By Charley Rockwell

Ten years ago, the only reliable way to produce more power from your engine was to purchase high performance parts, install them in your engine, and then test them on a dynamometer or race track. It as an entirely trial and error process. Some professional engine builders managed to spend enough on parts and testing to blunder into a special combination of parts and machining that produced winning horsepower.

Today, you do not need to perform so many trial and error tests to produce horsepower. With the aid of this series of articles and some modern computer programs, you can predict the power improvement of performance modifications with an accuracy of nearly 2%. These articles will cover the general theory of engine power improvement and inform the reader how to quickly evaluate engine modifications with a calculator. Computer programs are only required to get the accuracy down to a few percent.

PART I - Valve Area Determines Horsepower
The piston creates a vacuum as it moves down the cylinder during the intake stroke. The atmosphere pushes air into this vacuum through the intake valve. The faster the piston moves the faster the air has to flow through the intake valve. Simply stated, "The faster you rev your engine, the faster the air has to flow through the intake."

Engineers have observed that air cannot really flow through the intake at speeds exceeding 650 feet per second. This appears to be a critical speed at which it takes more power to shove air through the intake than you get by burning the air in the cylinder. For engine that burn very efficiently, the speed could be as high as 710 feet per second. an inefficient burning engine may have a critical intake speed of only 600 feet per second. An efficient burning engine would be a 4 valve per cylinder Cosworth Formula 1 engine. An example of an inefficient burning engine is a Ford Model T sidevalve engine. A maximum intake speed of 650 feet per second works very well for engine developing peak power between 4000 and 8500 rpm.

Now sit back and think about what you have just read because I will use it to explain the two most important concepts in engine design:

1. The faster you rev your engine, the more power you will make UNTIL the intake air speed reaches 650 feet per second.
2. The larger the intake valve, the faster you can rev your engine before the intake air speed reaches 650 feet per second.

The above two concepts lead to the most important conclusion:
YOUR ENGINE'S MAXIMUM POWER IS DETERMINED BY YOUR INTAKE VALVE AREA.

Yes, it is the engine with the biggest valves that wins the races, not the engine with the most cubic inches of displacement. A 180 cubic inch Formula 1 engine with 32 inches of intake valve area makes 720 horsepower and a 427 cubic inch racing Ford with only 28 inches of valve area can only make 600 horsepower. It is interesting to note that the Formula 1 engine has to spin at 13,500 rpm for maximum power and the Ford engine at only 6800 rpm for maximum power. This illustrates the following important concept:

Let me explain the above concept. Suppose we had a single cylinder engine with a valve area of one square inch, and at 5000 rpm the piston was moving air throught the intake valve at 650 feet per second. If I rev the engine faster, I will not make any more power because it consumes too much power to shove the air through the valve faster that 650 feet per second. If I double the area of the piston, than air will be going through the intake valve at 650 feet per second at only 2500 rpm. If the intake valve remains equal, the bigger the piston the sooner the intake speed reaches 650 feet per second. The horsepower will remain the same with the small piston at 5000 rpm or with the big piston at 2500 rpm.

Let me repeat the concept one more time:
Valve area determines total potential horsepower and displacement determines how fast your engine has to rev to produce maximum power.