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!
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).