Optimum Shift Points

I meant to include a restatement to clarify things a bit further:

The point of maximum torque is the point of maximum acceleration…in that gear! But if the choice is between holding it in a gear while the engine torque drops slightly, or upshifting so that the wheel-delivered torque drops significantly, you’re better off holding it in gear as long as you can get away with it.

aerodave, I’ll tend to agree with you given the following;
The specs for horsepower and torque for most cars are given at the rear wheels, so the transmission multiplier you refer to is already in play.
A six speed transmission is going to have pretty tightly sequenced gear ratios.
A smaller displacement engine, like the OP’s, is going to have a pretty unforgiving decline past the peak shifting range (point of diminishing returns).
**Shifting at max engine torque is not the fastest way to accelerate. **
Sometimes it is. Like you said, we need to see what the HP curve is doing as well.

If you get a nice solid tire chirp then you’ve found the sweet spot.

I’m not so sure about the numbers being reported at the wheels. It’s possible that wheel horsepower is reported, but even then I’d be surprised, since manufacturers would be more inclined to go with the higher number (bhp) out of salesmanship. Power isn’t multipled by the gears, so the only difference comes from drivetrain efficiency losses.

As for torque, the number at the wheels would be meaningless without the gear. (i.e. 150 lb-ft @ 3200 RPM in 4th gear).

Torque at the wheels is vastly different depending on the gear. The transmission keeps power (and thus work) constant, because it has to—the law of conservation of energy and all that. But it can negotiate the split between force and distance that goes in to making that work. That’s the whole nature of mechanical advantage.

In first gear, force is multipled greatly, at the expense of a lot of distance, so the engine spins a ton for little distance traveled. In fifth gear, distance is more important, so you can go 90 mph without needing a 40,000 rpm redline. The tradeoff is that you get less force multiplication. The point is that a lower gear will almost always provide more torque to the wheels at the same speed. That’s why downshifting works well for passing.

Let’s use my slow-ass Jetta as an example. If I’m cruising along on the interstate at 75 mph in 5th, I’m at my torque peak (3200 rpm). But if I want to accelerate as fast as possible, I put it in 4th (maybe even 3rd). Why? I make more torque at the wheels in 4th, even though the engine will make less torque at higher rpm. Here are the details for 75 mph:

Gear     Gear Ratio   Fin. Drive   Engine Speed    Engine TQ    Wheel TQ
5th         0.80         3.667          3179         135          396   
4th         0.97         3.667          3854         128          455
3rd         1.29         3.667          5126         112          530

The figures for torque at non-peak RPMs were guesstimated from a bad chart I found. But they’re certainly within a few percent. And of course, i ignored drivetrain losses in my calculations.

As you can see, at the same speed, I can generate a lot more torque (and thus acceleration) by dropping to a lower gear, as long as I’ve got room to run. Even though the engine’s torque drops off noticeably, I can generate significantly more wheel torque by downshifting. The torque dropoff could be much more severe and still alow me to benefit by downshifting.

And you mentioned tire chirp, which nicely illustrates my point. On my car, weak as it is, I can break the tires loose in first gear anywhere on the tach. Even far from the torque peak. But in a higher gear, not a chance. not even at the point of maximum engine torque. That’s because the wheel torque just isn’t high enough. As I showed above, in 5th gear, I get up to 400 ft-lb to the wheels from just 135 at the engine. But in first gear (3.45 ratio), I can get over 1700 ft-lb at the wheels. No wonder they light up!

Won’t this kill your engine, to go >75 mph in 3rd or 4th gear? I’d assume that it would be waaay past redline. :confused:

Remember, they are talking about a relatively light car (Ford Mustang) with a powerful V-8 engine. Not really comparable to the situation with the Scion.

I can remember the good old days of the muscle cars. They typically had a “four on the floor” and were so overpowered that you could start from a dead stop in fourth gear if you wanted to. Of course, most of the fun was running them through the gears.

I had a 1998 and a 2001 mustang with the V6 and unless you were accelerating hard, 2000 was the best feeling rpm to shift. Those engines redline at 5250 RPM, so 2000 isn’t all that low.

I also had a 1997 Dodge Avenger that redlined in the 8000-9000 range and in that car, I shifed at 4000 RPM. When I dumped the Avenger for the '98 Mustang, I didn’t really shift differently, but the RPMs fell a lot lower when I shifted. I guess it has to do with the gear ratios.

Oh…another thing…the '98 Mustang would do about 45 in 1st gear at redline. So shifting out of 1st at 2000 RPM meant about 15-18 MPH which is pretty fast for 1st gear.

I have a '87 Chevy Sprint with the 3-Cylinder Suzuki engine that I use as a delivery car. It has no tach, but I shift it REAL early for the gas mileage. There is a shift light on the dash that works in 1st and 2nd gear and it still comes on before I shift. The car feels like it’s lugging if I shift right when the light comes on. But I still get 40 MPG out of it in the city.

In my car, in 3rd and at 75 mph, i’m only at about 5100 rpm, per my chart above.

If I delay the shift until redline, I can make it to just about 90 mph in 3rd, and 120 in 4th. That’s with the factory redline. The aftermarket chip in my car raises the rev limiter, and would allow me to get to almost 7000 rpm instead of 6200.

All cars are different, so this may not apply to you. Many cars may well explode if you were going 75 mph in 3rd. As they say, YMMV.

I appreciate all the input, but now I’m really confused. Can anyone translate all of this into plain English geared toward the automotively illiterate? In short, I recognize that (up to a point) the longer I wait to shift, the better acceleration I’ll get. Actually, I already knew that. But is there an optimum set of shift points that will get me the best combination of acceleration, gas mileage, and engine wear?

The article Uncommon references seems to be fairly accurate, except that they have the price listed as roughly $10K more than it actually is. Yes, they had a fair number of options, but it didn’t look as if it added up to $10K worth. Odd.

On the whole, so far I’m thrilled with the car. It seems strange to me that they’re marketing this to kids, when it seems to me the absolutely perfect car for a family on a fairly tight budget. Go figure. It’s about as practical as a car can get, with a nice balance between cargo/passenger capacity and economy. And while I’m a long way removed from a Gen Y-er (being a 49 year old woman), I love the bizarre appearance. So far, I’ve had people refer to it as the BreadBox, the Yellow Submarine, a Soviet-era IceCream Truck, and That Yellow Thing. One guy said it looked like an exclamation point in the work parking lot. A lot of that, of course, is due to the yellow color, which is VERY yellow. Boy, anyone who plows into me had better not try the “I didn’t see it coming” excuse! Which, after all, was the whole point.

Does the car have a shift light? If you shift when the shift light comes on you should get the best economy, in theory.
As far as all around best shift points, you’ll never get an answer. Too many variables. You’ll have to shift when you feel comfortable doing so and that will come with experience and getting to know the car more.

I’ve already posted my opinion once here, but I’d like to chime in that you knew the answer when you first posted.
3000 RPM works just fine. You could go lower or higher, but you already knew the closest thing we have to a consensus opinion.

I don’t know. I think it’d depend a lot on particular car and engine, which I know aerodave has already said. (Warning: Anecdote ahead.) When I was test-driving my car, I took it onto the interstate to see how it accelerated, whether it had enough power to make quick moves, and so on. Well, I was going at about 75 MPH in 5th, hit a clump of traffic where I needed to at least gear down, missed 4th (5th to 4th is a hard transistion in my car) and accidentally dropped it into 2nd. Of course, I popped it right back into neutral within a couple seconds and then found 4th from there, and it hasn’t seemed to hurt the engine. Scared the crap out of me, though.

All I can do is reiterate what Uncommon Sense’s said a couple posts up. There is no easy way to find the optimum, because it’s hard to define just what the optimum is. Once you try to maximize or minimize multiple variables, it gets pretty tricky. And I’m sure that in-depth performance analysis isn’t what you’re looking for. Instead, think of it as a continuous tradeoff.

If you shift at or near redline, you’ll be maximizing your performance, but greatly reducing your fuel economy. You’re also increasing your engine wear (however, see my note at the bottom)

The earlier you shift, the better your gas mileage. (To a point…if you shift so early you’re lugging the engine, you’ll get poor mileage, poor acceleration, and probably increased engine wear.) Consistently shifting at around 2500 rpm and keeping the throttle as light as practical will do wonders for your mileage. However, you’re not going to win any drag races or have an easy time merging on the highway if you shift that low.

Somewhere in-between will, of course, give you results that are in-between. You may find, for instance, that delaying your average shift point to 3800 rpm gives you a mix of acceleration and mileage that you’re happy with. Perhaps you find it too sluggish on the on-ramps if you shift lower, and perhaps you feel you spend too much time at the pump if you shift later.

You’ll just have to experiment and see what feels right. Spend a tank of fuel shifting pretty early to see how much you can get your milage up. Make a game of it. Meanwhile, note how you feel about it’s performance. Then, run a tank where you spend much more time running it up the tach (don’t worry, you won’t hurt it if you’re not bouncing off the rev limiter all the time). Calculate how much your mileage has suffered, and note the performance difference. Decide how much you’re willing to pay in gas for the added acceleration, and adjust your habits accordingly.

When there’s no right answer, the one that makes you happy is the best one!

PS: I spent little time discussing engine wear, because I think it’s a moot point. You may be able to extend its life by babying it, but I bet it won’t even make a 10% difference. Unless you’re unreasonably hard on the car, it’s likely the engine will last far longer than you want to drive it. Toyota motors do last forever…and ever…and ever.

Ford Focus SVT

If you have ever done any kind of optimisation you know that the question doesn’t make much sense. Any set of shifting rules that fall between the extremes you have for those three criteria is an optimum, depending on how you weight them. Which is more important to you in a given circumstance? You will probably be sitting near the ideal condition for efficiency for most driving and then going all the way to the extreme for acceleration to get out of a bad spot or because you just feel sporty that day. The problem as you have phrased it is what we would call ill-defined.

Oh, and here is a good site that may make the math a little easier for you.

Thank you all for your help!