
New Porsche 911 Reaches Top Speed in 6th Gear; Not 7th or 8th - msnautomotive
https://www.motor1.com/news/297798/porsche-911-top-speed-sixth-gear/
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vish4
This is not limited to Porsche.

Most cars with 6+ gears hit top speed in 6th, as this gear applies enough
force at wheels to reach top speed. Some cars do it in 5th or 4th as well.

This happens because force needed to reach high speeds is follows the square
power law. Higher gears simply can't provide the force needed to counter air
resistance.

The higher gears, like 7th or 8th, are designed for fuel efficiency. They are
known as cruising or overdrive gears, because in those gears, the output shaft
is rotating faster than the engine.

Good info on this:
[https://en.wikipedia.org/wiki/Overdrive_(mechanics)](https://en.wikipedia.org/wiki/Overdrive_\(mechanics\))

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saltcured
The article is light on detail and somewhat ambiguous in its phrasing. "Can
hit top speed in 6th" and "can only hit top speed in 6th" are not the same
statement. If we assume the latter is what they really mean, there is still a
second question of whether top speed is engine power limited or not.

If the car hits the rev limiter in 6th gear, it may well have enough power to
go faster with an ideally chosen 7th gear ratio. But, the included 7th gear
ratio may be too tall, so that immediately upon shifting, the engine bogs down
to low in its power curve and the car loses speed.

If the car hits top speed in the peak of its power curve but below the rev
limiter, it is really out of power in 6th and no other gearing can help.

Beyond gearing and power, another potential top speed limit is due to
aerodynamics and grip. A car can be traction limited at top speed, with the
tires starting to slip instead of accelerating the car. But, being a high-end
Porsche, it probably develops the down-force required for traction, thereby
increasing aerodynamic drag and putting you back into the power-limited
regime...

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hinkley
I’m not quite sure I follow this explanation. Well more of a statement as it
doesn’t explain anything.

Speed is limited by a square power law. Internal combustion engines have an
rpm sweet spot for power to fuel ratio and a peak power that’s at a higher
rpm.

So are they saying that the power curve for the 911 engine peaks at much, much
higher rpms? So high that you can fit two gear ratios between peak power and
typical cruising rpms?

It’s been a while since I owned a manual but it used to be common to shift
down one gear to pass, for more torque and more power. Sounds like on this car
you might have to go for two.

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theluketaylor
Peak power is at high rpm in piston engines, but peak efficient power is at
low rpm (generally between 1000 and 2000 rpm).

The reason car makers are stuffing so many gears into transmissions now is to
keep the engine at peak efficiency far more often (to meet emissions and fuel
economy targets).

Trying to hit top speed needs power (mostly due to wind resistance going up by
its own square law) not efficiency. So the transmission will use the tallest
non-overdrive gear (6th) for top speed and the overdrive gears (in this case
7th and 8th) for efficient cruising.

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hinkley
So I'm partly stuck in 5 gear thinking. The gears are a lot closer together.

That said, Porsche's data sheet for the current 911 says peak power is at 6400
rpm, so the ratio in rpms between 'go fast' and 'cruise' is in fact a lot
wider than on an older, cheaper vehicle, where cruise might be 1800 while you
pass at something around 4400-5000 rpms.

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jandrese
Seems sensible to me. To hit the top speed you need the engine revved right to
the redline to generate enough power to overcome the wind resistance. The two
higher gears are for fuel economy and the engine doesn't have the torque to
rev up to the max in those gears.

It's a way for Porsche to have a fast car that isn't necessarily atrocious on
fuel economy.

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EngineerBetter
Ha! Gears! I remember those old fossil-powered cars had more than one of
those. Apparently in some you even had to choose which gear you wanted, rather
than the car doing it for you.

Hopefully soon people will look at piston cars like IT folks look at manual
deployment to dedicated servers.

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thisacctforreal
I think it’s closer to Linotypes vs Computers.

Sure they’re wasteful, but there’s an elegance to our complicated mechanical
contraptions that isn’t captured with electric systems.

