler in CA, super cool guy and they delivered it freight right to my house. Here is unpacking day:

[rjmcdermott81]

This thing is heavy. I was stuck trying to figure out how to get it from the crate to a furniture dolly until my neighbor showed up with an idea to use 2x4s



There we go

[rjmcdermott81]

At this point I got sidetracked because my bike frame cracked and I had to repair the CF.



Which gave me the perfect chance to try out a paint color I thought might look good on the RX-7. Long Beach Blue which I saw on an M2 in a parking lot:





That paint color was shockingly cheap. So maybe.

I'm also considering white and red





I love the red but man....painting it white would be so so much easier. Now that I have a little experience painting I realize that white doesn't show any dents and you don't really need a super high gloss so the cut and buff would be easy.

[rjmcdermott81]

I found a spare rear subframe on ebay for $350 and I figured it might be wise to cut that one up and leave the one in the car intact for reference. Starting to eyeball up what needs to be done:



Behold! I found somebody online doing the same thing. He posted a couple pics on diyelectriccar.com and has the driveunit mounted so I know there is a path forward

[rjmcdermott81]

When I cut this subframe in half it's going to be in two pieces. I figured the first thing I needed to do was put it in a jig. I think there are a couple of ways to do this. Were I to do it again I think I would just weld pars directly to the subframe. I chose to drill holes in barstock, attach it at the suspension points, and then weld it to a frame I built. With the idea being to maintain the same suspension points.







Experimenting with timelapse videos on the GoPro

[rjmcdermott81]

After that I added a bar across that will eventually have the motormount attached to it. I went with 1.75 DOM steel tubing that is spec'd for roll cages. I figured if it's strong enough for that it's probably strong enough for this application. When I start cutting the center section out it will help provide support.

[rjmcdermott81]

Now I'm starting to cut away the center section. Going a little bit at a time:





Even with that removed you can see there is a major issue. The motor and invertor are hitting the vertical supports for the suspension:





When I zoom in on the other guys subframe it is apparent that he's done some major major rework of this section. Nearly to the point that it looks like he's rebuilt the whole vertical piece:

[rjmcdermott81]

Sidetracked again - my son had made his pinewood derby car. I swear, he held the paintgun the whole time, but I definitely guided his hand when we put down the PPG clearcoat :)

[rjmcdermott81]

Back to the grind. Taking more metal away.





You can also see that I decided moving a 300# object for test fits wasn't working. I created a scale mockup of the drive-unit. Much easier to move around



I also put my sweet jig on wheels because I am lazy



Now, here is where it's at after that grinding. You can see the axle inputs on the mockup. They are forward of the suspension pillars but ~2 inches behind the stock location for the RX-7 differential. If you look at the rubber pads on the bottom of the subframe, that is where the axles typically pass





At this point, I don't want to take any more metal off without assessing more of the situation in the car.

[rjmcdermott81]

Pulling it in to where it will live for the next few months. Once I start taking this thing apart it will only leave with that motor mounted.

Quick note - I got a quickjack over the holidays that runs off of 12v. It is rad. Not as good as having a lift obviously but I love it and I feel so much safer using it.



Exhaust and driveshaft out:



Shocks out:



Pulling out the diff:

[rjmcdermott81]

NOW the big question. Looking for advice here in a big way. Can I move the axle attachment location toward the back of the car by a couple inches? The axles will definitely clear the vertical section of the subframe so no issues with the axles. What I don't know is will this cause binding? Increased wear on the joints?

Here are the best pics I could get.

Stock axle location











New location - moving it backward
















Here is a comparison showing the axle orientation related to the hub.

Stock



New location




Thoughts?

[dadsbmw]

Awesome idea and great start! Keep it montego blue!

[pandaboo911]

This is awesome! What are you going to do for batteries? I don’t see a problem with setting the axles backward, as long you don’t exceed their limits, usually 25-30 degrees. The wear will be accelerated, and slight drop in torque transferred, but minimal.