You might be surprised to discover how few horsepower it takes to keep your car — or even a big SUV — going.
Well, once it’s already going.
Last week, Dodge sent me a Durango SRT392 to test drive (the review of this beast is ). It has an interesting feature, a display that lets you know how much horsepower the engine is producing at any given moment — the number of them increasing or decreasing according to speed and load.
It doesn’t take much horsepower at all to keep even a 5,500 lb. SUV rolling. Getting it moving is what takes power (and eats gas). But once under way, as few as 40 or so horses out of the 475 that the Dodge’s 6.4 liter V8 is capable of producing are needed to keep the show on the road.
That’s about the same number of horses made by a classic VW Beetle’s much smaller four cylinder engine.
Of course, the difference is the Dodge can get to 60 about five times more quickly than an old Beetle — in four extremely enjoyable seconds or so vs. a seemingly endless 15-20. And the Dodge can go much, much faster — in the vicinity of 150 MPH vs. around 80 (maybe, if the wind is at your back) for the old Beetle.
The Dodge has a sur of horsepower. The Beetle has just enough.
But when both are just cruising along at say 45 or so MPH on a flat, level surface — the same 40 or so hp is all it takes.
In part this has to do with gearing — leverage, really — the rest with inertia.
People don’t often think of the gears inside a transmission (and drive axle) as levers — but functionally, that’s their role. To multiply the mechanical leverage so as to lessen the load on the engine — while maintaining a given road speed.
It’s pretty neat.
Open up a manual transmission and have a look inside. Automatics work on the same principle, too. The gears are like levers affixed to a wheel, in order to make the leverage continuous.
The longer “levers” (first gear, second gear) greatly multiply the engine’s mechanical power to get those thousands of pounds of metal and glass and plastic in motion from rest. As you get going faster, shorter “levers” maintain and increase speed with less and effort.
It’s the same principle that lets a man of average strength move an object otherwise too heavy for him to lift or even budge on his own.
That inertia — the tendency of something in motion to remain in motion unless something interrupts its motion — is why not much horsepower is needed to maintain motion. Think how relatively easy it is to keep say a very heavy log rolling once you’ve got it rolling.
Whether log — or car — once in motion, you have only to overcome friction to maintain the motion.
Modern cars make the most of both leverage and inertia via transmissions with multiple overdrive gearing. Once in the very top overdrive gear, it hardly requires any effort — that is, power — to keep on trucking.
I’ve test-driven a few that — because of their deep overdrive gearing — are idling at highway speeds. The Ford F-150 I test drove a week ago, for example, has a ten speed transmission that cuts engine revs to just over 1,200 RPM at a steady 65 MPH. It didn’t have a “horsepower meter” like the Dodge, but if it did, you’d be able to see that it, too, only needed about 40 or so hp to maintain that speed on a level road.
Using less horsepower also uses less fuel — the main reason for all those overdrive gears in the latest cars (and trucks and SUVs). An additional benefit is reduced wear and tear on the driveline — as well as reduced drivetrain noise, even at high road speeds.
Of course, if the road transitions from flat to hilly — or you want to go faster — then you’ll need more power.
If the engine in the car you’re driving hasn’t got much in reserve — like that old Beetle — you may not be able to go much faster and (if the hill is steep) might even find yourself slowing down, no matter how hard you mash the gas.