This morning I left the cage at home, and decided to come to work using a two-wheeled
vehicle. Since I’m only about 2 miles from campus, I took the long way round and rode
along beautiful Lake Sammamish Blvd, into Redmond, and then up highway 520 towards
I haven’t been riding much the past year or so, and every time I do, I’m reminded
why I should do it more. This summer, I sold my ancient Acura Integra, and bought
a used BMW 328i. The Integra had reasonable power for a small car, but the 328 is
a lot faster, and still fun to drive. But neither accelerate like my motorcycle.
One way to measure performance – a bad way, it turns out – is to look at horsepower
to weight ratios. If you do that for the vehicles my wife and I own, you’ll find the
- Ford Ranger – 27 lb / hp
- Subaru Outback – 23 lb/hp
- BMW 328i – 18 lb/hp
- Honda CBR400RR – 9.6 lb/hp (the wife’s bike)
- Honda VFR750 – 7 lb/hp
What does this tell us about the relative acceleration of the vehicles? Well, the
answer is “not a lot, really”, for a number of reasons:
The first is that torque is the important measure, not horsepower. Torque is what
gets you accelerating. Horsepower is only a measure of the work the engine can do.
It’s a good measure of top speed (assuming equal aerodynamics), but has little bearing
otherwise. So why is it the common measure? Beats me – it may be because it has “power”
in the title.
The second factor is one of rotational inertia. Big engines have a lot of rotating
mass, and when you accelerate the car, you also need to accelerate that mass. Small
engines (more correctly, engines with smaller pistons, connecting rods, cranks, etc.)
have less rotating mass, and therefore more of the torque can go into accelerating
the vehicle. This is one reason engines with more cylinders perform better (but not
the only one).
Power curves have a huge difference on acceleration. Race engines are tuned to have
lots of torque in a very narrow rev band, and they make lots of power there, but not
a lot elsewhere. Their peak power may be higher, but the peakier an engine, the more
work it is to extract the power in a useful manner (which, strangely, equates to both
“more fun” when you want to work at it, and simply “more work” the rest of the time).
Finally, the rev range of the engine has a huge impact on the amount of acceleration
it can produce. If you take two engines, one which makes 30 lb ft of torque at 3000
RPM and one that makes 25 lb ft at 6000 RPM, which one is likely to produce more acceleration?
The whole answer lies in gearing – you can take the second engine, gear it down by
50%, and have an engine that produces (ignoring losses) 50 lb ft at 3000 RPM. So,
small engines are better because you can spin them faster, which (within reason) gives
you a free lunch, so to speak, in the power realm.
So how do you really compare vehicles? Well, one obvious way is “at the dragstrip”,
which isn’t really a bad measurement, delta the differences between dragstrip use
and real use, and the launch characteristics. (If you’re curious, the BMW does 15.3
sec @ 83MPH, and the VFR does 11.4 sec @ 112 MPH). But a nicer way is to create what’s
called a thrust graph. For each gear, you take the torque graph, scale it based upon
the various gear ratios and the tire size, to finally arrive at a measurement of pounds
of thrust at the rear wheel vs road speed. You then overlay the graphs for each gear,
and come up with an overall “thrust graph”. If you compare the graphs for two vehicles,
you’ll get a decent idea of how they stack up.
There are only a couple of motorcycle mags that do this, and I’ve never seen it in
a car mag.
Somehow I got from a quick entry about my ride to a dissertation on comparing engines.
Not quite sure how that happened…