Rennolazine

Pardon my ignorance. Couldnt find where this has been covered before. If you get a tune (ie. Cobb), does it increase the top speed in each gear? Just curious as this would make it much harder to shift at redline on any public roads (one of the best parts of having a 911 is hearing the sweet engine music).

subshooter

Your top speed at red line will not change in any gear. However, for those gears that have a top speed limited by HP before red line (i.e. 6th and 7th IIRC) additional HP will increase top speed in those gears.

Rennolazine

Interesting. So in essence the extra pickup is perceived as a much quicker increase in rpm for the same throttle input? If you still need to shift at the same mph, that would be awesome. One of the things i would really enjoy is if the revs climbed more quickly..

GrantG

Yes, revs will increase more rapidly. The relationship between speed, rpm and what gear you're in is totally fixed and defined by gear ratios.

Rennolazine

Thanks for the info guys. Appreciate it

tgavem

PCA1983

Speeds in lower gears are limited by engine redline. Speeds in highest gears are limited by Hp.
Grunt, AKA acceleration, is boosted by higher torque, especially in lower gears.
For daily driving, torque is more important than Hp, for getting you quickly from stoplight to stoplight, or when squirting past another car in traffic. On high speed straights and Autobahns, Hp is more important.
Further, weight and traction, launch and transmission shift speed are also very important in acceleration, which is why the 991.2 Carrera X51 S and GTS can beat any Cameo, Mustang, and almost any Corvette or Dodge Henri from 0-60, even tho they are outgunned from a Hp and Torque standpoint.

Bob Z.

Horsepower is how fast you hit the wall, and Torque is how far you move it through it.

erko1905

I'm pretty sure the OP didn't ask about this but I'll point out one nuance - some Cobb tunes do actually move the redline higher a few hundred rpm, so that'd technically mean a higher "top speed" in lower gears but that's really not too relevant- just pointing out for completeness sake

subshooter

Since we are talking about HP and T vs Top speed.....

Top speed is limited by HP. In fact if you want to double top speed, you have to quadruple HP since drag is proportional to the square of velocity of the object. You can see this in the data I tabulated below.

In the case below, increasing HP from 370, to 580 (a 46% increase), only improves top speed by 12%.

C2: 370 HP, Drag Coefficient .29, Top speed 183
C2S 420 HP, Drag Coefficient .30, Top speed 191
C2GTS 450 HP, Drag Coefficient .31, Top speed 193
911 Turbo 540 HP, Drag Coefficient .31, Top speed 198
911 Turbo S 580 HP, Drag Coefficient .31, Top speed 205

And to geek out a bit.......
Mathematically, we can predict (i.e. calculate) the expected Top speed of the 911 Turbo S just by knowing the increase in HP and drag coefficient from the C2.
Assuming drag coefficient is constant would yield a top speed of the Turbo S: (square root of 1.46)*183mph = 221mph. But since we know that Drag coefficient increases from .29 to .31 (probably because of bigger tires and wing) then....
Predicted top speed of Turbo S = 221mph*(.29/.31) = 206.7mph. Pretty close to what Porsche advertises. They probably ran out of track distance..... or more likely the drag coefficients were only provided to two significant digits. lol

Bottom line is that you need to add a HUGE amount of HP to go above 200mph. Torque just gets you there faster.

PCA1983

Please calculate for my 991.2 C2S with X51 PowerKit. It should be faster than the GTS.

subshooter

Yes it should be faster (i.e Top speed) since the drag coefficient is less but with the same HP. (.31/.3)*193mph ~ 199mph. Wow.

Of course we are assuming the drag coefficient with the power kit also has the same drag coefficient as the C2S. I forget if they do something with the front spoiler lip with that power kit package. Obviously the aero bits on the GTS causes the drag coefficient to increase slightly.

Also of interest is that the drag coefficient increases slightly between the C2 and C2S. I'm guessing it's because of the bigger tires with the C2S.

freeman

I would like to ask the opposite question.

What are the MINIMUM RPMs that the 991 should be run in each gear. My 74 914, 87 911, 89 911 and 95 993 manuals all clearly articulated minimum RPMs in each gear before the driver is DRAGGING in each gear. My 2013 C2 991 had PDK but my 2015 991 GTS has 7 MT and the manual does not refer to the minima.

Can anyone kindly supply this information that I missed for the MT 991.

Thank you so much for providing the information kindly.

Valvefloat991

Keep in mind that drag coefficient is only one component of aerodynamic drag. The other is the cross sectional area of the car, known as frontal area. Drag coefficient indicates how smooth a hole you are punching in the air. Frontal area indicates how large that hole is. Total aerodynamic drag is called drag area, and it is Cd x FA.

So the GTS is aerodynamically worse than a base car or an S in two ways. It has both a higher Cd and it also has a larger frontal area, due to the wider rear fenders.

Also, earlier someone had stated that required horsepower goes up with the square of speed. It actually goes up with cube of speed. That's because drag increases with the square of speed, but power is drag times speed, hence the third power.

spdracerut

The concept you're looking to learn is gear ratios.

https://auto.howstuffworks.com/gears.htm

There's a whole section on drivetrains:
https://auto.howstuffworks.com/car-t...ms-channel.htm