# Planning BMW 750i Conversion



## dougingraham (Jul 26, 2011)

E38 750i has a curb weight of 3800 to 4820 lbs according to wikipedia. That is a huge range of weights and I don't know why that would be. Maybe the V12 is a lot heavier.

If you assume the /10 rule of thumb and the conversion comes out on the high end of the weight range that means 480 wh/mile and to get to 50 miles you would need a battery pack of 24kwh for a drop dead range with no accessories on. Same assumption on the light end of the range gives 380 wh/mile and a battery pack of 19kwh. Even if you strip out a lot of the stuff that makes these great cars to drive they are heavy and weight is one thing that kills EV range in stop and go driving conditions.

You will probably want an 11" motor of some kind. 150kw (output) is going to require a pretty good battery pack. At that power level the motor will probably be around 70% efficient so you will need to feed it over 200kw. You can get there by having a battery that can put out 1000 amps at 200 volts or a higher voltage at lower amps. Say you have a 340 volt pack that sags 15% under full load this would give 289 volts. A motor like the WarP 11HV can take 288 volts so to get your 200kw input you would need 694 amps. To get that voltage you need 98 cells. You could probably do this with 60AH calbs briefly. A 98 cell pack of 60AH CALB would give you an 18.8kwh pack which is close to the 19kwh minimum estimate. Using 100AH cells at 1000 amps and 200 volts would give a pack of about 74 cells for a 23.7 kwh pack size.

Guessing your usable range with this setup will be closer to 40 miles for the smaller pack and 50 miles for the larger one assuming 3800 lb car.

Not saying you shouldn't do this, but it is going to be more expensive than a smaller and lighter car. I don't think you are going to hit your budget numbers as the batteries alone will be in the $9000 range for the small pack after shipping and intercell straps, and $11500 for the large pack. A more realistic budget for this build would be about $20000 when all is said and done.

Best Wishes!


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## skooler (Mar 26, 2011)

Hi fellow uk converter!

Being blunt.... It's not possible to meet those specs (with any reasonable quality) with your budget in such a heavy car.

You could go for something lighter and it may be possible but even then I think you'll need at least £12k...

Why the 7 series?


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## Spir0 (Nov 26, 2013)

I didnt realise weight played such a large part in it?
What sort of weight would you suggest?


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## skooler (Mar 26, 2011)

I'd look for something under 1500kg

Suggest you search for 'Damien Maguire' on youtube 

Cheers,

Mike


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## Spir0 (Nov 26, 2013)

I watch his videos all the time, but his build is going to be 2100kg when it is done. Around the same weight as a E38. So why is he doing it then?


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## skooler (Mar 26, 2011)

Suggest you ask him that! I'd guess its because it is not his first conversion and he wanted a bigger car.

Bottom line with a heavier car is you will be spending more money to reach the same performance and range.

Is £10k a hard limit? Is the 750i a must?


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## Spir0 (Nov 26, 2013)

The reason I wanted an E38 is because I need a larger 4 door saloon for work. Would you suggest an E38? 10k isnt a hard limit but I dont want to put all my cash into it.


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## skooler (Mar 26, 2011)

Ah ok. Is that for image or practicality? Just trying to understand your thought process.

Cant really make the recomendation as I dont know the car.

The weight isn't a show stopper but its definately worth considering the options!


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## RIPPERTON (Jan 26, 2010)

I cant understand why the EV community insists on living in its Brushed DC rut.
An AC induction motor would make the 750 a feasible concept because of regen.
Remember kinetic energy is proportionate to vehicle weight.
All you would have to make sure of is you have a large controller that can channel all that current and a battery pack that has a large number of cells in series also to handle all that current.
Get out of the rut people, free energy is out there.


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## skooler (Mar 26, 2011)

RIPPERTON said:


> I cant understand why the EV community insists on living in its Brushed DC rut.
> An AC induction motor would make the 750 a feasible concept because of regen.
> Remember kinetic energy is proportionate to vehicle weight.
> All you would have to make sure of is you have a large controller that can channel all that current and a battery pack that has a large number of cells in series also to handle all that current.
> Get out of the rut people, free energy is out there.


It is, however AC systems are generally more expensive than dc systems. The budget for this build wont really allow for it.

The hpevs ac75 (biggest ac motor before the prices skyrocket (expect to pay £4-5k for it in the uk with curtis controller) is still half the power of the smallest ice fitted to this vehicle.


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## RIPPERTON (Jan 26, 2010)

skooler said:


> AC systems are generally more expensive than dc systems. The budget for this build wont really allow for it.


Short sighted
your not buying a lolypop here.
the extra expense now will be well worth the rewards later.
My friend here who has a Suzuki Carry, now wishes he had upgraded to an AC system instead of the bigger DC motor and controller he put in.
27% extra range in my Mira. Cant be ignored


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## RIPPERTON (Jan 26, 2010)

skooler said:


> The hpevs ac75 (biggest ac motor before the prices skyrocket (expect to pay £4-5k for it in the uk with curtis controller) is still half the power of the smallest ice fitted to this vehicle.


With big cars like the 750, its going to be difficult to reproduce the same performance figures as the ICE unless you find a Solectria AC90.
Unlike small cars it actually difficult to NOT improve on performance over the ICE version.


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## skooler (Mar 26, 2011)

RIPPERTON said:


> Short sighted
> your not buying a lolypop here.
> the extra expense now will be well worth the rewards later.
> My friend here who has a Suzuki Carry, now wishes he had upgraded to an AC system instead of the bigger DC motor and controller he put in.
> 27% extra range in my Mira. Cant be ignored


How is that short sighted!? I'm agreeing with you.

The issue is budget, not which is the better technology. 

If the OP can find the extra up front then AC is definately the better way to go. 

Dont forget that we'll be looking at about £7-8k of batteries to get a solid 50 miles in this!


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## dougingraham (Jul 26, 2011)

RIPPERTON said:


> I cant understand why the EV community insists on living in its Brushed DC rut.
> An AC induction motor would make the 750 a feasible concept because of regen.
> Remember kinetic energy is proportionate to vehicle weight.
> All you would have to make sure of is you have a large controller that can channel all that current and a battery pack that has a large number of cells in series also to handle all that current.
> Get out of the rut people, free energy is out there.


All it takes is a reasonably priced 150kw+ inverter and then you have something. There are plenty of motor choices but the limitation is the inverter. Higher voltages than the Curtis can do at 500 amps or more would help a lot

I would prefer to have an AC system myself because the regen energy is as you say free even though it does not extend range much. If I have to spend an additional $7000 for a 150kw inverter I would rather spend it on more batteries.

The dual AC-35 setup for $10000 would be able to power this car but it is only 123kw at the shaft and 176ft-lbs according to the HPEVS dyno tests.


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## Hollie Maea (Dec 9, 2009)

For a big heavy car like this, in which regen starts being a significant advantage, I'd probably look at the AC35x2. It's not that much more expensive than a Warp11+Soliton 1.


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## jackbauer (Jan 12, 2008)

Spir0 , you can convert any car. If you like big heavy beemers as I do then go for it. Don't sit at the keyboard and type yourself out of doing the build. I see this so often its depressing and one of the reasons I don't post on here and other forums much anymore.


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## Spir0 (Nov 26, 2013)

jackbauer said:


> Spir0 , you can convert any car. If you like big heavy beemers as I do then go for it. Don't sit at the keyboard and type yourself out of doing the build. I see this so often its depressing and one of the reasons I don't post on here and other forums much anymore.


 
Can I ask why people are telling me not to do this and you are? I dont want to come off rude.


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## dougingraham (Jul 26, 2011)

Spir0 said:


> Can I ask why people are telling me not to do this and you are? I dont want to come off rude.


Just to be clear, I am not telling you not to do it. I am just letting you know it will be more expensive than what you have indicated is your budget. I think it would be a really neat conversion and I would like to see it happen.


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## tomofreno (Mar 3, 2009)

Spir0 said:


> I didnt realise weight played such a large part in it?
> What sort of weight would you suggest?


 Acceleration is inversely proportional to mass: a = F/m, where F is the tractive effort – force the tyres apply to the road. The tractive effort is proportional to motor torque (depends on gearing and wheel size). Increasing mass by 20% decreases acceleration 20%, or you get a motor with 20% higher peak torque to maintain the same acceleration. More $.

Rolling resistance force is proportional to mass: F = C*m, C a constant. Increasing mass by 20% increases the work done per mile to move the car by 20% and decreases vehicle range by 20%, or you buy 20% more kWh worth of batteries to increase pack energy by 20%. More $.

2100 kg is about 2x the mass of my vehicle, so you will need 100% more peak torque and power, and 100% greater pack capacity for similar performance (assuming similar drag force, but yours will likely be higher due to larger vehicle cross section). My vehicle’s acceleration is likely not acceptable to you (0 to 100kph in 16 sec), so you will likely need more like 4-5x the peak torque and power. Many more $. 

An AC motor and controller with that peak torque and power will likely cost 2-3x that of a DC motor/controller. Many many more $

My motor’s peak torque and power are about 120 Nm and 48kW, so look for a series DC motor with 500 to 600 Nm peak torque, determine what current it requires to deliver that, and ensure it can deliver it to high enough rpm for 190-240kW peak power (motor shaft power = torque*rpm*2*pi/60, torque in NM), find a controller that that meets those requirements (likely a Zilla or Evnetics), and batteries with high enough C rate to deliver the required current. 

My pack is a bit under 21kWh and my range is about 75 miles mixed highway/city with 80% DoD. You want 2/3 of that, so 2/3 of 42 kWh (100% more mass), or 28 kWh minimum. If you want that in colder weather with a heater running, then add 10 to 30% depending on temperature and whether you will do mostly highway or city driving.

Look at components and run some numbers. Zilla is sold by Manzanita Micro and Evnetics has an add on the RHS of your screen. Netgain or Kostov are the most frequently used series DC motors.


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## RIPPERTON (Jan 26, 2010)

skooler said:


> How is that short sighted!? I'm agreeing with you.
> The issue is budget, not which is the better technology.


Im not saying its you thats short sighted ,
Generally what is short sighted is the whole concept of having a budget.
Its usually the first casualty in an EV build and sticking to one will more often ruin the design.
Budgets and deadlines, I always work without them.

By the time the OP is near the end of the build he will very likely have more cash then so I say keep designing and dont by anything yet.
I believe in the car, This thread has even got me looking at 750's on Ebay.

http://www.ebay.com.au/itm/1990-BMW-735IL-/181291847393?pt=AU_Cars&hash=item2a35d60ee1

http://www.ebay.com.au/itm/BMW-735i...-/321281338985?pt=AU_Cars&hash=item4acddc2e69

http://www.ebay.com.au/itm/1997-BMW...-/281210638072?pt=AU_Cars&hash=item417975cef8


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## jackbauer (Jan 12, 2008)

Spir0 said:


> Can I ask why people are telling me not to do this and you are? I dont want to come off rude.



Your not rude at all. I say it because you are entitled to convert the car YOU want. Just because it may not be the most efficient donor does not mean it won't work. Not everyone wants a small 2 door lightweight.


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## sunworksco (Sep 8, 2008)

A BMW 2002 would be a much lighter car and is still very spacious for batteries.
Find the lightest car you can lay your hands on and your budget will be much more achievable.


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## Spir0 (Nov 26, 2013)

How would a E30 coupe sound? They are anywhere from 1000kg to 1300kg? In a way I'd raather have that something a little more sporting yet still be classy enough to go to meetings in.


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## skooler (Mar 26, 2011)

E30 would work nicely. What are you looking for in the doner car? Is it a professional image, sporty ride, passenger/luggage space?


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## Spir0 (Nov 26, 2013)

I want something sporty, and something that is either a track killer or something that I can take people to meetings in and look classy. So I guess I have to choose the lighter car. So I was thinking has anyone ever built a kit car, and put an electric motor in it? I think it would be ideal. It would be a open and clean sheet.


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## sunworksco (Sep 8, 2008)

You might take a look at www.vintagespyders.com


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## Spir0 (Nov 26, 2013)

Yeah EVTV did a Roadster replica. mean more like a caterham. In the UK you can get a GBS Zero which is very simimilar to a Caterham. And it cost about the same as a good E30.


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## skooler (Mar 26, 2011)

If you want a cheap doner then I'd recommend the rx8. They are cheap as chips over here due to the engine problems (or to be more accurate, people not looking after them properly)


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## Spir0 (Nov 26, 2013)

I think I'm going to buy an E30, I think it would be a perfect car to convert. They are light sporty, lots of aftermarket parts and forums, fairly cheap and classy. Off to the yards tomorrow, gunna go see if there one fairly cheap.


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## skooler (Mar 26, 2011)

Good luck! Look forward to seeing this come together!


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## sunworksco (Sep 8, 2008)

In some respects, the Inrekor panel chassis of the 356 Porsche is better insulated, stronger, better crash protection, lighter and is easy to fabricate yourself.
The polyethylene foam panels are manufactured in China and the aluminum panels are bonded to the foam panels. The panels can be water-jet cut to design and size. You can also us Alulight all metal foam panels. These panels are very easy to assemble with epoxy and Ejot rivets or tig weld panels together.
The Inrekor 356 Porsche only weighs about 1300lbs.
I am using Alulight aluminum foam panels.

http://youtu.be/p0Bc6t96jPA

http://youtu.be/Q8_C2Az2VVM


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## tylerwatts (Feb 9, 2012)

Spir0 
Hello and welcome. Firstly, an ev is a big task to take on and will last you a long time so make sure you have the car you want and will enjoy all that time after building it. I like the 750 personally, weight aside. I see the appeal for you also and agree it is a good choice. Check which year they went all aluminium on the chassis as they had a good weight saving. Audi A8 is similar, all aluminium. But be aware of can-bus. It makes life harder in conversion terms and needs a bit more savvy... 

Secondly, I agree with Ripperton. Invest a little more now and you'll not regret it, especially in the right vehicle. May I recommend the Scott Drive AC controller to you. It has the options of BIG PMAC motors with loads of torque and regen or any AC motor you wish to tune it to. I know Scott has programming for the Siemens series motor now also. Ruckus is the forum member to get in touch with on that front. But for less than you'd expect you can have a high power AC drive with all the glory of regen and improved efficiency also, worth 10% extra range alone. 

You could even consider using the Chinese PMAC motor Scott uses capable of 200kw and huge torque with a home built controller at low cost to keep a sensible budget and meet your goals. I'm no expert but am looking at similar myself and am certain the extra effort and cost is absolutely worth it. 

Again welcome, and good luck. The forum will always support members even when some feel they'd do it differently, and we only share our opinions to help, the choice is ultimately yours and you must weigh up the pros and cons from each bit of input and your own research and calcs to be sure you are happy and confident in your decision. Look forward to the adventure, because it will be monumental and you'll love having done it in the end, no matter how you got there! 

Regards
Tyler


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## tylerwatts (Feb 9, 2012)

PS, a kit car is only ever a toy, I don't get the impression one would properly meet your personal needs. But that is my opinion from discussion so far.


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## jk1981 (Nov 12, 2010)

As others have said weight is not your friend here and the 7 series were never lightweight. To be honest if I were looking at an old 7 series conversion I'd be looking at swapping in one of the current generation high performance diesels (and I'd swap it into a 6 or 8 series) but there's no reason why you couldn't make it work with electric drive, it just requires more attention to detail to ensure you can afford adequate performance. Something lightweight will perform ok no matter how slapdash the spec/build is.

Personally, and this is just the way my brain works I'd set myself up a spreadsheet and build a model of your vehicle, drivetrain and of your commute, build a good understanding of the effects of changes you make to the design. Try some different vehicles (basically just mass and CdA data needed) Plug in some figures for other peoples conversions for a reality check then see whether you can afford a 7 series with enough poke and range. Some detailed research up front could identify and/or solve problems before they become expensive/insurmountable. My bet is that on the £10k budget you could just about do it with extensive DiY on the electronics side if you gamble on picking up a scrapyard EV pack or two a year or more down the line when prices may be falling but you won't get close with off the shelf kit.

There's nothing to stop you converting a heavy luxury car but the weight is against you. Then again, if that's what you want then an E30, no matter how brilliant they are as a driver's machine is unlikely to satisfy the brief, these days they feel distinctly basic. On the upside they're simple, becoming classic and are brilliant to drive.

jk


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## Spir0 (Nov 26, 2013)

Hi guys,
Sorry for the late reply, I am now sticking with the idea of the E30, I have done about of research and as far as I can see. I can just about get and E30 and convert it within my budget, I may have to buy some more batteries at a later point but I will get to that when I meet that problem. First things first I need to choose AC or DC I spoke with Mike from EVWest. And he can ship anything I need to the UK for around $800. So My plan is to buy from them unless its much cheaper to buy from someone else. I have looked on there website and correct me if I'm wrong but two AC35 motors are not much more than two Warp 11's. Would you guys still suggest the two Warp 11's? Because that's what I'm leaning towards.


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## Hollie Maea (Dec 9, 2009)

Spir0 said:


> I want something sporty, and something that is either a track killer or something that I can take people to meetings in and look classy. So I guess I have to choose the lighter car. So I was thinking has anyone ever built a kit car, and put an electric motor in it? I think it would be ideal. It would be a open and clean sheet.


There's a guy who is slapping an AC35x2 in one of those new Factory Five Project 818 kits. It looks like it's going to turn out quite nice.

I'd love to get an AWD kit (there are some Subaru based ones) and slap a 150KW EVDrive system on each wheel, with torque vectoring and make a death trap supercar. But:

1. I don't have the 150 grand it would likely take.
2. I'd probably be dead within a week of finishing it.


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## skooler (Mar 26, 2011)

Spir0 said:


> Hi guys,
> Sorry for the late reply, I am now sticking with the idea of the E30, I have done about of research and as far as I can see. I can just about get and E30 and convert it within my budget, I may have to buy some more batteries at a later point but I will get to that when I meet that problem. First things first I need to choose AC or DC I spoke with Mike from EVWest. And he can ship anything I need to the UK for around $800. So My plan is to buy from them unless its much cheaper to buy from someone else. I have looked on there website and correct me if I'm wrong but two AC35 motors are not much more than two Warp 11's. Would you guys still suggest the two Warp 11's? Because that's what I'm leaning towards.


Be careful with import duty, vat and fees... you can pretty much convert doller to pound when importing f4om the us


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## tylerwatts (Feb 9, 2012)

I repeat what skooler says. Not worth importing! If you can get second hand items that's different. 

May I ask why you want 2 11" motors? That is too much! One is excessive in an e30. One warp11 would compare to an AC35x2. Also, I'd suggest looking at having a transmission, an automatic even. I like the Yank boxes that have aftermarket manual valve bodies for manual shifting. That gives you control but still simplicity. Or use a stock auto with a simple control computer. 

Also, what time frame are you giving yourself? If you have the cash and want the build finished in 2 months that's one thing but if it will take a year anyway could you not spread the costs and spend a little more? 

If you plan to split up your battery costs you could start with a smaller pack and build it up for range later and then do a 750i conversion. Do you need 4 doors? Something like a clk or 6 series would be a bit lighter and still carry the image and prowess of the 750. Actually there is a fare bit of choice in that regard. Depends on you. I don't want to throw too many ideas and options out and prevent you making a decision. 

Ok one more. A Scott drive will have plenty power and cost similar to the AC35x2 setup with better performance. This would definitely be my choice if not going DC. Or a honking big industrial AC motor with a home built inverter similar to how ctscasemod has done his inverter. There are even kits for building these. Hope I'm not muddying the waters for you.


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## Spir0 (Nov 26, 2013)

Sorry for the super super late reply,

I was thinking just two 11's but just have them run at out of the box specs. Nothing OTT

Anyway I was looking down the interwebs and I found this......

http://www.ebay.co.uk/itm/1989-BMW-...2265923?pt=Automobiles_UK&hash=item51b793c5c3

(Dont know how to add images into posts)

I was thinking this would be better than a coupe or a saloon.


What do you guys think. He wants £2200 for it. I think thats abit high but Im sure I can get him to drop it off at my house for that price.

What do you guys think? The wheels alone are £1200.


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## tylerwatts (Feb 9, 2012)

Nice car that. 2 11s is too much. You would be better running 2 9s for the lower weight and space and higher revs! And the special is continuous power, peak is alot more depending on cooling used! So 1 would be plenty power!


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## skooler (Mar 26, 2011)

Be aware that with a brushed DC motor continuous use (such as motorway driving) will require some very decent cooling.

Either go oversize (a single 11 with forced air cooling would be good - e.g. kostov 11alpha or Warp11) or go AC if you intend on using it for long, non stop start type journeys (e.g. HPEVS AC74, AC75, AC76, AC35x2 or even siemens).

Cheers,

Mike


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## skooler (Mar 26, 2011)

BTW, car looks good but perhaps a tad overpriced.

The rust looks superficial and should be easy to treat yourself.


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## favguy (May 2, 2008)

Hi,
I disagree on the rust looking superficial, the sunroof panel appears to have rotted through from the inside out, be very careful when looking over this era BMW's, they didn't galvanise the steel and "small rust patches" can be much worse than first appearances. 

With the amount you're going to be spending to convert to an EV, it's just not sensible to start with a rusty car, also from trading cars, my experience is that owners who leave visible rust on a car untreated are generally running on a shoestring budget so how has it been maintained?

Really, you should be paying way under book value for a car with visible rust, getting it sorted out properly and permanently costs a lot of money!


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## skooler (Mar 26, 2011)

Good spot favguy, after a second look on a larger screen I'm in agreeance on the sunroof. Wheel arch looks reasonable though. 

Use it as a bargaining tool, if you need to replace a panel then make sure the buyer knows it when negotiating!


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## Duncan (Dec 8, 2008)

My tuppence worth on rust

Modern cars are amazing - metal treatment and paints have come on leaps and bounds,

Before that (and no I don't know exactly when it changed) you used to get rust bubbles and it was completely IMPOSSIBLE to eradicate rust - especially from area where two sheets of metal overlapped

INMHO 
Any car showing rust bubbles should not be considered as a serious long term project

BMW were always good, I remember being very impressed with the pain underneath a 2002 my friend was scrapping - but it was still being scrapped for terminal rust

Can you find a later model - after they have fixed the rust


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## Duncan (Dec 8, 2008)

Looking again at the pictures these rust spots appear to be external rust from scrapes

If they are you should be OK - check very carefully for the rust bubbles from the back


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## tylerwatts (Feb 9, 2012)

Look at rex1825 thread. He is converting the same 750 BMW. Ok a 740 but similar.


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## Spir0 (Nov 26, 2013)

Where did this guy find a damaged Tesla from?


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## tylerwatts (Feb 9, 2012)

Ask him...


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## epyon (Mar 20, 2008)

SpirO don't settle for the second place car of your dreams . You can make a 740 a light weight . The motor and gas stuff is above or around 25% of the weight . I had a THUNDERBIRD like the one I have now . I was on a scale an down to 3500 lbs with a ford big block and insolation . Besides , you will not need to beef up your car and add weight like all the other guys here had to do . That is the reason they say you should use a small car . Look at the work they did an you tell me . I took off metal on my car , and I now it's still 2 x stronger then most of the cars an some mini trucks here . With some lighter fiberglass (racing stuff)you can be 3200 lbs complete . I am building a custom AC motor . But you can over-volt a Warp 11 with a Soliton 1 ( 170 volts ) an call it a day . Your not going for speed , just keeping up with traffic . And it should be $4,000.00 Max , an put a 2-speed glide behind it .


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## Spir0 (Nov 26, 2013)

Hey guys, heres the plan, Im going E30 with two Kostov 9inch motors.

Now to find a car. Where would people suggest to find a project car? I don't want to be doing that much welding cutting and body work.


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