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The great horsepower hoax?

Newtonmeters are more important than horsepowers

02/04/2013 07:25 |  Comments: 

Editor

Our heavyweight champion, Balázs, or better known as Assur on the premises is the news writer of the mother site, totalcar.hu. Oh yes, he’s an automotive engineer too. They say there isn’t a data that has ever escaped his mind, all we know is that he can drop into any conversation about any car that was made after the war, and he’ll be able to add a ton of details you won’t ever be able to find in books. He has been thinking of selling his longserving Volvo for years, so he always has some used car adverts open on his computer, just in case. Has a wife and two (almost grown-up) children.

What’s the big deal? Open the sales brochures, google the web and you’ll know how powerful your car is. The more PS, the better. End of the story, isn’t it? No, you bet, it isn’t.

The more, the better. This must be one of the earliest measuring method of human history, probably dating back to the jolly good times of hunting and gathering. It is imprinted in our subconscious so deeply in fact that people still dig it up , when they go out to buy something new. This is the rule of thumb that makes you want more megapixels in your camera, whether it makes sense or not, this is why simply having more gears or more cylinders will make a car look so much classier, so much more attractive. Maximum horsepower is another typical measure of awesomeness. It also contributes greatly to the acceleration performance, the top speed, and the overall usability of any given vehicle.

Needless to say automotive manufacturers are keen to exploit the mesmerizing effects of pure strength. Stronger engines invariably cost more, even if (or rather 'when', since it happens all the time) you get exactly the same engine with a different control software, and perhaps a few cheap tidbits added or replaced. And yet I never ever questioned the almighty authority of Power Rating until I got around to driving test cars. Experience, however, has proven to me that two engines with identical power figures can in fact behave in completely different manners. Sometimes a car with 75 PS feels much more agile than another one with, say, 100 PS, even though they are the same size and weight. It is almost as if you could have good horsepower and bad horsepower. Of course that is silly – but you can indeed have engines of very different characters.

Before I carry on there is one thing I must set straight. I am not talking about high-powered supercar engines here which have enough power at idle to make it through a regular day and will put you into orbit as soon as you as much as pat the accelerator. The issue of good or bad PS arises with everyday cars where ten to fifteen horses can make a significant and tangible difference as regards driving dynamics.

So, how will you know the character of your engine? Definitely don't start by looking up the PS rating in the sales brochures. What is generally known as engine power is in fact a single point on a graph depicting the overall potential of the engine. It is the peak value that is only available at a certain rpm, with the throttle full open (in other words, with the pedal slammed to the metal). Let's look at a typical 1.6-litre inline four – brand and make do not matter in this case. This generic engine is rated at 110 PS, available at 5800 rpm. That is awfully close to the top end of the rev range where the delimiter kicks in. You will only reach that point when, say, merging onto the highway or completing a swift overtaking. The engine will be roaring coarsely – certainly not a pleasant noise even in a well insulated cockpit. It makes you want to gear up as soon as you can.

Ah, so much better. Back in lower revs the engine sounds pretty decent in fact. It also feels rather feeble, mind you. 26 PS at 1500 rpm, 38@2000, 61@3000. You just don't go higher than that in everyday traffic.

Now let's take another engine, a downsized 1.2 Turbo. It is theoretically less powerful at 105 PS and yet it feels so much more alive in everyday driving. Of course it does: at 1500 rpm it gives you 38 horses, 51 PS at 2000rpm, and as much as 74 PS when you rev it to 3000rpm. In other words it has about 10 PS extra across the lower and medium rev ranges. If you go by the fact sheets, however, you will end up with the less powerful car.

To avoid disappointment you need to explore the character of a given engine. Looking at the power and torque diagram (if the manufacturer provides one) might do the trick, at least it will give you a good idea of the general character of your engine. It does not take a rocket scientist to interpret these graphs. On the horizontal axis you have the rpm (increasing from left to right), while the vertical axis denotes engine power and torque.

Both graphs plot the same set of information, they are mutually interrelated and can be converted back and forth, yet they show two very different aspects of the engine. The torque diagram will give you a better idea of the character we are after: it tells you in which domain of the rev range your engine is more powerful and in which domain it is weaker.

However, if you want to know just how much of your nominal torque is being put to use – being manifested as actual driving force through the wheels – you need to consider the ratio of the current gear you are in, as well as of your differential. Power, on the other hand, is barely influenced by ratios, if at all. Friction is the only factor that takes away 2-3% of your power. So if you look at the power graph, you'll get a good idea of how fast, how lively the car equipped with the given engine will be even if you have no idea about gear ratios and all.

If you want to know the character of your engine, look at the torque curve. If it starts out low and reaches its apex at mid-range or even further to the right the engine will likely be unwilling to pull at lower revs, meaning that you will need to rev it hard to make the car go fast. If, on the other hand, the graph starts out at a high value, rising steeply and not dropping off before the last third of the graph that means you have an engine with plenty of low-down torque, so you don't need to rev it.

It would make your life simple if you could just overlap the two graphs for instant comparability. However, results would not be authentic since the dynamics of your car are determined not only by the torque pattern but also by the gear ratios of the car.

Manufacturers will often pair low-powered but high-revving engines with a short-ratio gearbox. That means you'll need to shift gears more often and the top speed in any given gear will be lower, but the car will be livelier – at least at lower speeds. However, low-revving engines with plenty of torque – diesels par excellence – are best paired with a taller gearbox which allows the car to convert its power potential to actual road speed. The only reason we have the courage to compare directly the two engines above – rpm to rpm – is because both their max rpm and peak power were similar, hence they could be paired with gearboxes of near identical ratios.

If you know your gear ratios and the characteristics of your engine you can get a pretty good idea of the actual driving power reaching the wheels (what is more, by adding just a few extra bits of information you'll also be able to estimate figures such as top speed and acceleration) but that requires complex calculations and is the realm of hobbyists and engineers, rather than the average customer looking to make an informed decision before buying a car.

What is more, even factory graphs won't tell you everything about an engine. For one, these are plotted at full load so you wouldn't know how the engine reacts at half throttle. And there are other significant factors such as responsiveness, i.e. the delay before the engine reacts to changes in the position of the accelerator pedal. This is one area where most modern engines perform despicably. Naturally manufacturers are fully aware of these data but you'd have to be an industrial spy to lay your hands on them.

Alright, so here is a check list of what you have to do. Decide on a car you like, check the torque and power curves, but before you seal the deal make sure you get a chance to test drive the vehicle. Nothing beats first hand impressions. If the car does not drive like you'd like it to, move on and try to find another model, even if the first one's power ratings are looking good.

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