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I was hoping someone could provide me with the formula or possibly a link to a shift point calculator. I'm looking to come up with shift points based on my dyno results and gear ratios.
Also from the dyno, I notice my stock tach is way off. I was taking the car to an indicated 7500rpms, which is actually only ~7100. I'm going to have to take that into account next time I'm at the track.
Looking at your dyno sheet....if it were me, I'd shift at 6,500 rpm (6,900 on *your* tach) on one run, then 6,800rpm (7,200 on tach) on the second, and see what changes on the timeslip.
This is why a trip to the dragstrip is soooo much fun. You get to figure stuff out.
1-2 shift comes under duress, as there's just so much going on with a turbo car at that point. My car spins the slicks and the shift light immediately comes on....but...if I wait it will grab again and *then* I shift to second gear.
Summation: For the gears where there is some degree of control going on, those are the gears to hit your shift point at. 1-2 is just a guessing game with all the spinning going on.
Um............Greg, your theory is *basically* headed in the right direction, but if you plug in his dyno chart with the actual gear ratios you get a much different set of shiftpoints than those you recomended.
Ill have to extrapolate a little for what happens past 7100rpm both times, but since most boosted SR20DE's tend to arc off smoothly and slowly in power, I think I can come up with a pretty accurate picture of what is going to happen.
Now, the first thing you need to know is the gear ratios.
For a stock SE-R/NX/200SX SE-R tranny, these are as follows.
1st 3.063
2nd 1.826
3rd 1.286
4th 0.975
5th 0.756
Now, you then calculate the rpm drop for each gear shift. 1st to 2nd drops you from a 3.063 to a 1.826.....(1.826/3.063 = 0.596)......this means that in 2nd gear, no matter what rpm you shift at, you will ALWAYS have just under 60% of the rpm you would in first............so a redline shift puts you at ~4470rpm in 2nd...........the actual tach will never drop that far down off of a hard shift because there is lag in the needle, but held at a steady 38mph (speed at indicated redline) you will sit at 7500rpm in 1st gear, and 4500 in 2nd.....
Anyway, looking at his 10psi chart lets try to see where the 1-2 shift should occur. If we claculate it for a 7000rpm shift, you will notice that there is approximately 242whp available (I'm using the non-iced run)..........unfortunately, going into 2nd gear, he ends up with only 4170rpm .............at which point he is making only around 165whp..........that is a HUGE drop-off........he obviously needs to shift higher. Anyway, the way that curve arcs across, he should have something like 225-230whp still available at 7500rpm. Since he doesn't make this much power until @5500rpm, he is obviously best off going until as close to the rev limiter as he possibly can (since the highest rpm he can get off of a 7500rpm shift is only 4470rpm).
Going into 3rd you have a similar deal......... 2nd (1.826) to 3rd (1.286) drops you to 70.4% of your 2nd gear rpm (closer)..........but even another 7500rpm shift will only drop him to ~5280rpm in 3rd.................we are getting closer to having the power curves intersect, but again, he is better off sinking the tach.
For 3-4 and 4-5, you WIL end up shortshifting a *touch* as there is a ton of midrange, but not to 6800rpm as was first advertised..........
Using the same method as above, 3-4 should go off at 7300-7350rpm. I'm extrapolating a bit here, but it looks as though there should stil be around 230whp at 7300rpm............shifting there will drop you to 5535rpm in 4th, at which point you ALSO have ~230whp available (actually, just less than this).............THAT is a good matched shift.
4-5 is even a touch closer yet, but not really enough to really change things...........best shiftpoint is approximately 7250rpm (again, I'm extrapolating the power curve a little).
So (and these are actual rpm.......add around 200-300rpm to account for high rev tach error)..........
1-2 7500rpm (or more if you have the JWT ECU)
2-3 7500rpm
3-4 7300-7350rpm
4-5 7250rpm
All quite a lot higher than the estemed Greg Perry originally recomended
Now............your homework is to figure out your own shiftpoints for the 1 BAR run
This WILL be graded.
__________________
92 NX2K Hardtop
Suspension, Intake, Header............it's slow, but it corners well
Well......he DID ask how to calculate shiftpoints................
As long as you know your gear ratios and have a dyno chart of your car (or at least one with virtually identical mods), you can calculate the shiftpoints for ANY manual transmissioned car.
Automatics are a little different though, since they are still accelerating DURING the shift (the torque convertor never FULLY unlocks), and you have to allow for this..............but even then, you can get VERY close.
Very interesting reply, BigBuls. And very informative.
However, you have forgotten one little item: torque. IMHO, turbo cars should drive/shift for torque benefits, since they make so much more than a NA car does, and the fact that most turbo cars often produce more torque than HP.
Especially at the dragstrip, where torque is what gets you there and HP is what keeps you there, to borrow a low-tech saying.
That being said... I would wager solid money that Jonathon's car will produce quicker ETs by shifting in the mid-to-upper 6,000rpm range than by doing so at 7,200rpm as you suggest.
No scientific/mathematic evidence to back up this claim, other than my experiments at the track with a car that is virtually identical to Jonathon's.
This is a solid debate, and very informative, nonetheless.
Well, I should point out that HP is nothing more than the WORK that the torque is doing.............and since it's the WORK that actually propels you down the track, I stand by my argument.
In addition, I should note that if you plot out the torque curves multiplied by the gear ratios (and then plotted against speed), you will end up with the EXACT same shiftpoints (this is simply plotting the actual FORCES [not work] at the wheels at a given speed). You see, the gears simply multiply the available torque (this is why the car pulls harder in 1st gear [a 12.791:1 overall torque multiplication due to the 3.063 1st and 4.176 axle ratio] than it does in 2nd [a 7.625:1 overall multiplication] and so on [obviously aerodynamic and tire drag play a part at higher speeds too]).
Both HP and Torque are actually the same thing............just expressed in two different fashions (one as a direct force, the other as units of work).
Either way........I still stand by my argument
As for other arguments that you may have ran slower with different shiftpoints (namely lower ones), I should point out that your dyno MAY in fact fall off more at high rpm than does the one in this particular thread. I HAVE seen a few DET's that die off more up top (normally ones with raised boost levels on the T25 die out past 7K rpm rather quickly).
I just did a little work with Mr Perry's dyno chart posted in his Nissanperformancemag feature.
Again, we see a dyno chart that cuts off at just past 7100rpm due to tachometer optimism.
BUT.
His T28 chart looks a LOT like most low boost T-25 charts do.............
I also did a shiftpoint analysis on his chart, and came up with the following for 9psi (assuming something between 190-195whp at 7500rpm):
1-2 7700rpm or CLOSE to it (I know you have the JWT ECU)
2-3 ~7400rpm (I'll assume just under 200whp at 7400)
3-4 ~7150rpm (drops you to @5400rpm)
4-5 ~7100rpm(not that you'll get there in the 1/4 with stock sized tires on this boost level)
Now.........notice Greg's shiftpoints come out a little lower than "MyprojectB13"'s did (subtle differences in setup can change things quite a lot on FI cars). And again.........these points are only valid for 9psi............with the 11psi setting that Greg used for his 12.800 run, he may very well be starting to push the high rpm limits of his setup, making power drop off more rapidly, thus necessitating somewhat lower shiftpoints.
Remember..............transmitted torque is what you FEEL, but your butt isn't a very good indicator of power ("I gained 25hp from my spark plugs...I FELT it")..........why would it be a good indicator of when to shift?
THAT is what dynos are for (amongst other things).
Either way............as long as you have a dyno chart and know your gear ratios, that IS the way you work out shiftpoints.
Go ask Dave Coleman if you don't believe me (he has published this SAME method of picking shiftpoints in SCC a couple of times in the last few years)
I would bet that given a nice track where repeatable times are possible (in other words, launching was relatively easy and consistant), that shifting higher WOULD net better times.
There is nothing mystical about turbos or superchargers that makes them different than NA cars..............they both have power curves, and they both have ideal shiftpoints............and that ideal shiftpoint is where the power curves for each gear intersect.
It's that simple.
The whole idea of shifting just after peak power, and ideally dropping you to just before peak torque is an old muscle car idea.............those cars often had a HUGE swell of midrange power with the power peak occuring just before a huge power drop-off, making the idea valid.
But that's just not the case with an SE-R........even a turboed one.
Again, I should point out though.............at higher boost (where most of you guys seem to run at the track, but hardly ever dyno at), you might just have more power drop at high revs..........and until I see a dyno chart, I can't say either way.
