Chevy Indy V8's Four-Valve Advantage

    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.