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Weird. My pdk is automatically shifting 5-6 at around 175mph as I cross the half mile mark at the airstrip events. And I have a taller 315 rear tyre so you would think my shift point for 5-6 would be more than 180mph. So perhaps the pdk auto shift point is actually a bit less than 7,200. I'll have to look at logs.
Weird. My pdk is automatically shifting 5-6 at around 175mph as I cross the half mile mark at the airstrip events. And I have a taller 315 rear tyre so you would think my shift point for 5-6 would be more than 180mph. So perhaps the pdk auto shift point is actually a bit less than 7,200. I'll have to look at logs.
Here's a hypothesis for what you are seeing:
For a well designed drivetrain at full throttle and high rpm, once the engine is past the torque peak, the transmission should upshift when the torque times the current gear ratio is less than the torque times the gear ratio in the next gear, considering the associated rpm drop. As the gears get more closely spaced, like 5-6, and if torque falls off quickly at higher rpm, the optimal shift point might actually not be at redline but might be lower in the rpm band. Maybe that is what you are seeing.
Just eyeballing the gear ratio graph above and torque curves, it looks like once you hit 7000 rpm in 5th, you are better off shifting than revving to 7200 rpm.
Last edited by catpat8000; 05-23-2019 at 01:10 AM.
For a well designed drivetrain at full throttle and high rpm, once the engine is past the torque peak, the transmission should upshift when the torque times the current gear ratio is less than the torque times the gear ratio in the next gear, considering the associated rpm drop. As the gears get more closely spaced, like 5-6, and if torque falls off quickly at higher rpm, the optimal shift point might actually not be at redline but might be lower in the rpm band. Maybe that is what you are seeing.
Just eyeballing the gear ratio graph above and torque curves, it looks like once you hit 7000 rpm in 5th, you are better off shifting than revving to 7200 rpm.
This is correct.. Under Full throttle in sport mode he PDK will shift through gears to maximize acceleration.
These are calculated optimal shift points: n
The curves below can show where these shift points should happen. Where the torque curves cross between 4th and 5th gear at 6,990 RPMs, that is the theoretical best shift point, upshifting from 4th at 6990RPMs will drop you to 5574RPMs in 5th gear, yet acceleration in 5th at 5574RPM is higher than 4th 6990RPMs.
Gentelmen, thanks for the additional analysis. As I run through the half mile my eyes are scanning the rroad and guages. i have opted to manually shift 1-2 but let the pdk shift automatically in 2-3 and higher. And it shifts above 7,000 rpm. Not sure exactly where though as things are happening quickly.
I just viewed my last dragy video of half mile run where I crossed the finish line at a little over 175 mph. The 5-6 shift automatically happened around 170 mph - no where near 180.
By the way my car's hp/torque curve is different than stock as I have 68mm turbos and meth and so my torque does not "fall off@ at high rpms. Which is why I'm going 25mph faster than a stock 991.1 through the half mile. I think the more interesting question is whether I could hit 7k rpm in 7th gear (Sport Mode) and how fast is that?
Gentelmen, thanks for the additional analysis. As I run through the half mile my eyes are scanning the rroad and guages. i have opted to manually shift 1-2 but let the pdk shift automatically in 2-3 and higher. And it shifts above 7,000 rpm. Not sure exactly where though as things are happening quickly.
I just viewed my last dragy video of half mile run where I crossed the finish line at a little over 175 mph. The 5-6 shift automatically happened around 170 mph - no where near 180.
By the way my car's hp/torque curve is different than stock as I have 68mm turbos and meth and so my torque does not "fall off@ at high rpms. Which is why I'm going 25mph faster than a stock 991.1 through the half mile. I think the more interesting question is whether I could hit 7k rpm in 7th gear (Sport Mode) and how fast is that?
Hi, If you look at my previous post you will see it shows 168mph@6,730RPMs as optimal shift point from 5-6th gear, which is in line with your logs, you are running 315s, it makes a difference of ~1.5mph at those speeds approx. so it is pretty close. Tire pressures, expansion of the tire with speed etc.. will make some difference. This is just an excel calculation.
The wheel torque curve shapes posted above correspond to a similar setup as yours. If you send me an xls sheet with your RPM/Speed/long G for that run I can show you your exact real wheel torque being put to the ground, much more accurately than any dyno.
In 7th gear@7200RPMs you would be close to 250mph from memory.
Unlike with higher gears, the best acceleration in 1st gear is limited by traction rather than amount of torque available.. On a normal road with good tires it is usually 0.8-0.9G, on the drag strip it can go up to 1.4G.
For a 991.1 turbo S (560HP), the graph below shows that throughout 1st gear (blue curve), your torque to the wheels exceeds the traction limits (dotted red line) so you have wheelspin all the way to the limiter. The best shift point would be at 3.5-4K RPMs which is the point where in 2nd gear you are the closest to the max traction.. If you shift earlier, your max torque to the wheels in 2nd is still below the maximum traction available so you are loosing time.
On the drag strip you will have grip in 1st gear almost all the way to redline, because traction limit is closer to 1.4G or 13.7 m/s2, so you don't want to shift earlier.
05-22-2019, 10:16 AM
