Chev Indy V8: All About Rev Limiters

Chevy Indy V8 Insights: The Hard Facts on Soft Rev Limiters

    NASHVILLE, Tenn., July 18, 2002 -- It is the nature of racers to
seek speed. Without the limits imposed by rulebooks and sanctioning bodies,
modern race cars could easily achieve levels of performance that would
breach the boundaries of prudence, safety and expense. As a result,
virtually every form of organized motorsports imposes limitations on engine
performance. These range from displacement limits and fuel allocations to
carburetor restrictor plates and turbocharger pop-off valves. In the Indy
Racing League, the final arbiter of horsepower is an electronic rev
limiter.

    IRL teams will be running at the limit this weekend in pursuit of
victory on Nashville Speedway's distinctive 1.33-mile concrete oval, the
site of Saturday night's Firestone Indy 200. The championship-winning Chevy
Indy V8 engines that compete in the IRL are governed by a trio of rpm
limiters. Two "hard" limiters effectively shut down the engine's ignition
system when it reaches the prescribed 10,700 rpm maximum, while a third
"soft" limiter progressively cuts spark to the cylinders as engine speed
approaches the legal limit.

    "The IRL limiter is called a hard limiter because when it is activated,
it shuts down the entire ignition system," explained GM Racing engineer
Ned Baker, the group's electronics specialist. "It's like an on-off switch.
When it determines that the engine has exceeded 10,700 rpm, it effectively
shuts off the ignition until the rpm falls below the specified speed. When
the limiter is activated, engine output drops to zero. The limiter doesn't
restore current to the ignition system until it has determined that the
engine is below the rpm limit, so there is a period when the car is
essentially without power."

    This stand-alone rev limiter that is supplied, maintained and inspected
by IRL officials is the policeman that enforces the series' rpm rule.
It calculates engine speed by measuring the intervals as four teeth on
the flywheel pass a block-mounted sensor. The circuitry can measure engine
speed with remarkable accuracy. At 10,000 rpm, for example, there are 666
pulses per second, permitting the limiter to detect changes in engine rpm
continuously.

    "The IRL limiter also has a powerful data-logging capability,"
Baker noted. "It records engine rpm in real time, so officials can examine
engine speed at any point during an event -- in practice, qualifying and
the race.

    "The IRL hard limiter is extremely harsh, so there is a second rpm
limiter incorporated in the engine control module (ECM)," he continued.
"This limiter is set just below the threshold of the IRL limit. The ECM
calculates engine speed just as the IRL limiter does, but uses its own
sensors and its own logic. When it is activated, the internal ECU limiter
shuts off the signal to the ignition module. The interruption may last only
a thousandth of a second, but it is usually enough to keep the engine speed
below the point where the IRL limiter intervenes.

    "The third limiter, which is also contained in the ECU software, is
called a 'soft' limiter because it is reduces engine power progressively by
cutting the spark to selected cylinders," Baker said. "Instead of shutting
down all eight cylinders like a hard limiter does, it drops one, two or
more cylinders per revolution. When the soft limiter is engaged, the engine
can still produce a significant percentage of its potential power instead
of going to zero as it does when either of the hard limiters is activated.

    "With the hard limiter, it's all or nothing; with a soft limiter,
you might still have 90 percent of engine power available to drive the car.
There is still the absolute boundary of 10,700 rpm that you can't cross,
but the soft limiter allows you to walk a little closer to the line while
maintaining a relatively high performance level."

    It is instantly apparent when a driver steps over the line because
the engine's exhaust note changes abruptly. When the ignition system is
shut down, fuel is still injected into the cylinders. The unburned methanol
goes out the exhaust system, where it is ignited by subsequent firings. The
result is a distinctive sound that can be heard in the grandstands.

    IRL teams must carefully balance gear ratios, aerodynamics and
track conditions to maximize vehicle speed without exceeding the engine rpm
limit. Fourth, fifth, and sixth gears are closely spaced to allow the
driver to choose a ratio that will keep the engine running as fast as
possible without exceeding the rev limit. In spite of a team's best efforts
to keep the engine below 10,700 rpm, the limiter can be activated when
engine speed spikes momentarily as the rear tires skip over bumps in the
racing surface. Other environmental factors also influence engine rpm --
running in the draft behind another car or getting a push from a favorable
tailwind can increase speed and activate the rev limiter. In night races
like this weekend's Firestone Indy 200, changes in temperature and track
conditions between daylight and darkness also impact speed and engine rpm.
In these situations, the driver must select a gear that will produce
maximum speed without exceeding the mandated rpm limit.

    The rpm limit has been revised several times since the series
implemented its naturally aspirated engine formula in 1997. Initially set
at 10,500 rpm when the series introduced 4.0-liter engines, the limit was
lowered to 10,300 in 1998 and to 10,000 rpm in 1999 to rein in rapidly
escalating speeds. The move to smaller 3.5-liter engines in 2000 saw the
limit raised to the current 10,700 rpm standard. In 2003 the
second-generation 3.5-liter engines will run initially at a maximum of
10,300 rpm.

    "The rev limiter is a key element in the IRL's commitment to
affordable technology," said GM Racing Group Manager Joe Negri. "Along with
rules that specify maximum cylinder bore diameter, minimum engine weight
and key engine dimensions, the rev limiter encourages manufacturers to
focus on fundamentals such as improving efficiency and minimizing parasitic
power losses. These are topics that have great relevance to GM production
engines."

    While racers invariably push the limits of speed and performance,
the IRL's electronic rev limiters provide a reality check that keeps the
focus on close competition and appropriate technology.