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Planning BMW 318ti conversion

2995 Views 5 Replies 6 Participants Last post by  VoltsCar
- Your skill level with auto mechanics and fabrication
Engineer/fabrictor/mechanic at a BMW performance shop. On hand MIG and TIG, sharing a building with a machine shop. Design automotive performance parts and software for a living. Have a reasonable electronics lab in house for "small" stuff, with a few good soldering stations, a hot air rework station, etc.

- The range you are hoping to get (how many miles/charge)
Initial target of 30 miles. Minimum of 25 (distance from home to shop). Longer term target of 60 miles if the lithium technology continues to drop in price.

- What level of performance you are hoping to get
doesn't need to be fast or quick. Needs to not hold up traffic off the line, and needs to be capable of 70mph on the freeway. Doesn't need to outrun a 1980's Mercedes 240D.

- How much money you are willing to put into your project
"less is more." At least for the first attempt. I'd like to keep this more of a "toe in the water" sort of test. so would love to target a budget of $5k. (Given my access to material and fabrication, the only "cost" would be the main parts. Batteries, motor, controller, charger, wire. The car is "free", and adding something like a gearbox would be as well.)

- What parts you've already considered, if any.
Batteries: AGM. likely PC2150's. Also trying to find info on "tempest" batteries. Such as the TR120

Controller: based on cost, and my love of building stuff, I would like to try the ReVolt.

Motor: There is apparently a very large forklift dismantler about an hour south of me. Would like to find something in the 8-9 inch range.

Charger: no idea.
BMS: also a ?.
I figure I'll need to decide on a battery first...? Though perhaps the cost of those sorts of components should drive part of the battery decision?

On the charger front, I would like to target a 4-6 hour recharge time. Max of 8 hours.

The donor car came with an automatic transmission. I could swap pedal clusters with a manual and plumb the clutch, but I would prefer to not. Clutchless with a manual is also an option. I have spare manual transmissions and appropriate driveshafts so installing one would be easy. My ideal solution (if it will work) would be to put the electric motor in the transmission tunnel and couple directly to the driveshaft. This would give the entire engine bay for batteries and the controller, and save the weight of the gearbox.

The car starting life as an automatic is to my advantage on the gearing front. The car came with a 4.44:1 differential. This gives me hope because the manual transmission 318ti's came with a 3.45, and things like the 328 came with a 2.93.
A 328 is OK off the line in 2nd gear with little throttle. (2.49 in the gearbox and 2.93 out back, so 7.29 total.)
I will try to do more testing on that front in the next few days (get a car to 1000 rpm in the closest gear to my target ratio, ~3rd gear in the 328) and see if the acceleration rate is in the ballpark.

Only other concern on gearboxless is the power draw for a given acceleration rate. I don't have any higher ratio diffs that I can easily use at the moment. Will direct drive be harder on battery/motor/controller/other parts? Or just slower?

On the battery front, I would love to do Lithium, and realize it is likely more cost effective long term, but I have difficulty justifying the up front expense.
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Hi Techno,

I am looking at a similar car (E36), and I have come to the conclusion that using the larger diff would not make a lot of sense.

I am going for AC (not sure about the typicalities of DC here), but in an AC motor you have the typical "knee" in the torque curve where the inverter voltage is maxed out and power no longer rises. Below that knee you will see more or less linear acceleration, above it the acceleration drops down slowly.

As a rule of thumb, it is wise to put that knee somewhere around your normal driving speed, e.g. around 100-120 kmh in Europe.

In the simulations I have done till now, I most of the time found out that the stock ICE diff was not that bad, when you want to match the EV performance with the ICE performance. I cannot explain this better, but it seems (or might be) that fundamentally the power curves of ICE and AC are a bit comparable integrally, even though a AC has almost full torque from 0 rpm upwards.

I specifically looked at the 4.45 diff for the E36, and that ratio was way to long to get adequate performance with my motor (your motor might be different behaving though). I have to add that I am limited by a max rpm limit of 4500-5000, so larger rations mean significant decreases in topspeed.

I could achieve an improvement by using the 3.72 diff from a E30 (apparently these diffs fit into the E36 compact). The change would be noticeable, but also not dramatic (at least in the simulator).

Just my 2 cts, feel free to comment, as I think you are in a much better position than I am (sitting on top of the gold so to speak, working at BMW).


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Hi - Welcome to the BMW Conversion Club :)

Two things I thought would be worth mentioning:

1. using lead is cheap, but would have made my 328i overweight unless I removed the back seat. If you don't have to worry about exceeding the GVWR in relation to being allowed on the road, this might not be a concern...

2. Not having a gearbox is possible, but you have to find a balance between being able to get moving from the lights, and not destroying your motor when travelling at high (i.e, motorway) speeds ((DC) electric motors have a tendency to self-destruct if they rotate too fast). Keeping the gearbox (with or without clutch) allows you to avoid this problem. I know of one (RAV4) direct-drive conversion that had trouble moving away from the curb if any of the wheels were touching it, for example.

Welcome to the party. E36's are good cars in my opinion. Light for their size.

To add the clutch to the auto chassis is not difficult. I did this on another conversion. The pedal box is the same on both cars. Just need the pedals, clips, springs and master cylinder. If you look at the brake master cylinder you should see a nipple that is closed off. THis is for the line to the clutch master.

Keeping the trans is good unless you have GOBS of power. Plenty of details on that in other posts. I had the same thought as you. Put the motor in the tunnel and ditch the trans but after more research decided it was best to keep it.

I have an e30 2.93 differential with an LSD in it that I'll sell cheap. (bought it for my race car, but never used it) If you use the driveline calculator at you can figure what RPM your motor will be at with your final drive ratio and tire size. Electric motors like to cruise at a fairly low RPM (~2000) so a tall differential is a good thing.
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