Shift Points for Maximum Acceleration

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Originally Posted by 986Jim

This is simply NOT TRUE!!!! Spend some time at the rack track. Your Porsche can make 250whp between 4500rpm and 6500rpm range steady but a car that makes 200whp starting at 4500rpm and makes 300whp at 6500rpm will be faster.

jim, you're missing a lot of points. when we say average horsepower at the WHEELS, we mean after GEARING is factored in. when you run a car on a chassis dyno, gear ratios are entered in order to calculate the RWHP that the ENGINE is generating. this takes the transmission out of the equation. in order to calculate a car's expected acceleration, we CANNOT ignore the transmission and the mechanical advantage it provides.

an example:

current torque at flywheel: 200 ft-lb
gear ratio: 2.00
rear end: 3.00
wheel diameter: 13.00 inches

the car is making 1108 ft-lb of torque at the wheels. now, let's say the car weights 3000 lb and is travelling at 60mph (88 feet / second). the car is performing 3000 * 88 = 264,000 ft-lb / sec of work. this is equal to 480 HP. a chassis dyno would have factored in the gearing and given us an answer of 200 ft-lb less drivetrain losses (~170ft-lb).

the car that is putting out the most average power at the wheels (including mech. advantage of tranny) over a set distance will get there the fastest, period. if you win a race, you HAVE DONE THIS. you had a set amount of work to do; you did it the fastest. you have maximized average power. in most cases, this does occur if one shifts at or near redline, but this is not ALWAYS the case.

imagine a car that's horribly tuned whose torque SHARPLY falls off at 4000 rpm. what if it redlines at 8000rpm? the car would drag ass from 4000 - 8000. using this RPM range would be a waste; the driver would want to sandwich the motor's sweet spot (4000 rpm) in the center of his shifts.

that having been said, engineers / tuners want to take advantage of the power that RPMs provide, particularly with sports cars. they will try to maximize this advantage by tuning the car to produce more torque in the high rev range, translating into more power. because most sports cars are well tuned by the manufacturers, it makes sense that your experience tells you to shift at / near redline. i'm just saying that this isn't always true.