Cars: Acceleration: Torque or power?

So, the specification sheet for my car says that it generates maximum power at 6000 rpm, and maximum torque at 4000rpm.

If I want fast acceleration away from the lights, do I want to be generating maximum torque, or maximum power? In short, should I change gear at 4000rpm, or should I wait until 6000rpm?

It depends on more than just where the peaks are, the shape of the power and torque curves are important. If the peak torque was at 4,000rpm, but then there was a plateau at just under that for another couple of thousand revs then you wouldn't want to change up that early.

I struggle with the relationship between power and torque, but as I understand it torque is more important for acceleration.

In practical terms (not worrying my pretty head with the theory) it is usually where you end up in the rev range after changing up that is more important. If you change up at 4,000rpm then you are going to find yourself fairly low in the rev range when you get to the next gear. The result of this on many engines is that you will find yourself in a range with low power and torque, thus slowing you progress.

On my MX5/Miata it is fastest if I hang on to nearer 7,000rpm (rev limiter at 7,200) even though the power and torque starts to fall off, because that puts me in a better place in the power and torque curves for the next gear.

Almost all my experience is from smallish four cylinder petrol engines. I suspect things are different with big V8 and diesel engines, where you can have huge amounts of torque low down in the rev range.

Some light reading for you:

http://www.allpar.com/eek/hp-vs-torque.html

For starters, torque is the number that directly correlates to acceleration (modulo gearing, which is explained well by furtive's link) . Shifting just when you reach your max torque means you're forgoing the region just above that when you're still getting good acceleration out of your car.

To an approximation, your car's velocity has a lot to do with the integral under the torque curve (as experienced by the wheels, not the engine). If you could plot the torque curves in each gear, where the X-axis is the car's velocity, the ideal shift point would be when the torque curves intersect. That would maximize the integral under the extended curve.

Of course, there are other tricks to getting a good 0-60 time that mostly involve destroying your transmission, clutch, and tires. From a lecture I saw recently on the mathematics of drag racing (an absolute blast, BTW), you get your maximum acceleration out of a tire when it's just starting to slip. To do that, you need lots of torque, which is hard to develop when your engine is still revving low. Solution? Rev high and slip the clutch.

In my clio, the magic happens at about 3500. So if hold on 'till about 7200 (that's when the little gearchange light goes on) before I change gear then I'm still in the powerband in the next gear. If that makes any sense.

In all the Focus' I've driven, it's redline all the way baby!

Finally a question I can answer! (I'm a mechanical engineer). The relationship between power and torque is described well in the linked article. Here is a cool thing to try, however:

To find your optimal shift points for accelleration, you want to maximize the torque to the rear wheels (meaning you need to take the transmission gearing into account -- to know the actual torque, you also need to consider the final drive gearing and wheel size, but these are constants so they don't change the optimal shift points).

As was mentioned before, to know the exact best shift point, you need to compare rear wheel torque at the engine speed of the current gear with rear wheel torque at the next higher gear. Ideally, that requires a torque vs. RPM graph of the engine.

However, you have two points of the torque vs. RPM curve, because you know the maximum power and torque and the engine speeds where they occur. You also know that the engine develops zero torque at zero speed. So, you really have 3 points of that curve.

It's not perfect, but you can assume straight lines between those three points and come up with a surprisingly good approximation of the ideal shift speeds. To do this, you need to know the transmission gear ratios.

You can write a very simple computer program or use a spreadsheet to compute the transmission output torque as a function of RPM (assuming the linear engine torque curve and multiplying by the correct gear ratios).

When do you shift? Simple. When you get higher torque in the next higher gear. Remember, the next higher gear will be at a lower engine speed.

Using the final drive ratio and the wheel size, you can have your computer program use a vehicle speed increment. I used 1mph in a program I wrote years ago. Wheel circumference x wheel RPM gives you speed, so with the gear ratios and the torque vs. RPM approximation you can express your ideal shift points in terms of MPH. If you do this, you get best results by measuring your drive wheels from the center of the wheel hub to the ground and using that as the radius. The tire is flattened at the bottom, so you want to use the distance from the wheel hub to the ground.

Jim

I am complety unqualified to give an answer to this question but the conclusion drawn in Torque and Horsepower - A Primer is

Repeat after me. "It is better to make torque at high rpm than at low rpm, because you can take advantage of *gearing*."

I don't know the answer to your question, but I think this truck has power and torque...

slip the clutch.

I have an issue with the term ``slip'' as I think you're using it here. Most race-type driving professionals I've heard refer to slipping the clutch as the way we use the clutch during normal driving. What I think you're referring to is what they call ``dumping'' the clutch; that is, to just completely lety the clutch pedal go without allowing the cluch plates to slip enough to get the drive axle up to engine speed.

Check out CarTest/CarTest 2000. CarTest is a free DOS program that will allow you to input data related to your car, most if not all of which is findable on the 'Net and in your car's manual. CarTest 2000 is a non-free Java app that does the same thing.

That thing being run simulations of your car. It takes gear ratios, displacement, wind resistance, etc. and produces an amazingly accurate representation of your car which can then be used to find the best shift points for your car for certain circumstances (0-30 might be different than 0-60, for example), amongst other things. Quite a cool program.

I'd do it myself except for I have no idea what year or version of Focus you have, so no way to collect the proper information. You need:

• Engine Displacement (cc)
• Engine Location (front, rear, mid)
• Engine Type (NA, Turbo, Supercharged, sequential turbo)
• Max HP (bhp or kW) w/ RPM
• Max Torque (lb. ft. or Nm) w/ RPM
• Compression Ratio
• Redline
• Gear ratios for all gears
• Final drive ratio
• Front, Rear, or All Wheel drive
• Weight (lb. or kg)
• Weight distribution
• Wheelbase (in. or cm)
• Tire width (in. or mm) (the 1st number in your tire size)
• Tire circumference (ft. or m) (optional)
• Wheel diameter (in. or cm) (the 2nd number in your tire size)
• Tire profile (the 3rd number in your tire size)
• Coefficient of drag (oddly often published spec)
• Frontal area (sq. ft. or m^2) (optional, but helpful if you can find it)
• Overall height (in. or cm)
• Overall width (in. or cm)
• Ground clearance (in. or cm)

Great answers above, and in addition - depending on your car, there is likely a web based forum dedicated to it, much as this one is for the empeg. And on such forums, there's almost always a section where people discuss racing, techniques, etc. If you want the fastest acceleration, read up on the experiences that other owners have at the track, and go from there. They have likely figured it out already with many actual runs, and it will probably be similar to but not the same as what you calculate or expect.

Repeat after me. "It is better to make torque at high rpm than at low rpm, because you can take advantage of *gearing*."

I don't agree totally with that conclusion. It's not that any of the math behind it it is wrong, but more simply that you don't really have much choice in the matter. Your torque and HP curves are somewhat dictated by the bore and stroke of the engine (leave forced induction out of the equation for the moment).
So maybe you take your stock 2.6L engine, and spend a small fortune overboring it and shortening the stroke to shift the torque up the rpm axis. Then you can go and spend another $1k+ on have the tranny dropped to change the gearing to take advantage of it. Great. In the meantime someone comes along with the same car with a stock 3.2L engine in it, and you're stuffed. Or a turboed 2L would also give similar results.

Speaking of turbos, they (and superchargers) are the easiest way to radically shift the torque curve around, assuming that the internals can take it. Basically a larger turbo can provide more torque higher in the rpm range (and peak power is therefore obviously higher)., whereas a smaller turbo provides torque earlier (less lag) but runs out of breath earlier too. Even here I couldn't agree totally with that conclusion - I don't spend my driving life at 5k RPM in 3nd gear. I wouldn't want to. Most of my driving is interstate cruising. Having a hefty punch of torque at cruising RPMs is really useful to me. Why would I want to downshift when I don't have to?

I reckon that for 99% of drivers 99% of the time, torque is more useful lower in the RPM range than higher.

Now if you want to talk specifically about drag racing, or want to discuss maximising racing performance from a given engine size, then I'd happily accept that statement. To be fair to that primer, I believe that it is approaching the subject from a drag racer's perspective. I just don't think it holds up well for the rest of us.

I agree. By this guy's definition, wouldn't the VTEC 2.0L in the Honda S2000 (which is incredibly high-strung, by most normal drivers' definitions) the ultimate engine ever made?

VTEC 2.0L in the Honda S2000 (which is incredibly high-strung, by most normal drivers' definitions) the ultimate engine ever made?

You've got to admit that it's quite a feat of engineering to get 240 odd horsepower out of 2.0L of displacement. It might not pull off the line like a muscle car, but as soon as you find a few corners you'll come out well ahead.

However, I just went to honda's web page to get the actual specs, and it seems they've moved the S2000 up to a 2.2 L engine. Anyone know when that happened? It's probably in an effort to get some more of that low end torque I keep hearing about.

Matthew

It's a 2004 change. Yes, there's more torque now, and the peak is at a lower RPM. Same horsepower rating, but also at a lower RPM. In fact the redline is also lower now. (I think down to 8,000, still pretty high!)

Honda now uses the 2.2L engine in the American S2000 and the 2.0L engine in the Japanese market. I have no idea what they sell elsewhere in the world. The justification for the higher displacement is more torque at lower RPMs, meaning that in daily driving you don't get bogged down or need to shift every time you want to accelerate. The trade-off of the larger displacement is a lower max RPM.

(Apparently, due to funny Japanese taxes, it's significantly cheaper for them to sell a 2L than a 2.2L engine in Japan. Conspiracy theorists talk about them wimping the car out for clueless U.S. drivers, but I think it has as much to do with the way the taxes work.)

Meanwhile, on "slipping" vs. "dumping" the clutch: what I meant to say was, to get a good launch out of your car, you don't necessarily want to just rev it up and immediately release the clutch. This might (a) destroy your transmission, (b) burn out your clutch, and/or (c) deliver more power to the wheels than they can deliver to the ground. Your goal, if you want the fastest possible acceleration, is to have the tires just slightly overpowered. Depending on your engine, tires, etc., this may change how fast you release the clutch and at what RPM. On my Z3 (2.8L 6cyl), the answer seems to be revving it up to around 3000rpm and releasing the clutch over about 1 second while keeping the engine RPMs steady. On a friend's Honda S2000, he has to do something like that at 6000rpm.

Dan, RE: 2.0L vs. 2.+L. This may not apply to Honda in this case, but many other cars are kept at 2.0L or lower so that they can compete in certain race classes. In World Rally Championship racing, the maximum displacement for an engine is 2.0L so that's why the Subaru WRX STi is a 2.0L in Europe and Japan, but is a 2.5L in the USA. (This also explains many 2.0L or smaller offerings from Ford in its Focus, VW in its 1.8L Turbo line, etc. etc.)

Another reason is fuel economy. Gas is a lot cheaper in the US, so we can sell large trucks with V-10's in them. But where fuel is moe expensive, it makes sense to use a high rev'ing small engine because when that engine is kept at low RPM's, you can get good fuel economy. Spin it fast, and you have power.

Finally, in Japan, it's very common for car makers to falsely underestimate their horse power ratings to keep insurance ratings low (US car makers did this in the 70's). 280hp seems to be the "magic number" over there, but many cars listed at that make far more. Displacement or engine size might also be a factor in insurance rates, I'm not sure.

In all the Focus' I've driven, it's redline all the way baby!

Well, I took the car for a day out up to Cambridge yesterday, and I'd have to agree. There's a lot more poke if you just rev the nuts off it before changing.