DIY Electric Car Forums banner

Rate my build plan (c3 corvette)

23835 Views 80 Replies 10 Participants Last post by  MattsAwesomeStuff
Location: San francisco, CA

Budget: Like 15k for the donor car, and hopefully less than 20k for all the conversion parts and any labor costs.

Donor car: 1975 c3 corvette. They're cheap, easy to convert the bumpers back to the wonderful 68-72 chrome bumper design, and have some decent safety equipment upgrades over the early cars. A clean 1975 is like 15k, a clean 1970 is 25k or more, YIKES

also, maybe some race car flair to it, need big rear tires to hold all that torque eh?

Motor: Tesla model S P85 large rear drive unit OR Tesla model 3 performance rear drive unit. The entire tesla rear subframe and all the suspension components, fabricating mounts so it bolts onto the c3 ladder frame. This could be relatively easy or the hardest, most expensive part of the entire project. The motor 3 motor is more efficient when not under heavy load, and FAR FAR better cooling capacity BUT nobody supports their firmware yet and it isn't clear how many years it'll be until the T-1C supports it.

Battery: 34kw of chevy volt batteries. I'd like Gen2 but the Gen1s are half the price and only slightly lower capacity. Some in the engine bay with a battery enclosure to make it look like a V8 because yummy under hood candy, like ICON's derelict mercury EV did. the rest of the batteries go where the gas tank used to be, which may or may not hang lower and thus cause the full size spire tire sling to have to go away. Volt batteries have excellent high performance because of almost no voltage sag. They're also cheap, work very reliably, have a nice log-like form factor, and have built in excellent battery cooling. I'm hoping for 80+ miles of range

Cooling: unsure yet, but the water pump and electrically controller diverter valve out of a tesla. chevy volt batteries and the tesla motors have built in cooling systems so no need to screw around making chill plates.

Controller: EV Controls T-1C, pretty close to turn-key, i'd buy it from EV west specifically so i can lean on them for tech support and general info. This controller's already been used in a few tesla motor/chevy volt battery builds

Contactor: unsure

Power steering: I really want to keep hydraulic power steering, electric pump driven. It's wasteful but it feels so much better than electric power steering. EV West makes a belt fed pump but i'll probably re-use the one that came with the car.

A/C: MVP might omit this since i live in a SF and A/C is kinda unnecessary

Charger: Not sure how to get there, but i absolutely want to shoot for a 50kw charger which interfaces with CCS. With such a short range this feels like a heavy requirement for this to be driveable. There's some noises about CCS charging for DIY builds but this might be a wait and see, hopefully this becomes a mature thing soon (within a year). For 50kw, maybe just stack 5 of those 10kw tesla chargers...?

Safety: Haven't researched much but i would really like switchable fuses between every 72 volts or so, so at any given moment during assembly or maintenance, if i make an oopsie, i only get shocked with 72v before a fuse cuts it. Also, inertial fuses so in a crash, everything is isolated from everything else. Also, a first responder safety line, mimicking tesla's implementation with a BRIGHT RED WIRE right under the hood with a really obvious label on it.

Timeline: purchase the c3 within a few months, spend about a year maintaining it, adding quality of life upgrades, more deeply researching the platform, more deeply researching the feasibility of my componentry and fabrication. HOPEFULLY buy most of the drivetrain bits in advance and make sure the entire thing works on a bench, THEN start converting in fall of 2020 and be done some point before fall of 2021

So, what am i missing, what bad assumptions or assessments am i making, etc
See less See more
  • Like
Reactions: 1
1 - 20 of 81 Posts
The Tesla Model S/X drive unit places the motor behind the axle, and that's where the fuel tank is. This could be a packaging challenge, depending on the orientation of the battery modules. The overall plan seems workable, but a realistic look at how everything would go together would be required.
i actually get a little lucky with this, the gas tank sits on top of the frame rails ...
Right - I forgot about that detail from the last C3 discussion. It would be good to put a module under that tank level - mass up high is bad.
Problem is, the leaf spring in the stock c3 uses the diff itself as a central suspension member! Not sure how to adapt that to the model S motor?
I assume that you're referring to the use of the final drive (diff) case as structure. Some sort of framework could probably be built, but most people would probably just do a conversion from the leaf spring to coils (coil-over shocks).

A bigger problems is the use of the axle shafts as suspension links (which has nothing to do with the leaf spring), so the final drive (diff) output bearings need to handle lateral forces (cornering loads) as pushing and pulling on the axle shafts. The solution is to not use C3 suspension... it's just hopelessly antiquated, and incompatible with any modern drive hardware. On the other hand, apparently swing-axle Triumph Spitfires have used Hitachi (used by Nissan and Subaru) diffs without destroying them... so some sort of hack might be possible.

If you don't want to use the Tesla suspension and subframe, an option would be the C5 components on a custom subframe; the Tesla powered Mercedes Vito shows this is possible with a large Tesla drive unit.
As far as I know only the Nissan Leaf does NOT have liquid cooling...
There have been others, but the Leaf is the only common modern EV without liquid cooling, and the only one without active cooling (there have been some with forced air).

the Volt and tesla packs have cooling chill plates essentially baked into their form factor
Yes, and they're the only ones as far as I know. Even Rivian's pack design for their proposed pickup and SUV use plates under the modules... and they even use cylindrical cells like Tesla.

Rivian does bring an interesting idea: apparently they are sandwiching chill plates with modules on the top and bottom of the plates, getting double use out of each plate and eliminating a large unused face. Of course that make the packaging challenges worse, especially since the plate must go on one specific face of the module.

And we shouldn't call them "chill" plates, because they heat the battery under some conditions.
Is something else going to "replace" the Volt packs?
I think that's a good question. The Bolt seems obvious, but has the same problem as other mainstream EVs: with 60 kWh pack capacity and the modules all in series, using a fraction of the pack which fits in a conversion means getting a fraction of the operating voltage. That might work out well for brushed DC motors and aftermarket motors intended for industrial vehicles (e.g. the HPEVS AC series), but is a concern for salvaged EV motors that are designed for the voltage of a full 96S pack.

Some people are starting to use the LG Chem modules from the Chrysler Pacifica Hybrid. Like the Volt, the modules total 96S but are only ~16 kWh in total, since (like the Volt) it is a plug-in hybrid. Logically the Mitsubishi Outlander PHEV would be a similar source, but I don't know if those will ever be common.
If that's a factor i'd look at the packs used in the VW architectures, personally, because they're pretty good and will be EVERYWHERE soon
the two BIG problems with those are
1) unknown platform, like, haven't been used in a lot of conversions
2) ...
It isn't just unknown, it's multiple platforms. VW has talked about their "MEB" EV-specific platform for a long time, but every VW Group EV produced so far has been on an adapted engine-driven platform, and even the coming Porsche Taycan - which apparently has an EV-specific platform - is not on MEB. It's not at all clear to me that that the EV components will be consistent between those vehicles and what they will be building.
priority wise: Horsepower, handling, fast charging, and simplicity of components. That means range isn't important, lower weight is a priority, and being able to push tons of amps consistently is important. Volt packs sort of fall out naturally from those requirements.
Volt batteries are pretty high power density
Volt packs have awesome power capacity, but terrible energy capacity.
For a given voltage, "tons of amps" means "tons of power": this with "range isn't important" is a description of a need for high power, not high energy capacity.
for a split second i was thinking of using two of those model 3 long modules, one through the transmission tunnel and another cocked at an angle in the engine bay but....the two short(er) modules are only 72v each, 144v nominal :/
I had the same thought, but those modules are almost 1/4 of the width of the Model 3 floor, so they would be too wide for a tunnel.

The Model 3 battery has three characteristics which are different from that of the typical modern EV:
  1. cylindrical cells (21 x 70 mm in this case), like all Teslas - this doesn't matter when working at the module level
  2. thermal management by fluid circulating in internal tubing, like the Model S/X - I see this as a good thing
  3. an abnormally small number of modules (two at 23S, two at 25S), meaning that each one is abnormally large and high-voltage - this makes no difference to the basic problem that half a pack is half-voltage, but gives less configuration flexibility
as you mentioned yeah, it's hard to get 400v out of half the packs out of some of these systems since the modules are all P and very little S
I would say that's the issue with all of current EV battery packs, not just some of them. If some production EV were using a much higher pack voltage then it would potentially be a source of modules that would total the desired 400 V or so in a smaller pack... but everything current is very similar, with most being 96S and the only a small variation around that.
See less See more
IMO, we'll have fusion power plants well before VW produces anything, other than their stalling-oriented BS, in volume...
The meaning of "in volume" depends on the observer. So far from the VW group there have been at least the e-Golf and the Audi e-tron SUV.

The e-Golf is somewhere between a stick-some-batteries-in-it conversion like the Focus Electric and the EV-derived-from-existing platform like the Leaf. At least at one point it was the best-selling EV in Europe, at a couple thousand units a month. Is that "in volume"?

The Audi e-tron SUV is also a production vehicle, but expensive, just being released, and intended to be relatively low volume. It is derived from the MLB platform, but nearly unique, so it almost the from-the-ground-up EV that the world is supposedly waiting for. We'll know in a year if it sells in volume.

While there are many thousands of production VW EVs out there, I think it will be at least a couple more years before we have an idea of what the VW commodity components turn out to be.
The Model 3 is by far the best-selling EV in the US at the moment, so those batteries won't stay expensive forever...
The modules will still be awkwardly long, though...
There's nothing wrong with the rear suspension design of the Tesla Model S/X, although the geometry is somewhat difficult to follow. It is essentially the same in design (although different in specific dimensions) as the "integral link" design used by Ford/Jaguar, Alfa Romeo, and others.

There's also nothing wrong with the leaf spring in the Corvette suspension (of any generation using them, which is C2 through C7 for the rear); it is just the spring, and not a control link, so it does not affect the geometry of the suspension at all. The C3 suspension is obsolete and undesirable, because it uses the axle shafts as control links and has horrible toe control.

I'm sure that the complete Tesla suspension would be a substantial upgrade compared to the C3's stock setup, but other designs might be easier to work with. If all you want of the C3 is the body, I'm pretty sure that there is at least one company which will sell you a completely new frame with C5/6/7 suspension front and rear (which is double A-arm at both ends), designed to fit a C3 body... but that chassis alone would consume your entire conversion budget.
See less See more
there are various full coil conversion kits for the c3 which eliminate needing the diff case but those are also incredibly expensive.
if i'm going to drop 6 grand on something i'd rather it be fabricating subframe mounts, since even with this coilover system.....still need to fabricate mounts for the tesla motor.
... and they only fix the small irritation of the leaf spring mounting frame, leaving you with the problem of axle shafts as suspension members.
... i think this detroit speed kit solves the axle shafts as suspension members problem
....for $8,800 :eek:
Yes, DECAlink completely replaces the Corvette's rear suspension with a sound multi-link suspension that does not use the axle shafts. It uses two longitudinal links per side, similar to the stock suspension in that respect, for compatibility with the C3 structure; as a result, it doesn't need an extensive subframe. If I were upgrading a C3 and wanting to keep the structure close to stock, I think this would be a good choice... but the probability that I would ever spend tens of thousands of dollars to upgrade a C3 is zero. ;)
Tesla Model 3 suspension and drive unit

Although this is making me lean a little more towards the model 3 setup, not sure if that has better adjustment though?
At least from the top, the Model 3 rear suspension appears to be a more typical 5-link-per-side design (with steel links rather than aluminum), first popularized by the Mercedes W201 series but now widely used on vehicles with performance as a high priority.

For those trying to model suspension kinematics, these suspensions work essentially like double A-arms, with the virtual ball joints at the locations where the lines of the links intersect, somewhere within the wheel volume; the track control rod is essentially the steering tie rod.

I don't see any adjustment in the upper links in that photo, and there is the expected cam adjustment on the inboard end of the toe control link; I have no idea if there is any adjustment in the lower links, but I would guess not.

You can see the subframe, upper links, toe link, one lower link (the one with the spring and shock) and swaybar in Tesla's image from the online configurator.

Note that the Model 3 rear drive unit places the motor ahead of the axle line (just like all of the drive units for platforms designed for transverse engines, including all front-wheel-drive EVs and the Smart ForTwo ED and Mitsubishi i-MiEV). The photo linked above is taken from ahead of the drive unit and suspension; note the motor on the left of the photo (right-hand side of the car), with the gearbox to the motor's right in the photo (left-hand side of the car), inverter on the end of the gearbox, and final drive (diff) on the back side of the gearbox.

the concern is, is the 3's track actually narrower? the problem with these images is..... it's measured rotor to rotor or wheel edge to wheel edge? the c3 image measures both separately which helps
hopefully this means the rotor-to-rotor of the 3 is 62, meaning i get a free 4 inches wider in the rear which is GOOD because i actually WANT to go for a widebody setup to keep this thing on the road!
Track is the lateral distance between the centres of the tires, as shown in the C3 drawing - not the same as the hub face spacing unless the wheel/hub offset is zero. So yes, the Model 3 is narrower than the Model S and X, but still (moderately) wider than the C3. The Model 3 seems like a good match for this project in this respect - fender flares would be needed to cover the few inches of extra track plus the extra tire width (stock C3 tires were narrow by today's standards).
See less See more
unfortunately the giant can impacts that crossmember bit, so that'll have to be cut off and a new crossmember welded up :(

another benefit of a model 3 subframe setup: the motor's quite a bit smaller
My guess is that crossmember is there primarily for the final drive and leaf spring carrier, so it may not need to be replaced at all.

The other obvious way to address motor bulk is to use the Tesla Model S/X small drive unit. While never used by Tesla as the only motor for a vehicle (it's only in AWD variants), and obviously less powerful than the large drive unit, it might still provide better performance than most stock C3s.

The Model 3 motor isn't just smaller - it's mounted ahead of the axle line, changing what it might run into. The Model 3 subframe is also differently shaped, due to the different suspension.

I see the name "corvolt" in an image filename. "Volt" made me think of "Bolt" - you could use the 150 kW Chevrolet Bolt motor instead of a Tesla, and not get anywhere near that crossmember. ;)
Your also have to give some thought about your Rims. The factory rotors and calipers are large and I don't think your going to be able to use a rim smaller they the factory (19").
Good catch. This is a common issue with modern components in older vehicles. The most common solution is to just use the large-diameter wheels, even if they often look strange with the old styling.

Using the smallest wheel size for a given model may require the selection of corresponding brakes.

Overall tire diameter can be an issue, too, but the G70-15 typical of this era of Corvette isn't far off the overall diameter of the tires on a Model S or Model 3 (and is actually taller than the 18" Model 3 tire). Of course tire sizes can be changed, but that affects motor speed for a given road speed.
Eh, small drive unit tops out around 300. It's certainly nice, but not enough for a proper american sports car.
CorVolt as in, Volt batteries. CorBolt honestly just sounds odd, also that motor is meh.
Right, you would need either boost the Bolt or Tesla small motor output, or build a two-motor all-wheel-drive C3 (with either brand of drive units). :D

And regardless of the motor or even the battery, "CorVolt" is a pretty good name for a Corvette EV. :)
similar diameter yes, but 19s give you a noticeably worse ride. I always always opt for 18s whenever possible because they still look quite good, but you have enough sidewall to properly ride.

the very old school 15" wheels with super meaty tires looks period correct but has its own handling problems, so i have no interest in that form factor either
Yes, appearance is only the start of the problems with excessive wheel diameter, or really inadequate sidewall height. Very short sidewalls are especially bad with suspension that isn't suited to them. The largest wheel / shortest sidewall combination for almost any production vehicle is not really suitable for normal street use.
... Importantly, all that rigidity makes the motor mounting itself FAAAR easier, i'd only need some thin steel bits to bolt into the motor casing's mount points.

if you look at the model 3's subframe
the actual parts which hold the motor into the subframe, they're just thin little bits, that's all. That'd be incredibly trivial to model in cad and weld to a good tube chassis, and again because of the massive stiffness in said tube chassis i almost don't have to think about where those mounts go in terms of stressing it. so, that negates basically all the motor mounting hardship
Yes, the drive unit mounts are relatively thin stamped plates, but they are strong in the vertical direction. Vertical is the important direction because the weight of the drive unit and reaction to axle torque are all vertical forces (although there is horizontal force in proportion to the drive unit mass and acceleration). These are not trivial forces, so mount design and placement of the mounts on the frame are important.

These mounts are easier than mounts for the subframe or direct mounts for suspension components, which take high forces in all directions.

There's some forum scuttlebutt which claims a 5 or 6 point roll bar can offer immense improvements in stability for not that much investment, on top of adding frame gussets here and there...
That makes sense. The flexibility of the stock frame results from the whole frame being in nearly one plane; the cage makes it into a big and much more sound box. Even in a unibody adding a cage can make a noticeable difference in flex and handling.
i'm thinking of cadding up some square-ish mount adapters...
imagine this shape but made of four thick pieces of steel intersecting and welding together into the boxy shape
The subframe mounts are so close to the frame members that this simple approach looks like it would be suitable. But why does the mount box extend to the structure of the subframe, instead of ending level with the face of the block in the middle of the mount? Or have I misinterpreted the image?
1 - 20 of 81 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.