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Old 01-08-2009, 02:46 AM   #1
Scooby Guru
Member#: 71092
Join Date: Sep 2004
Chapter/Region: MWSOC
Location: Licking County, Ohio
2005 2.5RS Wagon
Regal Blue Pearl

Default Optimal Shift Points and Choosing Mods Smartly

This is not one of my usual in-depth, critical looks at a new part or tuning option. This is simply me working through the data I have collected and thinking about that data in terms of how to get the most out of what I have now and how to make beneficial changes in the future.

My initial intent was simply to calculate the optimal shift points for the data I had. It was practice, really, because the dyno data I have is for a hardware setup I don't even have on the car anymore. But, I haven't dynoed the new setup yet.

Ok, so the first thing that I had to do is to figure out, as a percent of the maximum torque, how much torque AT THE WHEELS was being made as I rowed up through each gear. So I made a pretty graph (click to make bigger):

While it's interesting to look at how much torque you lose with each shift, that graph doesn't tell you anything useful about when to shift, so it has to be drawn with respect to wheels speed:

Now we're getting somewhere. Optimal shift points are where you see one line cross the next. What the graph is showing is that, at that point, you can upshift to the next gear and have the same amount of torque at the wheels, which is what you want. the lack of intersection between the 1st and 2nd gear plots means that the gear ratios are so far apart that redline comes before the ideal shift point. For my car, at the time it was dynoed (and remember, your car and my car now could be completely different), the optimal shift points work out as follows:

1st: Redline
2nd: 6200RPM
3rd: 6100RPM
4th: 6100RPM

Mission accomplished! Since I had slogged through all the numbers to make that happen, I decided to see what other interesting bits of data I could mine out of it. One of the things that I don't like is having to wring the engine all the way out to the fuel cut to get anywhere. Not to mention, in the kind of driving I do (hill country, TSD, RX, etc), you're rarely able to just pin the throttle down and hold it there for long enough for the tach to sweep from 3k to 6k. What's the implication of running in the 3k to 5k rev range (which is what's MUCH more likely to be used in real life). Presuming I upshift at 5k, I'm taking a 30-50% hit compared to shifting at optimal, depending on gear. Ouch.

Now we get to the fun part. What can I do about this? Well, first of all, anything I can do that just linearly adds more power (say more displacement, N2O, etc), isn't going to change the situation at all. Anything that adds torque above peak is just going to make the situation worse. Anything that subtracts torque from the top end will move my optimal shift points lower, but it'll do so artificially by making the top end suck more and making the car slower overall. The productive options are to move the peak toque lower without reducing top end torque or to boost torque below peak torque without sacrificing the top end (or at least not much).

I started exploring in that direction specifically because I have an interest in the GrimmSpeed Intake Manifold Spacers. The IMSes lengthen the IM runners by 8mm. In theory, this should boost the torque below peak torque, not move peak torque at all, and reduce the torque above peak torque. Now, the increase below peak torque sounds like a good thing but the decrease above peak torque worries me. If the low end increases enough that my optimal shift point moves below the onset of the high end reduction, I'm golden. However, if I have to run up into the reduced area a bit to hit my optimal shift, things get A LOT more complicated. At that point, you need to find the difference in the area under the curve. I'm sure that phrase brings back nightmares about Calc 1 for a lot of you. Sure does for me. Basically, if the change can boost the low and mid enough for a wide enough rev range, you'll still end up being faster even though your high end has fallen off (assuming you didn't end up with an extra shift in there). The advantage to this is 1) explosive torque whenever you touch the throttle even at normal driving around RPM and 2) you don't have to flog the engine to get somewhere, saving a bunch of wear and tear on the engine.

Now, BRR just posted a thread about his experiences with the IMS. Sadly, the way he collected the data makes it impossible to draw any hard conclusions from it, but it certainly at least provides a platform for speculation. The data he posts shows a clear and increasing gain from about 3.7kRPM on down and a clear and increasing loss from about 4.8kRPM on up. So the low end gain looks strong but the high end hit looks like a real mofo. That would definitely be in "area under the curve" land for wither or not it was a net loss or gain. It would also move the shift point down a lot. In fact, it might be enough to cause enough extra shifting to cause a net loss even if the area under the curve were bigger. However, there's an interesting thing being overlooked here. My peak torque is MUCH higher than his; 4250RPM for me vs 3600-3700RPM for him. If the pattern for my change is the same, and the theory says it should be, I should see a boost to a wider range of lower-than-peak RPM as well as pushing the worst of the loss higher. It might be enough to make it a clear win for me. Sadly, I'd actually have to buy the parts, install them, and dyno them to be sure.

Again, this wasn't a rigorous analysis of the hardware to be sure. However, I thought it might be interesting to you guys to see the work and thought I put in before I decide on a mod.
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williaty is offline   Reply With Quote