Optimal Shift Points

wolf370z

Lately I've been doing as much research as possible trying to figure out the theoretical side of the relationship between torque, horsepower, and rpms. As most of you already know, horsepower isn't a real force. It's a number that is calculated with a formula:

horsepower = (T*rpm)/5252

Essentially when your car is at 5,252 rpms your horsepower and torque numbers are identical. We all know torque is an actual force generated by the engine. Now torque is what really gets you to the end of a 1/4 mile in the fastest way possible. More torque = quicker 1/4 mile. Now the other question is what is the point of horsepower.

Since it has no actual real world use then why does everyone talk about how their car has for instance 370whp but in actuality they only make like 274 ft lbs of torque. It's misleading to rate things in horsepower so what's the point? Bragging rights?

So at last that brings us to the really important question. If you're trying to go down the 1320' as soon as possible do you shift at the point when your torque starts to drop off so you can shift back into the beginning of your "peak torque range" or do you shift at your peak horsepower?

Thanks.

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Jeffblue

for maximum speed, i think you are supposed to shift at peak horse power, which i believe, on our cars stock is at 7,000 RPM

WarmAndSCSI

To put it simply, the more power under the curve (i.e. the flattest torque curve possible), the faster the car. Power at a given moment is what matters - not torque - because it's what determines acceleration. The power band is what's important overall.

For the 370Z with our peaky V6, your optimal shift point is almost always at the redline (7500 rpm) or slightly later. This puts you back down at the beginning of the power band "hump" without losing too much power from going above the peak power point (7000 rpm isn't it?). It's all about maximizing the area under the curve. ******* calculus.

Torque is for bragging rights. Hence, diesel douchebags and their torque ****-waving fests. 1000 lb-ft hellz yeaaaa!!!!1one

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wolf370z

Let's take a look at this stock dyno chart for a 370z.



So you're saying that where you have the most power is where torque starts falling off and horsepower keeps increasing? I realize that torque isn't the only part of the force that's pushing you down the track. That's my real question... what else contributes to the pulling force (i suppose pushing force if you were in a rear engine vehicle).

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WarmAndSCSI

Let's just ignore wind resistance as a factor for the purposes of this discussion.

Acceleration is directly proportionate to power and weight, so it's definitely the only figure you should be concerning yourself with unless you're qualitatively trying to describe a power curve - then torque is useful. Torque is only a factor when you're talking about engine dynamics, engine internals, and drivetrain strength, etc. It is the instantaneous amount of force your engine is able to slam into the crankshaft - with the crankshaft throw acting as a lever - and subsequently to the wheels. Power is what the road sees.

For the purposes of the dyno graph, you're going to maximize the area under the power curve if you shift right at 7500 rpm (or slightly later) and end up dropping down to 5800-5900 rpm where there is more power available than at 5500 rpm, for example. With our power curve, you sacrifice a little bit of power output at the end of your gear for a lot more power and the beginning of the next gear.

Hopefully that helps.

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wolf370z

So technically you shift according to horsepower since it is the theoretical display of power you're putting to the ground? So that explains why from a 60 roll a 350fwhp honda that makes like 255 ft lbs of torque would embarass a similar weight car with 300fwhp and 300 ft lbs of torque?

However in the 1/4 mile the 300fwhp and 300 ft lbs torque would be faster because it would have more torque to push it off the line? So either way they'd both still be shifting at or near redline (whichever keeps them in their power band which is based off peak horsepower?)

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WarmAndSCSI

Exactly.

But a 300 lb-ft car may make that torque right at 5252 rpm, so who knows. Generally speaking, low-end torque (i.e. torque between idle and the start of your power band) will get you a better 60' time up to the limit of your tires and suspension, but nothing more unless you're pulling stumps lol

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spearfish25

These discussions will do nothing but give you a headache. It usually comes down to the semantics of discussing an abstract subject. Just buy the most powerful car you can afford and compare 1/4 mi times before you purchase. There are too many 'real world' factors such as weight, gearing, etc that make real comparisons of hp and tq impossible.

Anyone want to get a beater and put two different engines in it as a little test?

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Push370zzz

Horsepower is a measure of work...if your car had no horsepower, you would be going nowhere. Think of it as ability to apply torque over time...obviously more horsepower is beneficial. You need a certain amount of power, work, effort, energy to combat the friction on the road, drivetrain, etc, but namely air resistance at higher speeds.

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wolf370z

But I like headaches....

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wilsonp

While I like automatics, I have read a little

The theoretical optimum shift point is the point at which the transmission torque (engine torque at rpm x gear ratio) in your next gear will be higher than the transmission torque in your current gear.

Just like the NSX, the wonderfully flat torque curve of the 370Z means that the best shift point is as close to redline as possible, which also means you can maximize your time in first gear, which is how you can beat peakier torque cars - more time in lower gears = more acceleration.

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destinyZ

nice info.

wolf370z

Great stuff, thanks everyone.

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slammedsammy

This!

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