Hacker News Reader

Comment by gorgoiler | Hacker News Reader

Automatics traditionally had a single clutch and a viscous coupling to convert smooth the torque between gear shifts. Switching gears was slow and the transmission as a whole was less fuel efficient.

Modern automatic gearboxes have dual clutches and two separate sets of gears for even and odd numbers. You alternate between them as you shift up and down. Shifting is faster and smoother.

(The system isn’t perfect. If you switch to manual selection of gears the transmission computer sometimes guessing wrong — while you’re in 3rd and accelerating it’ll get 4th ready for on the even-numbers clutch, so if you suddenly brake and shift down there is a longer delay while the transmission shifts to 2nd instead.)

Here’s Audi’s marketing material on their version: https://m.youtube.com/watch?v=l_as29ym51M

View on HN

That’s not quite how traditional autos worked, they also have sets of clutches and bands, controlled by a complex hydraulic system. Modern automatics are electronically actuated and are basically killing the need for DCTs, with VW Group and Hyundai (and some exotic makers) being the lone holdouts in the U.S. market.

Honda used what was essentially a manual gearbox with automatic controls for years, that's why a 2000 civic got decent mileage

DCTs are still best for that sweet-sweet 0-60 time. The max stall RPMs on a stock torque converted transmission is probably 2300rpms at the most, while Porsche PDK can launch at redline and modulate torque delivery precisely.

I don't know of a car with a torque converted that can hit sixty in under 3.0 seconds, but the Corvette and 911 can both do so with RWD due to the bonkers launches these cars manage.

The BMW M5 has a ZF 8HP and pulls off sub 3s 0-60s in CS trim. The new xDrive M3 comes close. I’m sure there are others. The key obviously is to have a torque converter stall speed that’s in powerband.

Yeah, you're right. I realize that I eluded to it in the subsequent statement, but didn't outright say it, but I meant to say,

> I don't know of a RWD car with a torque converted that can hit sixty in under 3.0 seconds on street tires,

It's insane what the C8 can do with RWD on street tires. Being able to launch at full HP without destroying the rear tires is amazing. DCTs have near EV levels of torque management capabilities.

So with the C8 and 911 they can’t be characterized as merely RWD for straight line acceleration purposes because of weight distribution. One of the biggest issues with launching is that by the time weight transfer happens tires are already spinning, so to get a good launch you want a softer suspension which would ruin the car’s performance at corner exit. Being mid or rear engined helps there because you’re starting with 55-60% of the mass over the rear wheels which is closer to AWD than RWD for launch traction.

Reply on news.ycombinator.com