Optimum shift time in Boxster S

[Halfbreed83]

"Times have changed from the mustang and camaro days, with the RX-7's and the mitsu 3000GTs and the supra's... cars are more high-tech and built to last much longer and take more abuse."

Too bad my RX7 didn't make it Also, alot of RX7 owners are throwing in LS1's too make more power and reliability.

[blue2000s]

OK, here's the way it's done. This will probably lead to some useless torque vs power debate that I won't be participating in.

You look at the torque produced at the wheels in every gear for a given vehicle speed. This gives you a set of curves of torque vs vehicle speed. In fact, this will often give you 3-4 gears to choose from at a given speed. You choose the gear that gives you the highest torque at any given speed and you stay there until redline or until the torque curve drops below the torque curve of the next gear. This is part of the process used to define gear ratios and it's also the process you use to decide what shift pattern will give you the best acceleration. In the Boxster, as with just about every other high performance sports car, redline is where you shift in almost every gear.

Here's a spreadsheet showing what I'm talking about. Take off the .txt extension and unzip it.

http://986forum.com/forums/attachment.php?attachmentid=3282

Keeping RPMs between the torque peak and power peak is incorrect and shifting at power peak is incorrect. You ALWAYS have to look at the torque at the wheels if you want to answer this question correctly.

[Brucelee]

It clears the valves and simply allows the engine to wear correctly.

Not sure what you mean by this. Can you elaborate?

[BoxsterLewis]

Shift at 6k and you'll be good to go

[2000SoCalBoxsterS]

Originally Posted by blue2000s

OK, here's the way it's done. This will probably lead to some useless torque vs power debate that I won't be participating in.

You look at the torque produced at the wheels in every gear for a given vehicle speed. This gives you a set of curves of torque vs vehicle speed. In fact, this will often give you 3-4 gears to choose from at a given speed. You choose the gear that gives you the highest torque at any given speed and you stay there until redline or until the torque curve drops below the torque curve of the next gear. This is part of the process used to define gear ratios and it's also the process you use to decide what shift pattern will give you the best acceleration. In the Boxster, as with just about every other high performance sports car, redline is where you shift in almost every gear.

Here's a spreadsheet showing what I'm talking about. Take off the .txt extension and unzip it.

http://986forum.com/forums/attachment.php?attachmentid=3282

Keeping RPMs between the torque peak and power peak is incorrect and shifting at power peak is incorrect. You ALWAYS have to look at the torque at the wheels if you want to answer this question correctly.

O.K. so is this spreadsheet interactive where I can plug in my wheel size, tires, torque... and it will give me my shift points?

[blue2000s]

Originally Posted by 2000SoCalBoxsterS

O.K. so is this spreadsheet interactive where I can plug in my wheel size, tires, torque... and it will give me my shift points?

Yup. You put in the data in the yellow fields and it will output the estimated wheel torque curves. Then you choose the gear that shows the most torque at any given speed.

I think the torque curve in the spreadsheet now is for a 911. I don't remember what car I was looking at when I made that sheet.

[2000SoCalBoxsterS]

Originally Posted by blue2000s

Yup. You put in the data in the yellow fields and it will output the estimated wheel torque curves. Then you choose the gear that shows the most torque at any given speed.

I think the torque curve in the spreadsheet now is for a 911. I don't remember what car I was looking at when I made that sheet.

I will take my Boxster owners manual and look up the gear ratios and take the power and torque numbers from there and in put them. Why can't I adjust the size on your spread sheet? And I'd like to make the lines on the graph thicker so I can read them better.

[blue2000s]

Originally Posted by 2000SoCalBoxsterS

I will take my Boxster owners manual and look up the gear ratios and take the power and torque numbers from there and in put them. Why can't I adjust the size on your spread sheet? And I'd like to make the lines on the graph thicker so I can read them better.

It's locked so you can only edit the fields that I designated.

[2000SoCalBoxsterS]

O.K. I plugged in my variables. Shift ratios same as on your sheet. I have 18" wheels with 265/35 tires on the rear. My owners manual only shows my torque in NM at rpms. They look like this:

Enter Engine Speed, RPM Enter the engine torque, ft-lb
1500 240
2000 258
2500 265
3000 280
3500 285
4000 300
4500 302
5000 302
5500 300
6000 290
6500 265
7000 240

But the Drive Torque Chart output doesn't look right? The Wheel Torque Lb-ft scale is in thousands? Showing me that for example at 25 mph I am at 3500 Ft-Lbs? How am I supposed to read this? Use this info?

The stats on my Boxster from Porsche say I have 250 HP / 225 Ft-Lbs at 6250 rpm
Seems to me based on the graphs in my manual that 4,000 to 5,000 is the peak power band.

What am I missing here? I'm not being critical I just don't understand and I want to learn this stuff.

[SinnerDC2]

I come from the honda/acura world where we always shift at redline without any consequence. I shift right before 7,200 very often in most gears.

[2000SoCalBoxsterS]

I agree with Bruce Lee for all of the reasons he lists. It's only common sense. Try driving your Boxster by doing all your shifts at 6K + and if you're smarter than a 5th grader you'll quickly understand why it's not practical unless you are racing on a track.

[eqs]

If you want to know, take it to a dragstrip and do timed runs videoing your tach. Real life stuff, not theoretical. I race sailboats and VMGs and performance plots are one thing, but the race course is where it matters. Obviously one can extrapolate values from the Porsche owner's manual and determine intersection points btwn hp and torque and factor in variables like timing and lift off and such, but whatever....

That's too technical. Shift it when it feels good - for me that's just before fuel cut-off (because I dislike the feeling of the engine stuttering due to fuel starvation).

6000, 6500, 7000, whatever, just keep it pegged until it feels good to you, then shift and do it again. That's the "best" time to shift - the point when you're having fun - and the person beside you is smiling too (or keeping her eyes closed).

Sorry if this isn't technical enough. :dance:

Cheers