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Discussion Starter · #1 ·
So, I see that both the Cayman S and the Cayman GTS reach maximum rated horsepower at 7,400. But, the redline for both is at 7,800 rpms. In running my GTS in Sports Plus mode I see that it goes to 7,800 for each gear, why not stop at 7,400?
 

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Because if you shifted at 7,400 you wouldn't spend any time past the peak hp in that gear, but you would land at a lower rpm (well below 7,400) in the next gear after the shift and the average hp over the whole time would be lower.
 

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The optimal shift point is when the power available in the next gear, after you've shifted and the revs have dropped, is greater than currently available. Its a common calculation to make, I have a spreadsheet to do it. (I also have an iPhone ap to do it.)

If you're going along in first gear in a Cayman S, you have 325hp at 7400revs and about 300 at 7800revs. When you shift up you have about 220hp at 4300 revs (according to the published torque curves and specs). If you shift at 7800 revs you have 4500 revs and 244hp after shifting.
 

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So, I see that both the Cayman S and the Cayman GTS reach maximum rated horsepower at 7,400. But, the redline for both is at 7,800 rpms. In running my GTS in Sports Plus mode I see that it goes to 7,800 for each gear, why not stop at 7,400?
Well the Tacho is about as accurate as the temp gauge and only displays what you want to see to feel good. It shows 7800 revs on the tacho at the PDK shift point but in reality this is about 7400 - 7500 revs, redline on the tach is 7500 and the rev limiter will cut in before you hit 7800.

All tested on dyno runs. Ran till the rev limiter cut in at 7790 revs, about 8150 displayed on the tacho.
 
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The optimal shift point is when the power available in the next gear, after you've shifted and the revs have dropped, is greater than currently available.
I don't think the gear ratios are close enough to make that possible. With only 400rpm available past the peak hp, I'm pretty sure that upshifting from 1st into 2nd and from 2nd into 3rd (at a minimum), you will be making less power in the next gear than you were at redline in the previous. (i.e. if the redline was even higher it would probably be beneficial to shift even later than 7,800).

In most Porsches, if you shift at redline you will land at the torque peak or higher in the next gear and this is desirable (particularly in the lower gears).
 
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I don't think the gear ratios are close enough to make that possible.
They aren't. This is the first car I've had with no overlap on the torque graphs.

The next best thing is to shift at redline, there's not a lot else you can do. The point was that shifting at the power peak is not optimal for any reason.
 

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I agree - we need to shift at redline (not peak up) for best performance
 

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Discussion Starter · #9 ·
Thanks for the explanations, makes sense.
 

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Help me Understand, if Cayman S & GTS have Max Hp at 7,400, Why 7,800 WOT?

First and foremost, I was going to jump in here but I don't need to repeat anything said thus far as you guys have a solid grasp on this info. All I can add is that when we were racing these cars and dynoing things like the elbow intake, 997 plenum, 997 throttle body, etc when we bolted the larger 4" intake and throttle body on, peak power increased by 8 hp but astonishingly, power above 7100 RPM came up nearly 20rwhp. I think more air to these cars would really work to help that fall-off that occurs at 7500 RPM. Stock restrictions and lack of space are the reason we don't even make an intake for the 981. NOW...without stirring up too much of a poop-storm, I want to add the data point that I have seen some of that power loss lessen substantially when we began tinkering with the DME mapping. I know I know...more super secret squirrel / cloak n dagger talk but I want to keep you guys in the loop!


Sent from my iPhone using Tapatalk
 

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Re: Help me Understand, if Cayman S & GTS have Max Hp at 7,400, Why 7,800 WOT?

Will we also see inlet air side scoops to capture more air into the plenum a'la GT4?
 

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Re: Help me Understand, if Cayman S & GTS have Max Hp at 7,400, Why 7,800 WOT?

It's not about the HP, it's all about the torque. Torque falls off in the higher rpm's but when you shift to the next gear you're in the meat of the torque curve.
 

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Re: Help me Understand, if Cayman S & GTS have Max Hp at 7,400, Why 7,800 WOT?

It's not about the HP, it's all about the torque. Torque falls off in the higher rpm's but when you shift to the next gear you're in the meat of the torque curve.
It's about the power too, particularly at higher speeds.
 
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