Wednesday, July 02, 2008

Horsepower = ?

Horsepower Controls - Matt Yglesias

Being something of a gearhead I've taken a long look at things like horsepower and torque while wondering how the changes I make to my car will affect my fuel economy. Then again, I have a relatively small 1.8l turbocharged engine that in stock form had an average fuel economy of 22/28 city to highway mileage on 170hp. After replacing the air intake with a cold air intake I ended up with around 180hp but a fuel economy of 25/30. This is one area where horsepower was not the point but increasing the amount of cold air entering the engine was as it dramatically improved the running of the car without making it smaller or limiting the horsepower.
Focusing purely on horsepower is fairly ridiculous if you wanted to make an argument for more efficient cars. The argument you want to make instead is to point out all of the inefficiencies that exist in stock configurations. The tuning crowd is just as obsessed about weight reduction as they are about increased horsepower. Open your hood sometime and mind the effort it takes to lift the hood even with the help of a hydraulic piston.
A more fuel efficient car comes not just from limiting horsepower but from making the car as light as possible without losing the necessary structural rigidity. Carbon fiber hoods, lightened alloy wheels, lightweight aluminum flywheels, carbon fiber drive shafts, and metal sections cut up like swiss cheese are all common things on a tuner's vehicle. It's basic physics--the lighter the car, the less energy you need to get the car moving. Shedding twenty pounds here and fifteen there seems like ridiculous obsessiveness but when the pounds add up to the weight of an average-sized person then you realize how important weight reduction is in improving the efficiency of a car.
The reason why average horsepower has gone up in the last thirty years is not solely because of American desires for speed but because the amount of power it takes to move a three or four ton SUV around. Horsepower does not directly translate into speed. The three-ton pick-up truck that passes you on the highway may have a big V-8 with 300hp but anything less and you would have a vehicle that struggles to maintain highway speeds. Meanwhile, the little Mini Cooper that zips past you has a tiny engine but it doesn't need anything more to produce that kind of speed.
The point is Yglesias is wrong to assume that limiting horsepower alone would bring about some sudden leap in fuel economy. Certainly a smaller engine would do that but put a small engine in a heavy car and you will still have terrible gas mileage. What Yglesias should really argue for is a complete rethink in car design with an emphasis on improving the efficiency of the engine while making the car as light as comfortably possible. This means using more expensive parts and removing restrictions in the air flow into the engine. Oh, and there's one more way to lighten a car that few people think of--go on a diet. Losing twenty pounds on yourself can have the same effect as losing twenty pounds on the car. That engine has to lug your ass around so why not ease it's burden?