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Rate my build plan (c3 corvette)

23832 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
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hmm, what would you say is more reasonable for the ev bits if not 20k?
i guess, on the upside, i can sell the engine and transmission and exhaust/fuel system/tank to a good home to make a couple grand back
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 and the motor will be entirely under the rails, in a similar location to the stock diff although hanging out the back more

also, i did some rather rough fitment approximation using a c3 frame cad file i found somewhere, and a model s subframe i found somewhere else

the orange shaft is where i think the center of the axels are when the car's sitting on the ground

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Are you sure you have enough room for two full Volt packs? I love those batteries, I have most of one pack in my car, but they are bulky and heavy compared to Telsa modules. Two full Volt packs will run 720-ish pounds. A little shy of twice as much as equivalent Tesla modules. Volumetrics are similar.
think so, did some quick and dirty cad work and the vast majority of the two packs can fit under the hood as a fake V8, with another long module or two going down the transmission tunnel since there's room, then a final 2 or 3x short modules taking up the gas tank
if i have to (hopefully i won't), i can consume some of the letter box "trunk" space with another module or two

by comparison, a big block chevy engine with all the fixins is like 600 pounds, not including exhaust headers and such, so 600 pounds of batteries in the engine bay is within expected bounds for mass, the th400 transmission is like 135 pounds which is another module or two, and a fully loaded 20 gal gas tank is 121 pound of mass plus the a few pounds in the weight of the tank itself.
I definitely could go the more bespoke route like what Damien did but i'm specifically looking for more turn-key stuff to reduce the unknowns and more importantly, get tech support for if things go weird

drivetrain wise, i'm going for the model S rear + 34kw volt batteries because it's been proving in multiple projects by now, i really like things that have been done before because it becomes a solved problem with fewer unknowns.

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. As for the tesla motor, i just find it'd be more livable vs the old school warp DC builds, also more aftermarket support both now and in the future. There's a reason EV west is changing some of their kits over to a tesla small drive unit architecture instead of the siemens motors.

Model 3 motor would be better but...well....nobody is supporting that yet, and even if they supported it a year from now it'll be a while before it's proven.

also the volt price per kilowatt is crazy cheap. I can get 34kw for like, 4 grand at the most. 34kw of tesla packs. Even if i can get full packs for $1000 a piece, that's still $7000 for 7 tesla packs (37.1 kw)

And wiring up 37kw of tesla packs in order to get 350-400v nominal, that's a lot of S and not a lot of P, voltage saaaaaaag

examples of what 34kw of volt packs + p85 rear can is able to do:
this 818 uses the T-1C controller, it's their shop car

blue lightning uses a more traditional ECU by motec with a ton of custom software fiddling to make it into an ev management system

I'm waffling on adapting the model s subframe over though, it may be much less fab work to just weld motor mounts to the cross members around where the diff is in the stock c3 frame, the motor might need to sit at a weird angle though, sort of cocked upwards in the back.
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?
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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) no built in cooling so i have to machine a lot of chill plates and design my battery boxe(s) around those chill plates.

those are both huge mitigating factors imo, unless chill plate fab/positioning isn't as big of a problem as i thought
correct, they will have liquid cooling in the car
but if you separate out the modules, they do not have liquid cooling built into them. the Volt and tesla packs have cooling chill plates essentially baked into their form factor

see, this chevy Bolt battery pack disassembly video, the chill plate is designed to fit in that big pack but it doesn't split apart into smaller chill plates

the battery modules themselves:

so, if you want cooling, you'll have to make your own chill plates :/
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Flat coolant plates are easy to make

I made one for my controller - all you need is a thick aluminium plate and drill some long holes with liquid connectors

The actual Chevy ones will be lighter and cheaper for them to make - but we can easily make something that does that job
the plates are easy but you have to design your entire battery box architecture around including those chill plates in the right locations, and bolting the batteries to them in the right orientation etc.
with the volt packs it's just like, line em up and plug your coolant lines into the coolant ports on em

also, i literally can't find a chevy bolt battery module for sale, not even car-part has them yet
i'll admit i'm going off just stuff i've read.
volt packs have great power density, bad energy density, are VERY inexpensive per kilowatt, well researched and known, and have cooling capacity baked into them instead of having to engineering chill plate placement
so if a want a simple(ish) install that can run 400 volts and have really good power density i'm kinda stuck

BUT! If there's some alternative out there which does most of the same stuff but has a better energy density i'm all for it, i just don't know what's out there that'll satisfy that.
Pacifica battery? maybe? I just don't know

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 :/

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
You brought up handling as a concern. I too had that same concern and that's why I choose to NOT use the Tesla sub cradle. It's not a simple A arm design and I was having a hard time calculating the scrub radius/mounting height and camber. Just a word of caution. You might have already thought about those things.
i have not, i assumed it'd be good, and at the very least better than a leaf spring suspension setup designed in the late 50s

is the model 3's setup significantly better?
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.

they're like 17 to 25k depending on options
if i was keeping the gas engine i'd do something like that.

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.

also the problem with keeping the transmission/diff and using a DC motor in the engine bay is that'll take a lot of space that batteries could go in, and it keeps the sub-par rear suspension
ah, that's what you were getting at, yikes, i had not considered that

another thing i hadn't considered, i think this detroit speed kit solves the axle shafts as suspension members problem
....for $8,800 :eek:

Although this is making me lean a little more towards the model 3 setup, not sure if that has better adjustment though?

as for track width, there's this c3 image

and this for the model 3

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!

-better cooling
-better efficiency
-better adjustment(?)
-more aftermarket support(?)
-better track aligntment
-not nearly as well understood so far, no turn key systems support it and there's some skepticism if they even will for a while
-possibly narrower track(?)
-slightly lower peak power. Scuttlebutt is that the 3's rear drive can do reliably 500hp with some software tweaking, but that monster p90 rear melon can smash out 600+
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although, counterpoint, adding aftermarket adjustment systems to the model s suspension isn't too bad
hmm, double checked my cad model i have of a c3 frame

i recently checked out a pretty cool race built C3 and measured the inter-frame distance right by where the gas tank sits, reads a little less than 39 inches

i use tinker cad as a quick an dirty alignment tool and the c3 frame seems to line up perfectly, i scaled it down by 100 to fit into tinkercad, and it's 10mm between the same two rear frame members on the model, which is pretty much exactly one one hundredth of 39, so the c3 frame is very close to accurate.

the model s subframe piece cad though, not so sure about that one. I'll download this super accurate model and cross reference
really wish i could embed images from google drive
anyway, the motor model is quite validated and sits just right in the subframe model so that seems accurate too

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
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").
ah, thank you for the measurements
19" wheels are just too damn big for the c3, 18 is already pushing it, the model 3 can do 18s so i guess that's yet another reason to get a 3 subframe over an S :(
Eh, small drive unit tops out around 300. It's certainly nice, but not enough for a proper american sports car.

really wish i could get a 3d scan of a model 3 subframe/motor, but it doesn't seem to be a thing yet :/
the motor being a little forward just means it might intrude into the cabin fiberglass a little, i may need to add a hump into where the cargo area is

CorVolt as in, Volt batteries. CorBolt honestly just sounds odd, also that motor is meh.
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.
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

As for the name, i was literally planning to replace the stock lettering of "vette" with "volt", maybe add in a dash in the middle to keep the same number of letters

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Very short sidewalls are especially bad with suspension that isn't suited to them.
thankfully, 18s are well understood on c3s, they seem to work very well for good handling
19s.....much less so, wouldn't trust em
related unrelated, just shot out a few emails to a lot of the conversion shops to ask where they are in model 3 motor controller support. Polykup, zeroEV, stealthEV, HSR motors.
i know EV controls has it on their roadmap, and EV west hasn't bothered with them since the older large rear drive units and the small front drive units are fine for them for now.

apparently damien and group are doing a bit of a raffle on a motor to fund open source control board development, which is neat, and might be the only way to accelerate this

Zero EV has one taken apart but it'll take some time before they have a solution
057 technology says they're in the r&d phase for it, so it's on the horizon, but still quite a ways off
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