msauve

I posted some comments about shifting for maximum performace (acceleration) the other day, and that posting drew some comments which were, well, somewhat interesting.

In doing a quick search here, it appears that there has been quite a bit of (mis)information posted here regarding how horsepower and torque affect performance.

The physics are well understood, and have been for about 300 years. In essence, power is what determines acceleration.

Engine torque has a small influence, but only in first gear. Beyond that, horsepower is everything. Notice I said ENGINE torque, because wheel torque DOES determine acceleration.

The difference between the two is what's between the wheels and the engine - the transmission, which is a torque multiplier. If you put a 2:1 gear set in a drivetrain, you'll halve the rpm and double the torque, but horsepower is unchanged (ignoring frictional losses). That's why horsepower is such a useful measurement, it doesn't change like torque/rpm do when going through gears.

Some people are confused into thinking that engine torque is important to acceleration because the acceleration of a car will follow the engine torque curve in each gear. From this, one might think that the maximum acceleration occurs at the torque peak. That's not true.

Although acceleration IN A PARTICULAR GEAR is at it's maximum at the engine torque peak, for maximum performance we're really interested in the maximum acceleration we can achieve AT THE CURRENT ROAD SPEED. The two are quite different.

This sounds counter-intuitive, but is something everyone has experienced - if you're cruising, you downshift for maximum acceleration. When you downshift, you may be reducing the engine torque, but the transmission multiplies that reduced torque by a factor which _more_ than makes up for the loss. The result is MORE wheel torque at lower engine torque and higher engine RPM. That is horsepower.

It can be easily shown that the maximum acceleration at the current speed is ALWAYS achieved by shifting so that the engine is putting out as much horsepower as possible. In gasoline automobile engines, that almost always means higher revs in lower gears. (Which is why engine torque does make a difference in 1st gear, since there's no lower gear to shift into!)

For vehicles with Continually Variable Transmissions (CVTs), maximum acceleration is achieved by having the CVT adjust to keep the engine at it's horsepower peak, NOT it's torque peak.

To maximize acceleration, one should shift past the HP peak so the HP in the new gear matches the horsepower left behind in the old gear. You could do engine/gear/axle/wheel torque calculations for each gear, then overlay the results into a "waterfall" chart to determine the best shift points, but the results will be exactly the same. It's easier to just consider the HP.

Although engine torque doesn't have any direct bearing on performance, it is still a useful measurement. In general, a vehicle having a higher torque will also have a wider powerband. It's not the peak horsepower that determines performance with a geared transmission, but the area under the horsepower curve (as used in each gear). For engines with the same peak horsepower, the one which has a broader (flatter) HP curve will accelerate better.

This is also why close-spaced transmissions are used for performance, they allow fine enough control of gear ratios so that one can keep the engine operating closer to its horsepower peak.

Oh, torque is no more "real" than horsepower. Either can be calculated from the other (if the rpm is known), they both coexist. Anyone who says horsepower isn't real I challenge to hold their hand on a 0.0067 HP (5 Watt) light bulb and then on a 0.268 HP (200 Watt) bulb and tell us the power is "theoretical." (oh, and they can be powered by the same torque (amps), but different rpm (volts)) :-)

I suspect more than a few people here are familiar with the G-Tech, which measures acceleration and can compute various things. It can ONLY measure acceleration, but from that and input vehicle weight it can calculate horsepower. It _can't_ calculate torque, it simply doesn't have enough information. Chew on that if you don't think acceleration is more closely related to HP than torque. (hint: there is a flaw in this particular argument, but it's somewhat hidden)

S4iscool

=O

Jason Chen

I'm still chewing......

quietsTTorm

Thanx. Very Informative.

jop

... especially your comments on why peak engine torque is only incidentally important (because it generally indicates a wider power curve).

BTW, i've been working on an excel spreadsheet that lets you plug in a sampled powercurve (and some other variables) and estimates acceleration, the best gear @ mph, etc... It's not done yet, but it does put out some interesting info (including a graph of wheel torque for each gear).

Anyone who's interested, shoot me an e-mail, and I'll send it (it's all cell-formulas(e?), no macros, so no chance of viruses). Comments, enhancements, requests, corrections, etc. are welcome.

-Jon

mtbracr1

I can't think that hard on a Monday morning

Jim TT

..(not sure of the spelling). I have posted SO many responses including all the math that I don't want to do it again; I know this is a half answer but I suggest anyone who wants more info do a search, or get a copy of the book "Auto Math" or talk to a professional drag racer. JIM

msauve

Then you're disagreeing with Newton, LOL! It's fact, not opinion, and hence not subject to "debate."

Jim TT

You don't understand what you are talking about, that is very clear. JIM

jdh

In addition to all the fun chemical equations, we learned quite well all the stuff you just wrote. Thanks for the reminder!

Jason
2000 TTQ **180**