Quote:
Originally posted by JDMEddie
Racing is about taking it to the red-line, so you run out of torque, if you push it higher then shift your rev is higher in the next gear...understand yet???
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I don't feel like doing all the math that's involved right now but I may do it tomorrow or something if I have time. Here's a little piece taken out of a horsepower definition on finding the optimum shift point.
"So, how do you work out the shift points in order to achieve maximum acceleration? Well, to do this, you need a performance graph for the engine (showing torque against rpm, which you get from a rolling road session) and you need to know the gear ratios of the car.
At this point it's important to note that the table below does not take into account the final drive reduction, nor the transmission losses that would be experienced. This is because the table is purely showing the performance difference between gears, and the above factors would apply similarly to each gear.
This is based upon a standard Mk2 Vauxhall Astra GTE 16V, and the data is from the official brochure. I'd like to point out that due to the quality of the graph, these values are approximate, and may differ slightly from actual figures.
Torque 1st 2nd 3rd 4th 5th
RPM (ft-lb) 3.55 2.16 1.48 1.13 .89
1000 97...... 345 210 144 110 87
1400 108..... 382 233 159 122 96
1800 115..... 408 248 170 130 102
2200 120..... 427 260 178 136 107
2600 125..... 442 269 184 141 111
3000 130..... 461 280 192 147 116
3400 134..... 476 290 199 152 119
3800 141..... 500 304 209 159 125
4200 147..... 521 317 217 166 131
4600 148..... 526 320 219 167 132
5000 149..... 529 322 221 169 132
5400 148..... 524 319 218 167 131
5800 141..... 500 304 209 159 125
6200 131..... 463 282 193 148 116
6600 117..... 416 253 174 133 104
The first column in the table is the engine speed in rpm, followed by the torque output at the respective engine speed. The following columns have been calculated by multiplying the given gear ratio by the torque output. This calculates the actual amount of torque that is being sent to the final drive.
So, what does this table tell us? To make any use of it, you need to know the formula for calculating the change in engine speed going from one gear to another.
The formula is;
NEW RPM = CURRENT RPM X (NEW RATIO / CURRENT RATIO)
So, for the above car, if you were doing 3000rpm in first and you changed up to second, your new engine speed should be 1825rpm.
This formula is very important as it allows us to see the difference in torque based on gear changes. Now, lets say we were to take the advice that you should change at the torque peak. For this car it's at 4800rpm. So, we're in first gear at the torque peak, and the gearbox is sending 530Lbs-ft of torque to the final drive. We change into second, we're now at 2920rpm, and it's sending 279Lbs-ft to the final drive. Immediately we can see this was bad advice. If you look at the table, even if we left it in first for an extra 1000rpm, at 5800rpm 500Lbs-ft would be sent to the final drive. Yes we are past the torque peak, and yes the torque is dropping, but it's still sending more torque to the final drive than at any point in second gear.
This car happens to have good torque throughout the rev range, and it is apparent from the table that, with this car, in order to get the greatest acceleration you need to change as late as possible, about 6600rpm being the optimum.
Not all cars have such a good torque spread, and shifting may take place before the redline. However, the rule is that if you could be sending more torque to the final drive in a higher gear than the one you're currently in (by using the rpm formula), you've missed your shift point. As long as you shift at the optimum rpm, then your car will be accelerating at its maximum rate.
the water is now clear"