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Old 06-21-2013, 08:14 AM   #1
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Upshifting on the track

Sorry if this is a dumb question, but on a long straight at a racetrack, does it make sense to accelerate all the way to redline before upshifting? I'm driving a very nearly completely stock 1999 Boxster. I'm wondering, as I know the torque curve maxes out around 4500-5000 rpm, and the hp curve maxes out around 5500. Does it make sense to upshift around 5000-5500 instead of the 6500 redline?

Thanks in advance,
Jason
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Old 06-21-2013, 08:20 AM   #2
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I see no reason to shift much above 6,000. For a few reasons:

1) How much more ya gunna get above 6K anyways?
2) You have plenty to focus on at a track event, shift a little early (like 6,000-6,200) to stay out of the rev-limiter, and avoid range #1 pings and fuel/spark cut-off.
3) If your goal is 6,000 you will rarely ever exceed target, and bang the rev-limiter, if your goal is to shift at redline, guess what's gunna happen? You'll hit the rev-limiter.
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Old 06-21-2013, 09:37 AM   #3
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My 2 : for maximum acceleration, you would use a chart with RPM vs. speed in each gear, and a Dyno graph. Look at the first chart to find the RPM drop for each upshift. Shift points would then be calculated to maximize the H.P. "area under the curve". ie, rev. the engine past peak HP, so that when the next higher gear is selected, you're at nearly the same HP, but on the "uphill" side, it that makes sense. Hard to describe, but easy to see on a graph.

Now, whether you need that last bit of acceleration is debatable. Most just pick an RPM they are comfortable with.

If you bang the rev. limiter now and then, so what, that's what they're there for. Just try and avoid it.

Last edited by stephen wilson; 06-21-2013 at 09:42 AM.
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Old 06-21-2013, 10:50 AM   #4
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Trust me, with that 2.5L engine, I need *every* little bit of acceleration I can get.
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Old 06-21-2013, 11:25 AM   #5
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I think generally, yes, you should shift close to redline to maximize gearing and torque (and thus acceleration).

The goal is to maximize torque at the wheels, which is a function of the transmission and gear ratio. You almost always get better numbers at redline. It may seem like a complicated analysis to analyze torque values, but it really isn't that tough if you have a dyno sheet and know your gearing ratios.

If you crunch the numbers, it's pretty apparent that you have a net positive by shifting later. The graph below (from my other car) illustrates this. If you have a normal dyno graph (one that peaks at some RPM and then gradually drops), any shift will drops you to a lower RPM where the gearing and torque values are lower than where you were pre-shift. This means you shouldn't shift early.

Note: this leaves being nice to your engine and transmission out of the equation.

Yellow box is max HP. Torque is relatively flat.
Light green is max torque @ RPM [~280]
Orange is gear down location [i.e., shift from 8200 in 1st results in 4500 in 2nd, 5900 in 3rd, 6300 in 4th, 6800 in 5th and 7000 in 6th]
Yellow is Max HP @ RPM [373 @ 7300]
Blue shows where two gears overlap in terms of Torque. But this is not an illustration of where to shift. If you shifted at this point, you would drop to the light blue box in the next gear and lose Torque.

In the first column, you can see how shifting may move you into a lower engine HP range, but you can't really look at this independently of gearing.

-td



Last edited by 911_lurker; 06-21-2013 at 11:55 AM.
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Old 06-22-2013, 06:06 PM   #6
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Shifting at or near redline on a long straight nearly always make sense. Shifting through corners may benefit from leaving it in 1 higher gear depending on several factors. If exiting a corner in 2nd results in wheel spin, you are giving up all the torque advantage and adding 1 downshift and one upshift too many. A faster transition often results by staying in the higher gear (3rd) and focusing completely on maximizing a smooth racing line through the corner. I often run a corner both ways and compare the results later with a datalogger like Harry's.

As a rule I tend to keep the car between 4-6k in the corners and go to redline from 3rd and 4th on the straights. Minimum driver input usually wins the day.
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Old 06-23-2013, 07:45 PM   #7
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If you want to go faster than the other guy in an equal Boxster, I'd say you need all of the RPM's that you can get. Short shifting is rarely the fastest way around a racetrack. I pile on all of the RPM's that I can get. With practice, you can shift right at redline without hitting the rev limiter.
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Old 06-24-2013, 07:13 PM   #8
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Thanks for the input, guys. I've certainly been doing it that way (shifting at or near redline), but I was just curious if that was really the *best* way to squeeze all of the acceleration out of the car.

Example of my last (my second ever) DE below. Brainerd International Raceway has a ~1 mile long straight, into a banked, high speed right hand bend. The first half of of the straight is usually used for top fuel and/or funny car drag racing.

Part 1
First Fling 2013 1st Combined Session Part 1 - YouTube

Part 2
First Fling 2013 1st Combined Session Part 2 - YouTube

Thanks!
Jason
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