Piston Slap: Tranny Talk on Electrified Motors?

Frank writes:

So, reading about the Lithium Mustang with its completely superfluous six-speed gearbox got me and some friends thinking about when you need a transmission and when you don’t.

We all know that steam engines and electric motors torque from the git-go and never need transmissions, whereas ICE engines can’t do that. But then we realized none of us knew why. So, that’s the question — why can’t ICE engines torque like those others?

Sajeev answers:

Um, no: electric motors do indeed need a transmission. But they don’t necessarily need a multi-speed transmission. Especially at any normal price point, but I’m getting ahead of myself.

For example, Tesla’s Model S uses a 9.73:1 step-down transmission, which cuts the mustard thanks to the electric motor’s excellent powerband and NVH controls. A single gear could imply terrible acceleration, but unlike ICE’s massive metal reciprocating assemblies that barely run below 800 rpm, electric motors make full torque at idle… upon that first shot of electricity to their internals.

And from idle to the Model S’ 18,302-ish rpm redline, there’s either a tidal wave of torque or more-than-adequate highway horsepower, and it’s available instantly thanks to the lack of a reciprocating assembly.

That said, the Model S’ acceleration tapers off as highway speeds (70 mph) turn to Go To Jail Speeds (155 mph top speed), so let’s hypothesize that power/efficiency falters above 12,000 rpm. Which suggests lead-footed owners still have 6000-ish more revs before redline: that’s nearly triple the rev range of an average ICE. That assumption might be BS, but no matter, a “tall” one-speed gearbox is acceptable here.

The extra revving doesn’t matter to most, much like going to a sock-hop in a ’50s Buick with its Dynaflow box. But Dynaflow met its maker, relegated to the dustbin while its successor is still relevant today (well, in the enthusiast space). History is likely to repeat itself, but don’t take my word for it.

The ZF 2-speed gearbox is likely to prove that electric cars go faster, more efficiently with multi-speed gearing. If what I heard from an owner of a Porsche Taycan Turbo (not Turbo S) in Texas is any indication, the 2-speed gearbox significantly improves the long-distance (from Houston to Austin) range vis-à-vis the single-speed Model S. And his assessment isn’t unique.

While Tesla’s electric powertrains are a noble and wholly-impressive endeavor, they’ve wanted a second cog for a while. But this isn’t a slam on Tesla: standalone departments or third party suppliers get the funding they deserve because transmissions are terribly complicated compared to other componentry. If properly funded, the next-generation Model S will likely have that ZF gearbox, or similar.

Back to the price point: Google the Porsche Taycan’s starting MSRP. That’s a lotta stacks for two gears, Son!

I dream of a day when it’s possible to make a sub-20k electric family car (we gotta take 1/3 of the cost outta the Nissan Leaf, no biggie amirite?) with a single speed gearbox, and 2-3 speed units for buyers of 40+k machines. Who knows when battery prices shall fall in line with this dream, but a correlation between multi-speed transmissions and lofty(er) electric vehicle asking prices is likely.

[Image: Ford]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.