We’ve long been led to believe horsepower is what “power” is all about. But horsepower is rooted more in Madison Avenue advertising rhetoric than fact. In the power picture, horsepower doesn’t count for much, especially on the street. What counts is torque and when you have the most of it. Engines make torque when you feed fuel and air into combustion chambers and squeeze the mix. Torque is what gets us going, and horsepower is the force that keeps us moving at speed.
Engines do their best work when they reach peak torque where they are making the most low- and mid-range twist. When an engine is below the torque peak, it has more than enough time to completely fill the cylinder with air and fuel. When engine RPM rises above the torque peak, there isn’t enough time to completely fill the cylinders with air and fuel.
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The power you feel from an engine is torque multiplied by engine speed (RPM) to produce a number that tells us something about the engine’s output. This theory dates back to steam engines and James Watt who invented the steam engine in the 1800s. Watt’s theory was a simple one. It compared the work his steam engine could do with the same work an equal number of horses could do. Watt determined a single horse could pull a 180-pound load 181 feet in one minute. This formula figured out to 32,580 pounds per foot per minute. Watt rounded it off to 33,000 pounds per foot per minute. He divided this figure by 60 seconds, which worked out to 550 pounds per foot per second. And this became the standard definition for 1 hp.
As a result of Watt’s calculations, horsepower has become a measure of force in pounds against a distance in feet for the brief period of one minute. You can take this formula and apply it to an engine’s crankshaft at each journal throw to arrive at horsepower. This is based on the number 5,252, which comes from Watt’s calculations. 5,252 rpm is normally where horsepower and torque pass each other in a dyno pull.
Torque is the measure of an engine’s work. Horsepower is a measure of how quickly the engine does the work. Torque comes from displacement and stroke mostly. This means the real power you derive from an engine is expressed in a torque curve. A broad torque curve comes from making the most of the fuel/ air mixture across a broad RPM range. The broader the torque curve, the better the power package.
A broader torque curve is best accomplished with a longer stroke and a larger bore. And this is what strokers are all about: making the most torque across the broadest range. Truth is, you’re never going to get the best of everything, even with fuel-injected engines. Your engine needs to be planned and built based on the way you’re going to use it. What you choose in terms of a camshaft, cylinder heads, and induction system determines how your engine performs.
Giving Power Away
When you’re planning for power, you rarely stop to consider how power gets wasted in an engine’s design and construction. Friction is the power pickpocket hiding in all sorts of places inside your engines. Most of the friction occurs at the pistons and rings. Some of it gets lost at the bearings and journals. Yet more of it gets consumed at piston wrist pins, lifters and bores, camlobes and lifters, rocker arm fulcrums and valvestems.
Your objective needs to be compromise between having tolerances that are too loose or too tight. Piston to cylinder wall clearances are critical in order to have good cylinder sealing, yet not too much friction so you consume power. The same is true for rod and main bearing clearances. You want liberal clearances for good oil fl ow and heat transfer—yet less friction.
Another power-loss potential is engine breathing. You want an induction system that helps your engine breathe well at the RPM range it is designed and built for. This means using the appropriate intake manifold and carburetor. Go too small on carburetor sizing and you restrict breathing. If port sizes don’t match, you restrict breathing. Opt for cylinder heads where port sizing is too limited for your displacement and you restrict breathing.
The exhaust needs a scavenging system that makes sense. You don’t need long-tube headers for great breathing. Shorty headers do the job just as well, and without the shortcomings of long-tube headers. Go too large on header tube size and you hurt torque. Go too small and you hurt power on the high end. This is where your exhaust system has to work hand-in-hand with the heads, camshaft, and induction system.
Written by George Reid and Republished with Permission of CarTech Inc