Shifting at 9k will be quickest, but rev-drops would be very useful to know.

 

It mostly likely is true, but until we see the torque curve of the new engine, it's not guaranteed. Case in point - almost all turbocharged BMWs benefit from shifting before redline because drop of torque near redline is much bigger than drop at lower revs. It's more typical for turbo motors though, and you are most likely right.

 

Peak power is 8,250 and the goal when driving a road course is to minimize deviation from this target when accelerating. Shifting at redline is only 750 rpm over this target which will likely put you at more than 750 rpm below this target upon upshift (probably much more than 750 rpm for most gears). Shifting below 9k will exacerbate this issue on upshift.

I agree that things can change with a turbo car that cannot hold max boost til redline (not an issue here).

 

Technically manual transmissions have been well understood for decades. Big trucks have had ~13 speeds for years, and it would have been easy enough to make a seven speed manual in 1998, for example, either for a race car or a street car. No one did, however, because the math says that for fast cars (not slow trucks) you end up losing more in shift time than you gain in additional area under the power curve.

Since the 90s the main thing that's changed is that cars have gotten faster. And the faster a car gets, the less time you want to spend shifting. The only reason for 7 speeds in a very fast car today is either a) automated shifting, that eliminates the delay associated with the shift, or b) parts commonality with an auto box.

To this specific example, we don't know gearing or power band yet, but when we do we'll plug it into a simulator and answer the question definitively. You can think about the problem this way, however:

The PDK GT3 will do 100 mph in 7.5 seconds claimed. The 7 speed PDK (and presumably the manual version too) likely shifts 3 times along the way, each shift <100 ms, for a total shift time of <.3 seconds. The manual, however, would have a shift time closer to ~400 ms, slowing the 0-100 to say 8.4 seconds.

The alternative would be a six speed with one fewer shift. You'll lose area under the power curve, you'll gain back about 1% due to the 25 lb lighter transmission, but critically you save one shift, and this is more time than the extra average torque to the ground can make up.

Lots of people have done this math, and they always come up with 6 speeds. Porsche now decides that the math has changes, and that a 7 speed is optimum. It's just a coincidence that it shares >95% of its parts with their automated gearbox, where 7 speeds does make sense...

 

Math: A zero shift time and infinite number of gears is the optimum

 

7 sounds about right with such little torque.

Even the new Corvette with much more torque has a 7 speedmanual, but I dont know the gearing and it's V8 redlines at about 6500 I'm sure.
On the other hand the automatic Mercedes AMG CL 65 has 5 and it always felt if it needed about 4 but it has 734lbtq (Detuned from over 800lbtq to save the gearbox)

At most tracks we only use 3-4-5 as 2-3 was spaced out too far and 6 is a useless MPG gear.
With a bit closer ratio 2-3 can be shorter while 6 can be in place of 5 leaving 7 for the MPG gear.

 

A Dutch invention; CVT and it drives like ****.

 

Haha, yes indeed, but the math is right

 

They did the math and decided on 6 speeds when the 993 had a redline of 6,800 rpm and a max speed of 165mph. The 991 GT3 has a redline of 9k and a top speed nearing 200mph. That math has changed to allow for at least one more ratio.

BTW, the 993 has horrible gearing (particularly with the USA gearing) - 6 gears was never enough (at least those 6 - huge gulf between 2nd and 3rd) even 20 years ago...

 

Don't kid yourself. The corvette has a 5 speed manual with two huge overdrives. 5th gear is 200 mph, 6th is ~295, 7th is ~360 in the stingray, obviously for fuel economy.



So with that torque curve an power to weight the Vette guys have come up with a 5 speed, not 7. As soon as you talk fuel savings the number change, obviously.

 

That is true - that's how they get 26mph from a big displacement V8. I do think a Z06 with AT LEAST 6 close ratios would improve performance though. The Viper has 6 real performance ratios.

 

Realistically, only 2-4 gears will be useful on most tracks. Even on fastest tracks, I do not see many people (manually) upshifting at 150mph at the end of a streight just to downshift half a second later.

 

The 993 did 0-100 in 12.4 seconds. With 2 shifts, it spent 7-8% of it's time shifting.

Today a GT3RS 4.0, roughly as fast as a 991 GT3, does 0-100 in 7.9 seconds and spends 12-13% of that time shifting. You seriously think adding another shift and increasing that to 18-19% of the time while adding 25 lbs will make the new car faster?

You're off in the weeds. I'll do some area under the curve plots at some point when I have time.

 

I'm happy to be proved wrong (but that would conflict with my own empirical evidence, with respect to regearing and laptimes - I saved a bunch of time by increasing the number of shifts per lap in my 911, using the notoriously slow shifting 915 gearbox. Made the ratios closer together and was able to use 4 gears on a lap where I never exceed 130 mph). Faster laps than many GT3's that only use 3 gears and have higher trap speeds.

Btw, your estimate of .5 sec per shift seems a little slow - last estimate I saw was .3 sec with a modern gearbox.

 

OK, I ran some numbers for you. As with any simulation garbage in, garbage out, so the answer is all down to the assumptions one uses. However with the assumptions I'll list, the 6 speed fairly convincingly beat the 7 speed.

Now before I get into that, I'll mention two things. First, this is a straight line simulation. I no longer have access to my favorite lap simulation program. Second, when I've had this debate with a friend of mine who's a race engineer, he tells me that even when the lap simulations come out equal, drivers prefer having an extra gear for the added flexibility it gives.

That disclaimer noted, here are the assumptions I used:

In absence of dyno curves from the 991 GT3, I used a GT3 RS 4.0 dyno and made the redline 9000 rpm. I did this not only because the power and torque are likely similar, but also because the 4.0 also makes peak power at 8250.

For the 6 speed I left speed in each gear roughly the same as the 4.0, for the 7 speed I simply lowered the R&P ~13% and stuck another gear on the top to maintain the same top speed.

I left shift times at 500 ms- this is default for the manual in the sim program. I agree some gearboxes and some drivers can shift faster, at least sometimes. I added 25 lbs to the 7 speed (also helps in cornering and braking, not shown here). Otherwise the cars are identical.

The 6 speed is faster almost everywhere: .5 seconds faster to 100 mph. 3 mph faster in the 1/4 mile. If you reduce the shift time to 300 ms the gap drops to .3 seconds to 100 and 2 mph in the 1/4 mile.

It's interesting how much the extra 500 rpms help area under the power curve, by the way. If redline was closer to peak power an extra gear would be more useful.

I know you're not yet convinced, so what numbers or changes would you like to see?

Purple is the 7 speed, you can see the the speeds in gears, etc: