Second Shift

Alright, I wouldn’t go so far as to call this a retraction, but let’s call it a little clarification of the last article I wrote on the optimum shift RPMs for the Skip Barber car. Based on the calculations of horsepower and RPMs, I put together a prescribed set of shift RPMs for each gear change. But like all theories, it doesn’t hold much value until it is put to the test. The proof is in the puddin’.

So I set out to confirm if my shifting advice would put a driver at an advantage as they scream off the grid towards turn one. To do this I ran a number of starts at the beginning of an 800m long straight stretch and tried to hit a variety of shift RPMs and evaluate what shift technique was quickest to a given distance.

Well, it turned out that the run that was closest to matching my shift recipe was a few milliseconds slower than another run which was slightly off. So I decided to plot all the runs I did and compared them to the fastest run of the experiment. Then I looked at each shift from each run and evaluated whether it gained on the fastest run, or lost to it. I quantified the difference and came up with a much more reliable technique to see which shift RPMs offered the optimum acceleration.

The result is shown in the following graph:

It was odd how identical full-throttle starts resulted in different launches. It appeared that when the engine was increasing in RPM after the brief drop due to the rev limiter, and was close to 6300 RPM, the result was the optimum launch off the line. This was confirmed when the RPMs were held close to the rev limiter without triggering the rev-limiter. Thus, you could take your chances with a full throttle start, but you may get a slightly more reliable result if you hold it close to the rev limit during the launch into first gear.

The shift from first to second did not seem as critical, since it was changing in RPM so rapidly that the difference in shift points was not pronounced, but the trend did indicate that 6200 RPM was best. Both second to third and third to fourth shifts were optimized when just over 6000 RPM, but the most critical shift point appeared to be the final shift into top gear. Shifting too early is a big disadvantage here and you would need to get close to the 6300 RPM before you lose the advantage.

So the new shift advice for the Skip reads as follows:
Neutral to 1st: 6300 RPM (without triggering the 6400 RPM rev limiter)
1st – 2nd: 6200 RPM … not as critical as the others
2nd – 3rd: 6000 RPM … plus or minus 100 rpm is ok
3rd – 4th : 6000 RPM … plus or minus 100 rpm is ok
4th – 5th: 6200+ RPM … stay well above 6000; shifting lower than 6100 will slow you down a lot compared to people shifting at higher RPMs.

So my old shift points were not that far off the mark, and followed the same trends as was found in the actual testing. Only the second to third upshift was off by about 150 RPM.  Comparing this new routine to someone who starts on the rev limiter and shifts always at 6100 RPM would result in an advantage of about 1.16m after 800m.

But you know how the old saying goes: Even a broken clock is right twice a day.

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