mmarshall

Torque usually wins drag-races, while HP wins high-speed track-type races. That is because torque, the engine's twisting-power through the crankshaft/flywheel into the transmission, destermines how fast you will accelerate from a standing stop and at low-speeds. HP, on the other hand, determines how much ultimate power (and work) the engine can actually produce, and is more of a factor in overcoming air-resistance at high-speeds. This is also reflected in the fact that, all else equal, an engine's torque-peak will be generated a couple of thousand-RPM below the HP peak.

dj.ctwatt

^ With no disrespect, I have to disagree.

If you translate rotational forces to linear, torque=force, HP=power

1)Force = momentary measurement
Power= Change in (Force x Distance) / Change in Time

And when applied to rotation:
Torque = Rotational measurement of force = R x F = momentary measurement
HP = Rotational Power = Torque / time

This is why 1 HP = 550 ft-lbs / second

2) Most people are only familiar with the conversion of torque and RPM to HP, but that's RPM stands for minutes. And without a measurement of time, there is no power. Reduce the equation, and you get 550ft-lbs/second.

3) So basically HP is a function of torque (depending on redline). In other words, although they are related, they are two completely different forms of measurement. Secondly, through the use of gearing, torque can be converted to HP, and vice versa. It's the same principle that allows you to life a heavy car with a jack, or use torque and an underdriving gearset to create HP.

dj.ctwatt

To add to that, the limitations for HP->Torque and Torque-> HP conversions using gearsets is dictated by maximum RPM, and thus why there is an ideal ratio. And because RPM's are have a finite limit, it means you have to eventually change gears (which can slow you down during the time it takes to change gears). If CVT's were perfect, then this wouldn't be a factor.

So there is an ideal ratio HP vs Torque vs Max RPM vs Gearing. Where that ratio is depends on the particular application, max speeds, wind resistance, and I wouldn't even have a clue as to where to begin on that anyway.

Lastly, the reason torque peaks before HP has more to do with the physical/mechanical limitations of peak RPM than the maximum available torque. For example, if you were able to increase maximum RPM (by altering the physical/mechanical properties and NOT by only adjusting the computer's safety cutoff), you would also shift the peak HP curve.

bitkahuna

iTrader: (20)

great explanation. i think.

mmarshall

Fine, but that doesn't change what I said earlier. In general, torque determines acceleration; HP determines top high-end capabilities. A vehicle cannot accelerate simply based on the amount of power generated in the cylinders....that power has to be converted to rotary-motion in the crankshaft and flywheel.

Let's not get too hung-up, though, on the classroom-formulas, or too-far off-topic. The original thread-topic is the comparison of these three family-sedans, one-to-another. I only brought up the subject of torque-vs,-HP because of the comments on the power-vs.-acceleration for the tested-vehicles' stats.

dj.ctwatt

Sorry, didn't mean to threadjack. The generalizations are correct. I was only disagreeing with the parts in bold

At top speeds, wind resistance is a momentary measurement, and max speed is determined by momentary measurement torque, not HP. I've threadjacked enough. . . I just have a passion for the physics aspect (sorry again!) and I always see the same torque vs HP claims over and over. /threadjack

The point I should have made (and didn't do well) is that it's the gearing that matters. Hence why the more powerful and lighter Hyundai is outperformed by the Camry. Toyota may not have the most powerful engines, but they often make up for it with proper gearing (barring some of the base economy models like the Yaris that STILL have 4 speed Autos).

lexusls430

Turbo lag? Yes it exists but it shouldnt be that slow. Sorry. Most likely has something to do with the transmission or a really crappy power band.

bad co

turbo lag is a big issue on a small displacement pig power motor, its only about 20hp shy of a evo. have you seen the evo's 0-60 numbers w/ brake boosting and just plain old flooring it?

mmarshall

OK. I understand.



Again, OK. But I myself am not an engineer...I'm just going by what auto engineers say. They are the ones that actually design engines.

Definitely...gearing plays a huge role. But then, again, you have compromises. All else equal (which, of course, it rarely is), tall gears mean lower RPM, less engine-noise, better economy, and, unless the engine is lugging, lower wear on its components. Shorter gears increase RPM and give better acceleration and hill-climbing ability, but add to wear and engine-noise. That's why automakers are geneally going to transmissions with more gears (or CVTs)...the more gears you have, the more flexibility. But, IMO, if you are going to use more than about 5-6 gears, you might as well go to a CVT because the transmission then becomes too complex and it shifts up and down too much, increasing wear. CVTs, because of durability issues, used to be compatible only with low-torque engines, but are gradually being adapted to larger powerplants now.

mmarshall

The easiest way to get around turbo lag is with dual and/or sequenced-turbos, where one unit spins at low-RPM and is ready to give an immediate boost while the other one is spooling up, and then the other one takes over and continues the force-feed.

The old flooring/brake-boost technique, BTW, is effective, but it puts enormous stresses on the transmission and torque-converter (if equipped), drastically reducing its life. Of course, the other method with a three-pedal manual-transmission (high RPM, clutch pedal all the way to the floor, and then clutch-out quickly) places big stresses on the clutch-disc, pressure-plate, U-joints, and differential.

lexusls430

I can see where there would be lag 0-60 but that doesnt explain the really low trap speed of 91. Heck a solara with the 3.3 3mzfe traps at that.