msauve

In an earlier post, I corrected some mistaken beliefs with regard to torque/horsepower and acceleration. In short, the maximum acceleration at any given speed is ALWAYS achieved when the engine is running at its horsepower peak. HP determines the maximum acceleration, and torque simply doesn't matter.

Now let's talk about whhen torque does matter.

Even when you are interested in max acceleration, torque figures can provide a good idea of how wide/flat the (horse)powerband is. With geared transmissions, that is significant, because such transmissions can never run at all times at the optimal RPM for acceleration (= HP peak). In general, a wider powerband allows one to have wider gear ratios and achieve the same real-world acceleration.

Notice how both peak HP and peak torque figures are (almost) always given along with the RPM? That's significant, too. What most people don't realize (or think about) is that when you consider torque at a particular RPM, you are actually considering horsepower. That's because horsepower is the combination of torque and RPM. If pure torque were as important as some like to claim, there would be no need to quote an RPM to go along with it!

If the subject of interest moves away from maximum acceleration and toward drivability (i.e. who wants to drive a Honda S2000 in rush hour traffic? :-) ), then torque becomes VERY interesting. If we have lots of torque down low, we can have a very flat powerband.

If you've looked at the TT power/torque curves, one thing that is very interesting is that the torque curve is very flat. The power curve increases very linearly until around redline. Audi programmed the ECU that way.

Before I had my TT, I drove a VW with a TDI (diesel) engine. The power/torque curve for that engine is exactly opposite - the horsepower curve is very flat. The torque curve is high at low RPM, and linearly decreases, until it collapses at redline.

The difference in character is very apparent. If you are cruising along and you want to accelerate in a TT _right now_, you have to downshift to place yourself closer to the horsepower peak, where the maximum acceleration at a given speed occurs. Remember, the torque curve is flat, so if torque were determining acceleration, you would merely leave it in the current gear and push the throttle.

If you're cruising in a TDI, and you want to go faster _right now_, you simply push the accelerator (no throttle on a diesel, you know). Since the HP curve is so flat, downshifting doesn't get you significantly more HP than is available by simply pushing "go."

The TDi is much more "driveable." But the TT has much more horsepower, so it can accelerate much faster. Two entirely different forms of "fun." In fact, if I could get, say, a 175 HP TDI engine in the TT, I'd prefer that to the current 225. It would probably be quite close in _overall_ acceleration, too, although the 225 would obviously have greater _peak_ acceleration. Chipped US TDIs, which make about 115 HP, can run pretty closely with 150 HP 1.8T's. (because with the TDI's flat HP curve, it's always running near its HP peak, whereas the 1.8T goes "on and off" peak as you go through the gears.)

Note that in the above, when we're talking about how useful torque is, we're talking specifically about torque _down low_ (widely separated from the HP peak). In actuality, we're still really interested in horsepower, but we're using "torque" to mean that we have a wide, flat powerband. It is still the case that engine torque, by itself, is meaningless to performance. It's only torque _at a particular rpm_ (which is really horsepower) that matters to performance.

HACK THS

You just like picking fights don't you?

Good info by the way, thanks ;-)

msauve

"You just like picking fights don't you?"

No, but I certainly won't walk away when someone else swings first. You can review the record, Jim TT started out by saying I was wrong, without any explanation or proof.

INTT

in the earlier post and I agree on some of the things that you have stated. Where I disagree is when you state torque is not important for maximum acceleration. We always used the torque curve and RPM data to select the best shift points for maximum acceleration. In relation to this curve if you shifted to soon or to late your ET would suffer. We never shifted at the maximum HP point because this is usually above maximum torque. You do RPM above the maximum torque point because on a shift you have what we call fallback RPM. Another words when you shift into the next gear the RPM will drop off. We always selected shift points that would let you work the upper range of the torque curve. The closer you could stay to this peak in the torque curve the lower your ET would be. That's about the best job I can do to explain my thoughts on this.

msauve

"We always used the torque curve and RPM data to select the best shift points for maximum acceleration"

You can do it that way, but it's more work than necessary. You have to work starting from the engine torque curve (which is actually restated HP, because RPM is a factor), and then calculate torques as they are multiplied by the transmission and final drive ratios.

It's much easier to simply watch the horsepower, and the results will be identical - for maximum acceleration, you shift where the HP you are "leaving behind" equals the HP you are "moving to." (modified for the time it takes to shift, as you say).

"We never shifted at the maximum HP point because this is usually above maximum torque."

Correct, because you want to MAXIMIZE the area under the HP curve. You shift PAST the HP peak (which is WAY past the torque peak). I'm assuming we're talking about _engine_ torque here, rear wheel torque is different, and IS directly related to acceleration. There's a transmission (AKA torque multiplier with multiple ratios) in between. Note that even if you do all the calculations based on engine torque through rear wheel torque, you never end up with the result that max accel is achieved by shifting around the torque peak! (unless the torque peak is near the HP peak, in which case it might _appear_ that way.)

I created a spreadsheet which illustrates this. It is discussed and linked to in the linked post.<ul><li><a href="https://forums.audiworld.com/tt/msgs/544468.phtml">link to spreadsheet posting</a></li></ul>

HACK THS

I know, just giving a hard time :-)

smallTTs

We seem to go around with this every so often, don't we?

Torque is a quantity which can be measured, as is rpm. Horsepower, on the other hand is calculated from torque and rpm; in other words power is just torque at an rpm, as you sorta said. Unfortunately you said it backwards; when we talk about "horsepower" it is torque at a given rpm.

Now for the biggie: torque at the drive wheels is what accelerates a vehicle. The more area under the torque curve at the drive wheels in each gear, the better that vehicle will accelerate. In fact, the peak longitudinal g (acceleration) occurs at the torque peak in each gear....not at hp peak.

Gears multiply the engine's torque; that's why downshifting gives more acceleration...you are multiplying the torque to the drive wheels. Just don't down shift so that the engine rpm is at/near the hp peak in the lower gear and expect the car to accelerate much. Try it. It won't. Use your "g-tech" or your g analyst (from Valentine Research) to measure it. I have.

I think you believe you are correct, and to a point you are. Unfortunately you are not exactly correct; oh hell, you are flat wrong about some things like your first statement. Personally I don't care what you want to believe, but I feel badly that you mislead folks.

In the whole scheme of life, this isn't crucial; I just don't like seeing half-truths or worse spread around as gospel.

BTW, JimTT rarely posts anything technical which disagrees with how things work in the realworld. He's correct about this subject also.

My $.02.

INTT

Agree!

OTOF

Good weigh in, Jon. ;-)

quieTT

Msauve's first statement is exactly what is stated in a tech article that JimTT posted on 4-22-02.

Maximum acceleration AT ANY GIVEN SPEED is achieved at the maximum HP the engine is capable of generating AT THAT MOMENT IN TIME.

Basically this is the same thing as saying that maximum acceleration AT ANY GIVEN SPEED is achieved at maximum torque at the wheel (not engine torque).

That is why you downshift to accelerate -- to increase torque at the wheels (by taking advantage of the torque multiplier effect of a lower gear). But if you think one step further, what this actually does is increase horsepower becuase you increase engine RPM while keeping engine torque within the same range (you may lose a little engine torque). The overall effect is to increase horsepower.

Msauve's statements are somewhat confusing because thinking in terms of horsepower is not intuitive while thinking in terms of engine torque and wheel torque is much more intuitive. But he has not said anything incorrect -- just indirect.

plastt