Where to shift

1713 Views 6 Replies 5 Participants Last post by DocNrock, Oct 12, 2011

DocNrock

Discussion Starter · Sep 5, 2011 (Edited by Moderator)

Hey track fiends! Below is a copy of my dyno plot. The lines in red are my track map (17 psi peak tapering to 14 psi at redline). I hit my peak hp at about 6300 rpm. I've previously been shifting at about 6800 rpm. But after looking more closely at my plot, which I haven't done previously, 6800 rpm is a point on the curve where the hp is falling off.

The question is, should I be shifting the point of peak hp, ie., 6300 rpm? I know this is a really basic question and I'm overthinking it. Truth be known, I haven't been to the track since May and its highly unlikely I'll be able to get to the track again until next year sometime. So I'm jonesing for a track day. At least thinking about things I can do better next time out makes me feel better.

Anyway, thoughts?

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2009 Obsidian Black Nissan GT-R Premium. 550 awhp, 544 awtq (Mustang dyno, 91 octane). Dyno tuned by Jon at Cobb SoCal

Auto Club Speedway ROVAL 1:51.450, Willow Springs (big Willow) 1:34.457, Buttonwillow 13cw 2:02.858, SMMR 3.1 2:41.811

Go: Akuma BM intakes with Outerwears prefilters, AMS FMIC, Harman Motive intercooler piping, iD 1000cc injectors, dual Walbro fuel pumps, NGK racing plugs, AAM decatted downpipes, Cobb catted Y-pipe, HKS Legamax exhaust, and Cobb AP with custom tune.

Stop: AP Racing J-hook rotors, Endless ME20 pads, Stillen stainless brake lines, Endless RF-650 fluid.

Handling: Stillen sway bars, Dunlop SportMaxx 285's x 4, KW Coilover Sleeve kit, custom track alignment.

Accoutrements: Traqmate Complete/Chase Cam, GT Motoring tow hook, ARP studs, Project Kics R40 lugnuts. Passenger seat mount Halon extinguisher.

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shawnhayes

· #2 · Sep 6, 2011

The trick is to shift so that the "area under the curve" is the greatest. Shift too early, and the rpm you get to after the shift is too low, so you aren't near peak hp enough to take advantage of the fact that you were at, or slightly over peak hp.

500 rpm over peak sounds fair as a good shift point, but depends on power/torque required for the next turn too. "overselecting" for the next corner puts me in an area of the torque curve where I can apply power very early without oversteer. This smooths out the corner and shaves tenths off laptimes. Each track is individual, though.

Shawn

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[email protected]

· #3 · Sep 8, 2011

shawnhayes said:
The trick is to shift so that the "area under the curve" is the greatest. Shift too early, and the rpm you get to after the shift is too low, so you aren't near peak hp enough to take advantage of the fact that you were at, or slightly over peak hp.

500 rpm over peak sounds fair as a good shift point, but depends on power/torque required for the next turn too. "overselecting" for the next corner puts me in an area of the torque curve where I can apply power very early without oversteer. This smooths out the corner and shaves tenths off laptimes. Each track is individual, though.

Shawn

To make things even more challenging, Doc, the dyno plot only applies to that one gear, so unfortunately there is no easy answer. Area under the curve, and boost and trq in each gear will vary The best way, I believe to determine the shift point, is by analyzing your Traqmate data. The other thing to note, is if I told you that you could pick up 2 tenths of a second by shifting at 7000rpm vs. 6700rpm would you do it? All things being equal, your engine will last longer shifting at the lower RPM level.

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shawnhayes

· #4 · Sep 10, 2011

[email protected] said:
To make things even more challenging, Doc, the dyno plot only applies to that one gear, so unfortunately there is no easy answer. Area under the curve, and boost and trq in each gear will vary The best way, I believe to determine the shift point, is by analyzing your Traqmate data. The other thing to note, is if I told you that you could pick up 2 tenths of a second by shifting at 7000rpm vs. 6700rpm would you do it? All things being equal, your engine will last longer shifting at the lower RPM level.

Amongst the maintainance and oil crew I float with, they refer to RPM in one uncertain term.

Ruins
Peoples
Motors

every little bit you can reduce the rpm's by, everything else being equal, unless "lugging", the engine will last longer. So, I can echo Sharif, if you're trying to maximize FUN, and not SECONDS per lap, then choose the lower RPM shift point.

Shawn

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yearn2burn

· #5 · Oct 9, 2011 (Edited by Moderator)

DocNrock said:
Hey track fiends! Below is a copy of my dyno plot. The lines in red are my track map (17 psi peak tapering to 14 psi at redline). I hit my peak hp at about 6300 rpm. I've previously been shifting at about 6800 rpm. But after looking more closely at my plot, which I haven't done previously, 6800 rpm is a point on the curve where the hp is falling off.

The question is, should I be shifting the point of peak hp, ie., 6300 rpm? I know this is a really basic question and I'm overthinking it. Truth be known, I haven't been to the track since May and its highly unlikely I'll be able to get to the track again until next year sometime. So I'm jonesing for a track day. At least thinking about things I can do better next time out makes me feel better.

Anyway, thoughts?

You have such a broad power peak that it doesn't matter too much where you shift. The problem with shifting at peak HP is that the next gear would be well bellow peak HP, so acceleration will drop off after the shift. On the other hand, shifting well after peak HP, like 6800 RPM in your case, results in a drop in acceleration before the shift. The best balance is to shift when the power is the same before and after the shift. I sketched a figure conceptually showing a shift from 5th to 6th gear shifting at peak HP and at the right time after peak with a peakier power curve. Note that you need to know the gear ratios to estimate the best shift points, but I would guess yours would be around 6500 RPM.

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GTB/ZR-1

· #6 · Oct 9, 2011

yearn2burn said:
You have such a broad power peak that it doesn't matter too much where you shift.

Gotta agree... Unless your doing a max attack competition lap, save your engine & let the TQ do the work...

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DocNrock

Discussion Starter · #7 · Oct 12, 2011

yearn2burn said:
You have such a broad power peak that it doesn't matter too much where you shift. The problem with shifting at peak HP is that the next gear would be well bellow peak HP, so acceleration will drop off after the shift. On the other hand, shifting well after peak HP, like 6800 RPM in your case, results in a drop in acceleration before the shift. The best balance is to shift when the power is the same before and after the shift. I sketched a figure conceptually showing a shift from 5th to 6th gear shifting at peak HP and at the right time after peak with a peakier power curve. Note that you need to know the gear ratios to estimate the best shift points, but I would guess yours would be around 6500 RPM.

I'd be inclined to agree with you here, especially given what Shawn and Sharif have said about max RPM. On one hand, revving beyond peak hp the acceleration will beging to drop off. On the other hand, if the upshift revs fall to a lower part of the hp curve, then acceleration may drop off. However, this *might* be offset by the fact that the car would be in a higher gear ratio if not too low on the hp curve.

The next time I make it out to the track, whenever that may be, I think I'll start with shifting around 6500 - 6600 rpm and then experiment from there. Like Sharif said, I can then use my Traqmate data to see what is optimal. End of straight top speed may also give some more immediate feedback.

Thanks for the thoughts, guys!

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Where to shift

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