# Cost/Feasability of BMW 328i e90 conversion



## BozkurtlarOrdusu (Sep 3, 2020)

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## brian_ (Feb 7, 2017)

When asking a bunch of unrelated questions in the Technical Discussion section, it would make more sense to use separate threads for separate issues. The alternative is to put one thread in the Builds section.



CfRP said:


> I miss driving this car, I really love the road dynamics, crispness of cornering. SO I guess I would only need to preserve the hydraulic power steering. I could care less about A/C and accessories.


If you keep the stock hydraulic rack, the straightforward way to handle the conversion is to replace the engine-driven pump with an electrically driven pump, which is a common feature of cars built in the generation before almost all production cars switched to electric racks or columns. There are several discussions of this in this forum, so you can search for them.

I don't know if you consider brake boost an "accessory", but if the car has vacuum boost then you need either an electric vacuum pump or a different master cylinder with electrically driven assist.



CfRP said:


> Factory weight was listed as 3200lbs, but I think its closer to 3000, due to the way BMW lists curb weights. Just found out the straight 6 in her weighs in at about nearly 400lbs.


How does BMW list curb weights which is incorrect?

Straight six engines are nice in a few ways, but they are heavy for their displacement, which is good news for anyone doing the weight budget for a conversion. 



CfRP said:


> Not sure if keeping the auto tranny is necessary.


Not only is this not necessary, it is highly undesirable to use any typical automatic transmission. If your motor choice requires a multi-speed transmission, use the manual transmission for that model of car (just so that it fits and is easy to mount).



CfRP said:


> Would the easiest conversion route be to keep it front motor rear drive? or are there small enough units to fit direct to axles?


The least change to the vehicle is to replace only the engine with an electric motor, keeping a transmission... but that is also heaviest and leaves the least space for a battery pack. This is the traditional approach, and still commonly done.

All production EVs use a drive unit (motor or motors plus transaxle) mounted at the axle location. Most from the front of cars have the electronics stacked on top which makes them impractical for this car's rear axle unless the electronics are relocated; Tesla drive units have the inverter on one side, making them low (but wide). Any drive unit can be difficult to fit between the parts of the rear suspension, but it can be done, depending on drive unit and suspension design. The Tesla Model 3 has essentially the same suspension design as the E90, so the Model 3 drive unit would be a good fit... except that DIY control of the Model 3 motor and inverter is not well worked out yet. Also, if you want a BMW 3-Series with a Tesla powertrain, it makes more sense to just buy a used Model 3.

The in-between solution is to leave the rear end alone, keep the motor out of the engine compartment, and put the motor where the transmission originally was. This requires either a very high-torque motor, or a reduction gearbox on the motor output.

All of these conversion approaches have been used on various cars of similar layout to the E90. It would make sense to search this forum for BMW conversions.



CfRP said:


> Most important question. What kind of motor do I start with? grab a used production EV drive unit off ebay? go the forklift motor route? (off subject but What can anyone tell me about advancing windings on a motor?)


That depends on your budget, your performance expectations, your abilities and willingness to learn about new technologies (such as the computerized control systems of production EVs), and your chosen drivetrain layout.

Windings are not advanced - brushes are. It's almost trivial, so I would suggest not worrying about it until (and unless) you choose to go with a brushed DC motor.



CfRP said:


> So project goals for battery would be:
> 
> 45 - 60 miles of range.
> So aiming for a 15(+1.5buffer) - 20(+2buffer)kWh worth of batteries, (with an assumption of 333 watts per mile consumption)...
> ...


The energy consumption rate assumption is somewhat reasonable, but I don't think that you should expect production EV efficiency in a DIY conversion.

Using a complete pack is attractive, because you get a well-designed housing with all of the desired safety features without building anything... but none of them will fit well in any car not designed to fit them, and most won't fit at all. The Pacifica Hybrid pack is a tidy rectangular box sized and proportioned to fit under the central floor area of a van, but where would that go in an E90... the entire trunk, perhaps?

There isn't much fundamental difference between the performance of the various battery systems used in production EVs, as long as they are liquid-cooled. But with any of them, power handling capability is essentially proportional to energy storage capacity, and 300 kW (even briefly) from a 16 kWh pack seems unreasonable - that's a discharge rate of almost 20 C (power at 20 times the capacity per hour), and a Volt pack is rated by GM for 120 kW (7.5 C) for only 10 seconds.




CfRP said:


> Also charging wise, if I also pull the onboard charger from the battery donor car, would I be able to use that charge rate? (looking to get minimum 7.4KW on AC)


Yes, the battery and charger don't care if they're not in the original car, as long as all of the computerized control features are handled (chargers talk to the battery management system, which talks to who knows what).



CfRP said:


> Lastly is it also possible to incorporate DCFC into conversions? (would it be as simple as transplanting the hardware into the new car?) I've got a lot of stations nearby and would be great if I could take advantage and top off a smaller pack like this in 20-30 minutes.


If you keep everything from one donor EV, yes. If not, this is a non-trivial question of charging standards and control hardware. There have been several discussions of this issue in the forum - look for references to CCS and CHAdeMO (the industry DC charging standards).


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