You can't really go by "rules of thumb" like 3.73's for automatics and 4.10's for M6's..... you need to spend the time to estimate the ultimate performance of the vehicle, particularly trap speed. Then you need to put a tire under it that will allow it to reach its potential.
You need to wrestle with gear ratios, engine redline, tire diameters, tire growth, and convertor slip. I did a fairly decent Excel spreadsheet that reflected all of these, specific to my tires, my convertor, etc. On a chassis dyno pull, we plotted MPH vs. engine RPM and I compared theoretical RPM from the MPH to actual RPM, to get the convertor slip at each engine load. Then I estimated tire growth from the fact that until I moved the rear axle back a little, the tires would hit the front edge of the wheel opening at approx 120MPH trap speeds, closing up the 1/2" gap that normally existed. All this was cranked into the spreadsheet.
Everything tells me that with the 4.10's, I can just barely make it to the finish line, in the 1:1 3rd gear of the TH400, with the engine at the 7,000rpm shutoff point for the nitrous. But there are still a lot of "guesses" in all this, and if I am off by a little, the penalty will be the need to shift the Gear Vendor into O/D at the end of the track. But I've already done enough experimenting to know that it isn't going to hurt me as much to shift into O/D as it would have cost me to cross the line with 3.73's and be 10% below the "redline".
You need to wrestle with gear ratios, engine redline, tire diameters, tire growth, and convertor slip. I did a fairly decent Excel spreadsheet that reflected all of these, specific to my tires, my convertor, etc. On a chassis dyno pull, we plotted MPH vs. engine RPM and I compared theoretical RPM from the MPH to actual RPM, to get the convertor slip at each engine load. Then I estimated tire growth from the fact that until I moved the rear axle back a little, the tires would hit the front edge of the wheel opening at approx 120MPH trap speeds, closing up the 1/2" gap that normally existed. All this was cranked into the spreadsheet.
Everything tells me that with the 4.10's, I can just barely make it to the finish line, in the 1:1 3rd gear of the TH400, with the engine at the 7,000rpm shutoff point for the nitrous. But there are still a lot of "guesses" in all this, and if I am off by a little, the penalty will be the need to shift the Gear Vendor into O/D at the end of the track. But I've already done enough experimenting to know that it isn't going to hurt me as much to shift into O/D as it would have cost me to cross the line with 3.73's and be 10% below the "redline".
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