New Porsche 911 Reaches Top Speed in 6th Gear; Not 7th or 8th(motor1.com)
motor1.com
New Porsche 911 Reaches Top Speed in 6th Gear; Not 7th or 8th
https://www.motor1.com/news/297798/porsche-911-top-speed-sixth-gear/
12 comments
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...
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...
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.
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.
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.
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.
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.
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.
> 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?
That isn't how gearing works. Gearing allows car designers to tradeoff speed vs torque. You can design a gear so that when a car is running at 100 mph, the engine is at 800 rpm.
I suspect Porsche has designed the 7th gear is for normal cruising (65 mph at 1000rpm), and the 8th is for autobahn cruising (165 mph at 1400-2000 rpm).
That isn't how gearing works. Gearing allows car designers to tradeoff speed vs torque. You can design a gear so that when a car is running at 100 mph, the engine is at 800 rpm.
I suspect Porsche has designed the 7th gear is for normal cruising (65 mph at 1000rpm), and the 8th is for autobahn cruising (165 mph at 1400-2000 rpm).
I came to the opposite conclusion: you could probably start the car in 3rd and still pull an oak tree outta the ground. /shrug
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.
It's a way for Porsche to have a fast car that isn't necessarily atrocious on fuel economy.
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.
Hopefully soon people will look at piston cars like IT folks look at manual deployment to dedicated servers.
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.
Sure they’re wasteful, but there’s an elegance to our complicated mechanical contraptions that isn’t captured with electric systems.
Also top speeds. Why would anyone would wanna go over 100mph? How is it even legal to produce cars like these? There's like 2 roads in one country where you'll ever use it.
Flashing your horsepower numbers just shows your lack of understanding of physics or childish pursuit for top speed. Unless you are a racer, torque is what matters much more.
Flashing your horsepower numbers just shows your lack of understanding of physics or childish pursuit for top speed. Unless you are a racer, torque is what matters much more.
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)