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# Optimal Shift point for 1/4 mile

### Tech Articles

Discussion and questions related to the course Race Driving Fundamentals

Based on the dyno graph below, can we determine best place to shift for fastest 1/4 mile time? 5 speed manual. Is it worth it to rev past peak power?

Hello good question, it depends on your gearbox ratios, how many gears you have and the weight of the car, the optimal shift point is normally based on what rpm you don't want it to drop under,

but based on your dyno sheet and no other information 7500-7600 looks to be the right spot

You can calculate the proper shift point, given the gear ratios, and engine torque curve. Assuming you have enough traction, and if you want to optimize for the lowest time (fastest acceleration) to a given speed, then you can select gear ratios to achieve that.

If you can provide your existing gear ratios, final drive and tire diameter, I can run this through some software to give you the shift points.

I will say that generally a torque curve like that shown (which is limited by RPM), will probably end up shifting at your top RPM (7800) unless you have particularly tall gear ratios. I don't know what your limitation on RPM is (piston speed, valve control?), but after running the numbers you may want to focus on raising that limit.

Thank you both for the replies. Here is the gearing and final drive information:

1995 Mitsubishi Evo III

1st 2.75 Single

2nd 1.684 Double

3rd 1.16 Double

4th 0.862 Double

5th 0.617 Single

Final Drive 5.433

Front Differential 4.200

Rear Differential 3.909

Transfer Box 1.074

4.199 Combined Rear Final Drive

Total Final Drive Ratio: 5.358

Tire Size:

215/45/17

I believe this was discussed in one of the courses? Maybe in a gearing one?

The optimum shift point, in theory, is where the dropping off of power past peak is the same level as the increasing power in the next gear - you could use torque, but it's more complex and doesn't add anything.

In practice, this is usually at the rpm limit of the engine, especially in the lower gears where there is a large rpm drop. In the higher gears, or where there are close ratio gears used, the rpm drop may be small enough for the shift point to be before the rpm limit.With some engines the power drops off rapidly before the mechanical red line, so an earlier shift may help there, too.

There is a school of thought that slightly past the cross-over helps with a slight 'flywheel affect' kick and/or to compensate for the shift time, but I can't comment as mileage may vary.

I don't know about software options, but the 'old school' method I used to use involved taking a base-line value for the x axis for top gear (or 1:1 if O/D box) and redrawing the power curve for each gear by dividing the x axis by the ratio. This will give a series of overlapping curves, with the crosspoints being the 'best' shift points.