COLORADO SPRINGS, Colo., June 13, 2002 -- Just as athletes work out at the Olympic Training Center in the thin air of the Rocky Mountains to improve their on-track performance, Chevy Indy V8 engines will be breathing deeply in this weekend's ...
COLORADO SPRINGS, Colo., June 13, 2002 -- Just as athletes work out at the Olympic Training Center in the thin air of the Rocky Mountains to improve their on-track performance, Chevy Indy V8 engines will be breathing deeply in this weekend's Radisson Indy 225 at nearby Pikes Peak International Raceway. While long-distance runners depend on their lungs to sustain them, the naturally aspirated Chevrolet racing engines that compete in the Indy Racing League rely on a complement of 32 titanium valves to supply their eight cylinders with oxygen and fuel.
The Chevy Indy V8 has the advantage of four valves per cylinder, with two intake valves and two exhaust valves to regulate the flow of fuel and air. In contrast, the Chevy SB2 small-block that powers Team Monte Carlo in the NASCAR Winston Cup series and the Chevy LS1 small-block that will propel the Corvette C5-Rs at Le Mans this weekend have one intake and one exhaust valve for each cylinder.
"The first advantage of a four-valve design is that the valve area is optimized so you get more airflow through the engine," explained GM Racing engineer Roger Allen. "The second advantage is that four small valves are lighter than two big valves, so the valvetrain is more stable at high rpm."
Allen is an expert on the art and science of airflow. He led the design teams that developed the SB2 small-block for NASCAR competition, the Aurora V8 racing engines that dominated IMSA and IRL competition for seven years, and the Chevy Indy V8 that has won all six IRL races this season. Allen is the lead engine designer for the all-new Gen 3 Chevy Indy V8 that will make its competition debut in 2003.
Like an athlete who trains to increase lung capacity and the ability to assimilate oxygen, the Chevy Indy V8's advanced four-valve cylinder head design increases the amount of fuel that can be burned to produce power.
"The four-valve design produces significantly more airflow at low valve lifts than a two-valve layout," Allen noted, "Airflow through the Chevy Indy V8 engine approaches 1,000 cubic feet of air per minute -- enough air to fill a typical suburban bedroom in 60 seconds."
It is the pressure produced by the weight of the earth's atmosphere that pushes the mixture of air and methanol into the Chevy Indy V8's cylinders. At sea level, standard atmospheric pressure is 14.7 pounds per square inch (psi), but at Pikes Peak International Raceway, which stands 5,357 feet above sea level, the pressure is only about 12 psi. It's this "thin air" that leaves flatlanders gasping for breath and reduces engine output by almost 16 percent. A Chevy Indy V8 that produces 675 horsepower at a sea-level track like Homestead-Miami Speedway tops out at 570 horsepower at mile-high PPIR.
The Chevy Indy V8's intake and exhaust valves are forged from titanium, an exotic material once reserved for supersonic spy planes and nuclear submarines. Stronger than steel and as light as aluminum, titanium is an ideal material for the hellish environment inside a racing engine. The intake valves are about the diameter of a silver dollar, while the exhausts are the size of a 50-cent piece.
"The intake valves are larger than the exhaust valves because in a naturally aspirated engine, you only have atmospheric pressure to push the air into the cylinders," Allen continued. "On the exhaust stroke, the residual pressure of combustion expels the burned gases out of the cylinders, so the exhaust valves do not have to be as large. There must be a balance between intake and exhaust flow so that what goes into the engine can get out of it."
If four valves are better than two for high-speed performance, why not have five, six, or a dozen valves?
"You quickly reach the point of diminishing returns by adding more valves," Allen explained. "The goal is to maximize the valve area for a given bore diameter, which is limited to a maximum of 93mm by the IRL rules. If you had five valves -- three intakes and two exhausts, for example -- you end up with about the same area as with four valves. With six valves or more valves, you actually have less valve area because you have to leave room for valve seats and space between the valves. Additional valves also means more complexity, which can add weight and reduce reliability. The IRL rulebook specifies a maximum of four valves per cylinder, so these other layouts aren't an option."
The Chevy Indy V8's 32 valves are operated by four overhead camshafts, while its Winston Cup and road racing counterparts use a single cam buried deep within the block. The improvement in high-rpm performance offered by the overhead cam design more than offsets the weight of four steel camshafts and the chains and gears that drive them.
"A conventional block-mounted camshaft requires pushrods and rocker arms to open the valves," Allen said. "These components are flexible to some degree, which ultimately limits how quickly you can open and close the valves. The overhead cam design eliminates these parts, so the cam profiles can be much more aggressive. The overhead cam also allows more freedom in cylinder head design because you don't have to compromise the ports for pushrod clearance."
The four-valve Chevy Indy V8 is designed to operate over a narrow rpm range, typically between 9,500 and 10,700 rpm. The two-valve Corvette LS6 production engine, in contrast, must operate smoothly from idle to its 6,500 rpm redline, while meeting both customer expectations and government emission standards. The Chevy Indy V8 consumes a rich diet of five parts of air to one part of methanol, while the Corvette motor produces maximum power with a 12:5 ratio of air to unleaded gasoline. The differences between the two engines are as dramatic as the contrasts between a sprinter and a marathon runner training in the rare air of the Rocky Mountains.