Earlierapex

Most of you true gearheads know that torque peak is where the engine makes the most torque, but horsepower peak is where the engine/gearing combination can deliver the most "gear multiplied torque" to the wheels. The balance between these two occurs because the mechanical advantage of increased gear-based torque amplification with increased RPMs adds more "multiplied torque" than is lost as the "engine torque" curve decreases with increasing RPMs. Horsepower is a mathematical calculation of the point at which these two independent variables (engine torque and theoretical gearing) are maximized.

In a perfect world, the maximum mathematical acceleration would occur with an engine spinning at the horsepower peak with an infinitely variable frictionless transmission.

On one hand, current variable transmissions are inefficient. On the other hand, the rate limiting step in adding more gear ratios to a manual is the ham-fisted human being shifting gears with big feet and a hairy chest.

While the journalists have touted the obvious advantages of PDK (keep both hands on the wheel, quicker shift times), perhaps the real advantage is the ability to manage more gear ratios in less time to maximize the mechanical gearing advantage available when one keeps RPMs as close to the horsepower peak as possible?

Most of us use gears 3 and 4 on most tracks on most manuals most of the time. What if you could add another gear by keeping 3rd the same and making 5th the same ratio as 4th without any loss of function by having to wave your arms and feet around like a madman between southbend and oaktree at VIR? What if you could add 2 gears?

I've never bought the argument that a manumatic gives a huge advantage in acceleration because shift times are faster - you don't lose half a second of velocity, just half a second of acceleration time. While better, the argument that the computer can heel-toe more consistently than humans is equivalently marginal in my mind (and less fun).

But what if PDK moves the game on by doing something human beings literally don't have the physical prowess to do? Human beings don't have 4 feet and can't replicate ABS. In the same manner, we don't have quick enough, non-chassis-disturbing enough shift capability to manage 50% to 100% more meaningful gear ratios.

I'm sure someone will point out that the 991 Carrera pdk has basically the same ratios as the 7spd manual, and all Porsche has told us is that the GT3 has "shorter ratios and reaches top speed in 7th rather than 6th." The increased RPMs available with the GT3 create the perfect environment for gear ratio optimization.


What if PDK really gives us access to 10% to 20% more usable torque via increased and optimized gearing over 30% to 40% of the track?

But what if PDK-S doesn't just do what we do better, but does what we can't?

jumper5836

I would expect that pdk if left in auto, should change gears at the optimal rpm prior to it losing the peak hp/tq or keeping it in gear until red line if it is losing power where changing gears wouldn't be beneficial in the new gear that it is going into. But would change in the case where it had more power then the one it is coming out of.

Figuring out optimal gear changes is where we can't do this our selves very well and why it's best just to leave it in auto and not bother with paddles.

RobSpyder

Perhaps at a drag strip. On track the computer can't possibly be as accurate as we are heading into a corner. I'd be quite happy if auto-mode was omitted from the GT3 paddle box.

RobSpyder

duplicate

Earlierapex

Given the limitations of current gearing, you are always better off holding a gear to redline (unless you are in a diesel).

With a really slick PDK with 25 gears, you could have the computer shift at the RPM point where the reduction in wheel torque in the next higher gear is equivalent to the rate of reduction of engine torque.

fbirch

Why stop at 25 gears? A CVT has the ability to change ratios infinitesimally. The belts they use are getting more and more robust. It's only a matter of time before you'll be able to bolt one up to a 500 HP engine, and take the concept you are describing to its logical conclusion.

BTW, Williams used a CVT in an F1 car, with great results:

http://www.f1fanatic.co.uk/2007/05/0...nsmission-cvt/

Mike in CA

Based on my experience, I think you would be amazed at how well even the first generation PDK is at anticipating and being in the right gear at any given time. Sport + auto mode is incredibly "intuitive" in track and fast road driving situations. A very skilled human driver might be able to match it, but would be unlikely to beat it. PDK-S is at least two generations on from the original and should be that much better.

wanna911

I agree with the OP for the most part, but still believe the overall affect is quite negligible for a talented driver. I think the biggest benefit is a lesser dependency on gear ratios being right, shifting mid sweeper where in a manual you would probably be better off just staying in one of the gears as the time/balance lost shifting while turning increases dramatically. I know I skip some shifting opportunities because it's just plain faster not to, and because I can't use the additional torque from the lower gear at the edge of traction anyways.

I think the affect depends on how much torque the car is actually putting down. In a straight line, it's pretty much identical, but mid corner, an increase in torque will not help unless you have more grip than power. Which is quite hard to do in a street car on street tires with no big DF.


I have been amazed how it's not in the right gear and downshifting mid corner while watching the Cayman Interseries guys drive around in auto mode. Pretty much every one of them going through turn 1 at Road Atlanta, you could hear the car kick down a gear likely from 4th to 3rd.

Sometimes it's beneficial to be in a higher gear because you can't put down more power in the lower gear anyways so you aren't feathering the throttle in 3rd vs planting it in 4th.

Mike in CA

Sometimes it's beneficial if you can't get the power down, but other times it is. If the Interseries guys aren't finding it to their advantage it makes you wonder why they are leaving it in auto. I assume their PDK boxes are also capable of temporary manual override just like the production version.

jumper5836

There was a thread about this in the Racing & Drivers Education Forum. There where a few guys saying to just leave it in auto and telling students to leave it in auto. It's easier on the car and it does a better job. Your correct that even in any auto mode you can still override it and shift manually when you want to.

Mike in CA

That makes perfect sense.

MaxLTV

With variable valve and intake technologies, the area around peak horsepower is getting rather flat. The wider this flat area is, the smaller the benefit of more gears.

Modern turbo cars push it to extreme, developing same power over 1000+ RPM with rather gentle drop offs on either sides. For example, the new M5 puts practically the same power to the wheels between 5750 and 7200 PRM, so once you get to the point where shifting at 7200 would put you at or above 5700, adding more gears will no longer have any benefit, only more heat and wear. And even dropping down to 5100 would be unnoticeable in terms of total power output. I do not have numbers in front of me, but with that type of torque curve, there is probably not much if any benefit in going beyond 7 closely-spaced gears. But if you want to have 2-3 overdrive gears for economy, 9 or even 10 gears may make sense.

Earlierapex

Good point, but I certainly know a lot of really fast drivers who don't downshift to second at oak tree because they lose time and momentum trying to get both the 3-2 downshift and turn-in point perfect in tight spaces like oak-tree at VIR.

fair enough, but I don't know too many people with the described skill that are still on street tires.


I agree that it is easier to balance right on the edge in a high speed sweeper in a higher gear. However, that is only faster if you can actually induce oversteer in the higher gear. If you can "plant it in 4th" you aren't at the tire's capacity in the higher gear and would be faster in 3rd where you can apply more torque.

Earlierapex

I'm sorry, but this isn't mathematically correct. The drop in gear torque from 7200 to 5750 is 20%. If the gear ratio difference were only 10% (from 7200 to 6500), the available torque will be 10% higher with your assumption that engine torque is the same. The rate limiting step will be based on the trade-off between shift times and extra torque available.

In a nutshell, that is a very simple explanation of why more gears and a computer that can manage them is better for acceleration. it also explains why a 9000rpm gt3 is as fast as a 7000rpm turbo with 25% more engine torque.

MaxLTV

I assure you it's correct - the HP (power) is the same through that entire RPM range, because torque falls down at the same rate as revs go up. Here is an illustration (the blue dashed line). You can see that the turbo practically emulates CVT effect.



Acceleration is determined by average power to the wheels through the used rev range, and if the power to the wheels is not changing much through the used range, the average will be very close or equal to peak power. Once that point is reached, adding more gears will not increase average power to the wheels and thus will not help acceleration.