Horsepower and Torque

We’re thinking about getting a second car – Yay!

We’ve been looking rather longingly at the Mazda3 5-door. But when I look at the numbers, I get a little… confused.

Mazda 3 (2008 5-door)
Outlander (2003 AWD)
2983 lbs 3461 lbs
4.8 inches ground clearance 8.3 inches ground clearance
Fully independent 4-wheel suspension Fully independent 4-wheel suspension
2.3 liter engine 2.4 liter engine
156 hp @ 6500 RPM 142 hp @ 5000 RPM
150 ft/lb torque @ 4500 RPM 157 ft/lb torque @ 2500 RPM

Take a look for a moment at those numbers. Notice anything… odd? The Mazda has a slightly smaller engine… yet it develops more peak horsepower! How can this be?

And look at those torque numbers… the Outlander develops more torque at lower RPMs. Unfortunately I wasn’t able to find a graph of horsepower and torque curves, so we can’t compare them like that, but still, it makes you wonder.

We’ve taken the Mazda3 for a test drive – it’s definately a spunky little car with lots of “get up and go.” Moreso, I dare say, than my beloved Outlander. And yet… those torque numbers continue to befuddle me.

Horsepower vs. Torque

Horsepower and torque are confusing ideas – we all tend to think we understand them, but when you look at their definitions, you can’t help but feel a little confused. We often tend to think of more horsepower as good, because it makes the car faster, right? Well, horsepower is “work done over time.” Now try to work out in your head how being able to do “more work done over time” makes your car faster.

Same thing for torque – which few people even pay attention to. Torque is just rotational force – which gets really confusing when you realize that even if something is not rotating, it can still have torque! For example, when you try to turn a stuck bolt, you’re applying torque – even if the bolt isn’t turning.

Given that, you’d think that torque would be a very important number for cars – more turning force seems to imply that you could turn the wheels faster, right? Well, yes and no. You see, it’s not just raw turning force – you’ve got to consider that your car produces different amounts of torque at different engine speeds (RPM), and then you’ve got the gear ratio to consider (different for each gear your in, plus the gear ratio of your drive train). If you’re a casual car buyer, trying to figure all this out can give you a major migraine.

There’s got to be an easier, more objective way to measure things, right?

Power to Weight Ratios

Looking back on the Outlander vs. Mazda3 chart, I realized that it may just be the weight numbers that are throwing off my perceptions – the Outlander is quite a bit heavier, due in no small part to it’s (fantastic) all-wheel-drive system. So how can we compare?

A little bit of digging on the subject turns up the term “power to weight ratio.” Ahhhh, here’s what we’re looking for!

Mazda 3 Outlander
Torque/weight ratio 0.050 0.045
HP/weight ratio 0.052 0.041

Ah, now that’s a bit better. We’re still ignoring the final drive ratio (produced by the drive train, etc.), but this is much better for comparing power “at a glance.” And now we can see why the Mazda3 feels “zippier” – it’s power/weight ratio is a bit higher than the Outlander’s.

Of course, this reveals another anomaly – the Mazda’s HP/weight ratio is higher than it’s torque/weight ratio, while with the Outlander the opposite is true. But it’s not really much of an anomaly if you look at the rated towing capacity of the two cars – the Outlander is rated to pull (much) more than the Mazda.

So there you have it – the means to (somewhat) objectively compare horsepower and torque ratings between cars, so as to get a sense for their performance. It’s not the total picture, and of course you should still drive a car before you buy it, but perhaps this will help you narrow down your choices (as it sort of did with me).

Explore This

Heh heh heh... I see this explorer on my way to work sometimes. The hood is held down with ROPE. Heh heh heh...
Heh heh heh… I see this explorer on my way to work sometimes. The hood is held down with ROPE. Heh heh heh… fsckin’ FORD… ha ha ha!


Discs & Drums

Recently, I've been wondering why in the world we still use drum brakes on our cars.
If you know me, you’ll know that I tend to obsess over a topic until I know everything about it. Witness my entries from a couple of months ago about cars, AWD, and so on. Recently, I’ve been wondering why in the world we still use drum brakes on our cars. I think it’s obvious to everyone that disc brakes are superior in their stopping power; most 4×4 trucks come with 4-wheel disc brakes standard, and so do many performance cars. Motorcycles also use disc brakes exclusively, but not necessarily for the same reason (where would you put a drum on a motorcycle anyway?). So, with that in mind (and with knowledge that even my car has drum brakes in the back), I set out to find the answer to my question.

It was surprisingly hard.

Nobody seems to say WHY drum brakes are used, specifically. There’s much talk of converting drums to discs on all types of cars, there’s discussions of the superiority of disc brakes over drums, and whole volumes of stories about older cars using 4-wheel DRUM brakes and how terrible they were braking under adverse conditions. So, why are we still using them? After an exhaustive evening of web searching, forum prowling, and just general Keith-style research, I’ve come to the conclusion that we still have drum brakes on our cars for two reasons:

1. Price

2. Parking brakes

Let me deal with these in turn.

Price: it’s an accepted fact that drum brakes are CHEAP. Cheap to manufacture, and cheap to install. This is a free-market economy & all that, so the cheapest effective solution wins. But, you might argue, they’re not THAT much cheaper these days, right? Well, you’re right on that point – the driving issue here isn’t necessarily just the pure price of drums vs. discs… which segways nicely into point 2.

Parking brakes: every car’s got one, right? It’s mandated by some highway safety regulation. A car must have a parking brake. So what does this have to do with the disc vs. drum issue? Everything. Turns out, a parking brake just has to be a manually operated brake system that can function when the engine is off. Now, since modern brakes are all hydraulically assisted, when the power goes out – and the brake pump with it – braking becomes VERY hard. Go ahead, try braking sometime when your engine is off. Not much fun, is it? And what if there’s an emergency and you need to stop the car, but the engine’s off? Enter the parking brake. (Sometimes referred to, logically enough, the “emergency” brake.) Since it needs to be manually operated (i.e. no power assist), and the simplest method of doing that is via a cable attached to a lever or foot pedal, you need a brake system that can be operated by a cable. And the simple fact is, you can’t work disc brakes with a cable (at least not easily – and certainly not CHEAPLY). So you see, the reason we still have drum brakes on the rear wheels of most cars & trucks is because it’s more expensive to put 4 discs on a car vs. 2, and because even if you DID put 4 discs on the car, you need to come up with some way to operate the parking brake manually – and although there are a few different ways of doing it, none of them is simple or cheap (relatively speaking). And since in a car with the engine up front (as in almost every car), under hard braking the front wheels do something like 60-70% of the braking, it seems like an almost logical decision to put the “good” disc brakes up front, and keep the inferior drum brakes in the rear, which not only saves you the cost of putting more discs on the car, but allows you to have a cheap, simple parking brake solution.

Who woulda thunk it?

And don’t even get me started on why big trucks (I’m talking BIG… semi-trailers & buses) use ONLY drum brakes… because I STILL don’t know for sure, but it seems to be price (again), combined with the logistics of operating brakes on air pressure vs. hydraulics. But that’s a topic for another day – or never, since I don’t drive a car with air brakes!

I could chew your ear off with more discussions of drums and their mechanical shortcomings; something about what happens to the brake under hard braking, due to inertial forces, heat, and a whole bunch of other technical mumbo-jumbo… but I won’t, because I hardly understand it myself. I’m just satisfied to know that there is a reason (albeit not a great one, but at least one I can understand) why drums are still on so many cars today.


X Marks The Spot

Lots of X's in SUV names these days, it seems.
Suzuki XL-7

Nissan Xterra

Cadillac SRX

Infinity FX35, QX4

Acura MDX


Volvo XC90


Lexus RX330, LX470

Can you find the common theme in these SUV names?