Gearbox Question. Max torque and can I run it backwards?

[Jonny H]

Hi Folks,

I have just bought an electric 914 that has a 901 gearbox and a high torque electric motor. I don't have motor specs to hand but:

1)Max torque from the motor will be from standstill. What sort of torque can I expect each gear to cope with?

2) I can achieve reverse by running the motor backwards. Is there any aspect of the gearbox design that would cause damage if run backwards?

I'm asking on this forum because there are more people here and it's a 901 gearbox anyway.

Cheers

Jonny

[Matt Monson]

The crownwheel and pinion are definitely directional. You do not want to run it backwards. It's not designed to load that way.

Max torque? No car with 901 family of gearboxes installed (901,911 and 914) ever exceeded about 200ft/lbs. I'd have to look up the 71 ST engine specs but it was likely the most powerful. However people do v8 swaps in 914s so clearly they can take some more. The question always becomes for how long and how does it get driven. YMMV.

[Flieger]

It's not clear what you intend to do in reverse. If you are trying to turn the box around for a 911 then no, you need to flip the differential.

If you are just wanting a reverse gear for backing out of the garage then that is fine. The ring and pinion is loaded in reverse any time you select reverse gear in a normal car, and it is also loaded in reverse during engine/compression braking.

[Tippy]

An electric motor and 901 IMO sound like they should never be connected! Something about full torque at 0 RPM just seems too much.

[Matt Monson]

Originally Posted by Flieger

It's not clear what you intend to do in reverse. If you are trying to turn the box around for a 911 then no, you need to flip the differential.

If you are just wanting a reverse gear for backing out of the garage then that is fine. The ring and pinion is loaded in reverse any time you select reverse gear in a normal car, and it is also loaded in reverse during engine/compression braking.

Thanks for clarifying my confusing response. I've edited it. I was assuming he wanted to run an actual 901 in a 914. And I wouldn't want to run it constantly that way.

[stownsen914]

Matt's is about the most expert gearbox opinion you'll get!

One thing to consider is heat, since heat is a big transmission killer. I don't know much about electric cars, but I wonder if there might be less ambient heat transferred to the trans from an electric motor than from an internal combustion engine. Especially with a 914, where the trans is behind the engine, and the exhaust is right next to it. With an electric motor, you'll still have the heat generated inside the trans, of course, but there may be less heat overall, which could mean a 901 could withstand somewhat more torque. Just a thought.

Scott

[Flieger]

Yeah, torque is the enemy. If you can use some drop gears to reduce the torque before it gets to the trans it would be good (since you say high-torque motor). You want power, not torque with that transmission.

[Flieger]

Originally Posted by Matt Monson

Thanks for clarifying my confusing response. I've edited it. I was assuming he wanted to run an actual 901 in a 914. And I wouldn't want to run it constantly that way.

Actually I misread it too. I thought he had a 914 trans and wanted to run it in a 911. But regardless, we both were concerned about trying to run the forward gears in reverse.

[Flieger]

Originally Posted by stownsen914

Matt's is about the most expert gearbox opinion you'll get!

One thing to consider is heat, since heat is a big transmission killer. I don't know much about electric cars, but I wonder if there might be less ambient heat transferred to the trans from an electric motor than from an internal combustion engine. Especially with a 914, where the trans is behind the engine, and the exhaust is right next to it. With an electric motor, you'll still have the heat generated inside the trans, of course, but there may be less heat overall, which could mean a 901 could withstand somewhat more torque. Just a thought.

Scott

The torque limit is the strength of the side cover, pinion gear, etc. Purely metallurgical and geometrical. Heat definitely shortens the life but a cool gearbox won't withstand significantly more torque.

It will however be able to take more power with an oil cooler and spray bar, etc. because of the better lubrication and less friction/heat for the gears, bearings, etc.

[Matt Monson]

I've always considered the gears themselves the weak link in the 901. I forget the exact number but they are a size able amount narrower than 915 gears. The 901 7:31 isn't a lot different in size than a 915 r/p, with 914s and 72-73 915s sharing the same differential.

Some ratios are stronger/weaker than others. It's all about the distance from the ID to the gear tooth root. I don't deal with 901s much so don't know which off the top of my head like I do with 915 and gt3s.

Scott, you are too kind, but if you saw my unedited comment you would have seen that even I can sound like a dumbarse at times.

[Jonny H]

Thanks for all the answers. Yes it is a 914 with the standard 901 gearbox. It has a normal clutch, an adapter plate and a 9 inch diameter electric motor.

I do have the gearbox reverse obviously but had heard that some folks run the motor backwards instead. That didn't sound very wise as you have confirmed so I won't do that!

Based on what I've read, I can probably just leave the gearbox in third gear for most journeys. When a motor is coupled with a manual gearbox, gear selection is more about battery consumption. The car will accelerate faster in a higher gear, hence my fears of overloading it.

It will certainly take a bit of getting used to!

I collect my first retro EV from the docks next week. Unsurprisingly it was built by a cool dude in California.

Cheers

[Flieger]

Originally Posted by Jonny H

Thanks for all the answers. Yes it is a 914 with the standard 901 gearbox. It has a normal clutch, an adapter plate and a 9 inch diameter electric motor.

I do have the gearbox reverse obviously but had heard that some folks run the motor backwards instead. That didn't sound very wise as you have confirmed so I won't do that!

Based on what I've read, I can probably just leave the gearbox in third gear for most journeys. When a motor is coupled with a manual gearbox, gear selection is more about battery consumption. The car will accelerate faster in a higher gear, hence my fears of overloading it.

It will certainly take a bit of getting used to!

I collect my first retro EV from the docks next week. Unsurprisingly it was built by a cool dude in California.

Cheers

Just to be clear, the 914 has a variant of 901 transmission. The differential (and it's crown/ring gear) have been flipped to make the transmission have 5 forward speeds in a midengine application.

Best range is achieved by using the gearbox. Otherwise it is just worthless weight and you should just get a differential from a BMW or something.

To maximize range you want to keep the motor at it's peak efficiency point and since road speeds need to vary the gearbox will help with that, just as it does for a combustion engine. The side benefit is you can achieve higher speeds or better acceleration than with a single compromise ratio. It is interesting to look at the Formula E cars this year as some have 1 speed, some have 5 I believe, and everything in between.



Your motor will obviously have at least 120 times that torque and probably 30? times the rpm but the curve is similar in shape/relationship for all DC motors. Basically keep it 5 to 50% power.

[gliding_serpent]

So this fall I had the pleasure to drive a 2100lbs 87 911 with x3 electric motors in the back linked to a 901 gearbox. Each motor was good for 150hp, 200lbs/ft torque (450hp, 600lbs/ft). At max discharge he is producing over 2000amps. He has 3 world records in electric drag racing, runs 8.8 quarter miles, and never lost a slalom he entered.

He removed 1st gear as there was no need.



50:50 weight balance.


Super nice guy. I am sure he would love to chat. PM me and I can send him your contact info. He did everything himself in his garage. If I remember correctly, he is on his second transmission in 4 or so years but don't quote me on that.

His car has a button on the steering wheel that locks up the front brakes to allow burnouts, which are pretty easy with 600lbs/ft torque when you breathe on the throttle, front brakes locked or not. Talk about head snapping torque... you could not wipe the smile from my face that day!

[Flieger]

Yeah but what kind of range can he get?

[gliding_serpent]

Originally Posted by Flieger

Yeah but what kind of range can he get?

About "three days of regular driving" (not sure miles), or 12 quarter mile runs at the drag strip. His car has batteries and engines that outperform the BMW I8 electric motor in hp/torque, discharge rate potential (key for max torque), and total charge,. Actual range was much closer to "ear to ear". I helped it get down to 60% power in short order in the local neighborhood. My face hurt I was smiling so much. I kept it in 3rd mostly, but shifting worked fine, even if it was eery as hell.

I was at a local BMW club meeting recently. A automotive engineering professor did a talk on basically why electric cars suck... and calculations of how much gas is equivalent to the range of an electric car. If memory serves, the BMW I3 with "range extender" gets you about 220kms, which was about the same as putting 8L in the tank of your typical car. That range tapers off quite a bit with temps 30oC and over, and under 15oC. Optimal reported mileage is only seen at 20oC. You need to read the fine print to learn that one.

Then I got up basically said, you are doing it wrong... electric cars can be awesome! I then showed pictures of the electric 911, listed his quarter mile times and specs, and "said screw battery life and range, when what we really want is crazy acceleration, break neck slalom, and world records!" Just don't take the car to LeMans. You will be first going into turn 1, but will be dead last by turn 2.

Back to the 901 transmission. Like race engines, I am pretty sure he measures the life of his transmissions (g50 or otherwise) in hours when he is drag racing.

[Matt Monson]

The 901 1st gear is cantilevered out in the Tailcone and on a very thin shaft. It would break for sure if he was drag racing on it with that kind of power. The v8 conversion guys remove it as well. And then usually look for the slightly taller GA mainshaft found in the 70-71 911 gearbox.

[stownsen914]

Thinking about this more (and yes I am showing my ignorance here), doesn't an electric motor yield roughly the same torque at all RPMs? So doesn't need to take advantage of a multi-speed gearbox? Or at least doesn't need as many gears? Would you use just 3rd, 4th, 5th, or just pick two gears "low" and "high" or something like that? At a minimum you'd skip the weaker first gear ...

[Matt Monson]

Yes, constant output and this my earlier comment about how the couple guys I've worked with went single gear. They were commuter cars with a certain expected average speed. They picked their primary gear based on operating load as described by Flieger intending to maximize range.

One guy, just did the math and usually doesn't shift the car. The other guy geekier out to the point of removing gears and making spacers to drop rotational weight in the gearbox for Max efficiency.

[Flieger]

Originally Posted by stownsen914

Thinking about this more (and yes I am showing my ignorance here), doesn't an electric motor yield roughly the same torque at all RPMs? So doesn't need to take advantage of a multi-speed gearbox? Or at least doesn't need as many gears? Would you use just 3rd, 4th, 5th, or just pick two gears "low" and "high" or something like that? At a minimum you'd skip the weaker first gear ...

No, torque is linearly related to rpm as you can see in the graph, although being more industrial it is a little turned around from what you would see on a dyno plot (this is rpm vs. torque instead of torque vs. rpm). You can see that it has maximum torque at 0 rpm and it falls off linearly until it reaches it's no-load/synchronous speed.

Power is rpm*torque and it has the parabolic shape curve that peaks at 50% max speed (and 50% max torque). It is still ok on efficiency at that point actually but the sweet spot is around 25% load.

I think the primary reason EV's often have only 1 speed is that you can make it stronger and lighter than if you used a multi-speed gearbox with reverse and a clutch. You can get going with your cruise gear at the cost of some inefficiency unless you take off easy and accept the slower acceleration. But then once up to speed the single speed has less friction and weight than a normal transmission. It can also be made stronger.

[wwest]

Is the third gear "set" robust enough to handle the start from stop torque that the motor can produce?