How does Porsche determine redline RPM on various engines?

[WP0ZZZ]

Originally Posted by Bill Verburg

I have to disagree, HP is derived from torque and rpm, and does indicate the ability to move the car.

Hi Bill, we have already discussed this in the past. I'm not saying that using torque for calculations is wrong. But some statements about torque in this thread are just plain wrong and it turns out that by using power this discussion gets simpler and more elegant.

Power is not torque's little sister. In physics, energies are at least as fundamental as forces and power is the rate at which energy is delivered. The effects of power can be measured very clearly. From another thread:

If you can measure speed and how fast you gain it (relevant for performance cars) you are also measuring kinetic energy and how fast you gain it, i.e. useful power converted into speed gains. You measure energy by looking at the state of a system. It is as direct a measurement as measuring torque via the proxy of the deformations of elastic elements in a load cell.

The fact that we are first taught forces and then energies does not mean that forces are more fundamental. If someone masters the ways of torque and can understand all its subtleties I think it’s great. Good engineering is not defined by the tools we use. On the other hand, there are many people who understand neither torque nor power and make wild claims about the matter...

[Bill Verburg]

Originally Posted by WP0ZZZ

Hi Bill, we have already discussed this in the past. I'm not saying that using torque for calculations is wrong. But some statements about torque in this thread are just plain wrong and it turns out that by using power this discussion gets simpler and more elegant.

Power is not torque's little sister. In physics, energies are at least as fundamental as forces and power is the rate at which energy is delivered. The effects of power can be measured very clearly. From another thread:

If you can measure speed and how fast you gain it (relevant for performance cars) you are also measuring kinetic energy and how fast you gain it, i.e. useful power converted into speed gains. You measure energy by looking at the state of a system. It is as direct a measurement as measuring torque via the proxy of the deformations of elastic elements in a load cell.

The fact that we are first taught forces and then energies does not mean that forces are more fundamental. If someone masters the ways of torque and can understand all its subtleties I think it’s great. Good engineering is not defined by the tools we use. On the other hand, there are many people who understand neither torque nor power and make wild claims about the matter...

yes and still folks don't get it

yes some concepts are more fundamental than others

in Physics the fundamental quantities relevant here are
Mass(M)
Time(T)
Displacement(L)
angular displacement (θ)

we use M, L, T to avoid confusion caused by the various measuring systems
Angular frequency and speed are θ/T
Energy is (MLL)/(TT)
Torque is (MLL)/(TT)
Work is (MLL)/(TT)
Power is (MLL)/(TTT) = Angular speed * torque or angular speed * energy or angular speed * work hence my assertion that power is derived for torque & RPM
Velocity is L/T
Acceleration is L/(TT)
Force is (ML)/(TT)
Momentum is (ML)/(T)
Angular Acceleration is θ/(TT)


Everything else is derived from those 4 things


torque is the thing that moves the car, increasing ke or momentum or what ever you want to call it. The rate at which torque is applied is the HP, that's why HP is derived from torque and rpm, so skip the HP and go right to the important things

100lb-ft @ 2000 rpm is way less useful than 100 lb-ft @6000rpm because of gearing
200hp or 300hp or 400hp tells you little by itself 200lbft @5000rpm going through 12.00 gears into a 24" tall tire tells you a lot

[Walt Fricke]

One thing to remember here - in what context are we discussing things?
The optimum shift chart presupposes you have the torque curve you have, and the gears, too. That tells you optimum RPMs for shifting.

If you are looking at something other than what you have, like changing the gearing, or changing the engine's components, that is something else. Shifting the torque curve up 1000 RPM would give you a huge HP boost. But just reving the engine you have more isn't going to give you more than it has to give.

Bill's curves for a lighter vs. a more powerful car are interesting. I have wondered if having more HP isn't the ticket for fast tracks - HP to push aside the air which is increasingly resistant to being pushed. Vs being quicker off the corners due to less mass, where such a car might shine on tracks which have shorter straights. Vs COTA, with quite a few 2d gear corners, but about an equal number of top gear straights (think PDK). One of my engineer racing friends says lighter is always better, assuming equal wt/hp ratios.

Mind you, all this is fanciful when thinking about street driving. I used to out-accelerate a lot of cars with my VW bus just because I tended to put my foot in it more when the light turned green. Big deal. My 07 Turbo has a lot of power I can't really use very often. More than my 2.7 had, and I couldn't use its power all that often either.

[Bill Verburg]

Originally Posted by Walt Fricke

One thing to remember here - in what context are we discussing things?
The optimum shift chart presupposes you have the torque curve you have, and the gears, too. That tells you optimum RPMs for shifting.

If you are looking at something other than what you have, like changing the gearing, or changing the engine's components, that is something else. Shifting the torque curve up 1000 RPM would give you a huge HP boost. But just reving the engine you have more isn't going to give you more than it has to give.

Bill's curves for a lighter vs. a more powerful car are interesting. I have wondered if having more HP isn't the ticket for fast tracks - HP to push aside the air which is increasingly resistant to being pushed. Vs being quicker off the corners due to less mass, where such a car might shine on tracks which have shorter straights. Vs COTA, with quite a few 2d gear corners, but about an equal number of top gear straights (think PDK). One of my engineer racing friends says lighter is always better, assuming equal wt/hp ratios.

Mind you, all this is fanciful when thinking about street driving. I used to out-accelerate a lot of cars with my VW bus just because I tended to put my foot in it more when the light turned green. Big deal. My 07 Turbo has a lot of power I can't really use very often. More than my 2.7 had, and I couldn't use its power all that often either.

The answer is it depends, the engineer friend is right, lighter is always better but some cars are built to compensate for the added weight. On a track the highest average speed is the faster car and that more often depends on the driver than the car.


here's a 964, red is hp blue is torque(converted to thrust) in gears correlated to rpm. An effect of gearing is to flatten the useful torque band.

The power band is always from 4500 to 6500 and because of the gear spread you want to take the engine to red line in 1st to stay in it, in 5th it is the same rpm width but the gears are closer together so you would short shift to stay in the fat part of the power band longer. The lower the gear the more the car needs to be rev'd to stay in the power band the taller the gear the less revs are needed because the gears are spaced closer together. Most drivers will naturally do this because the but tells them to shift as much as the tach. A car w/ a torque curve shaped differently would be driven differently, a pure race motor will usually have a narrower power band at higher rpm gearing and shifting would be much different for that of a typical street motor. This is where the newer cars w/ varicam shine, they have the best of both worlds, broad usable torque band that narrows but gets carried to higher rpm simply by changing the valve events



Hee's what a 991 GT3 & GT3RS look like

[raspritz]

Originally Posted by Walt Fricke

I think it helps to consider two "redlines." The mechanical redline is the RPM above which things might break....

The other "redline" is the upshift point - the RPM for each upshift, beyond which you are getting less acceleration than you could if you had shifted earlier (but not too early, if maximum acceleration is what you want)....Where to upshift for max accell is determined by your gears, and your torque curve.

It is great that after 31 posts in this thread somebody finally mentioned torque and gearing.

[kent olsen]

Great info from the guys who have played with this on the track Walt and Bill. However I stuck my nose in because the OP first was interested in reving his engine well past the redline.

I've seen many who think that will not be an issue and will help acceleration rather than shift, and it sounds cool. As I hope these comments have shown, first knowing what your engine's horse power and torque performance is will make one able to get the maximum performance from our Porsche engines without an expensive lesson.