After years of enthusiast speculation, General Motors has officially announced
the Corvette ZR1. Propelled to a top speed of over 200mph by the new LS9 V-8,
the new ZR1 is the fastest production car General Motors has ever built.
The LS9 is a supercharged 6.2L V-8 based on Chevrolet's classic small block architecture
that debuted in 1955. The final calibration and certification of the LS9 output
won't be completed until early March 2008, but Sam Winegarden, chief engineer
for small block V-8s at GM Powertrain, says it will produce a minimum of 100hp/L
or over 620hp.
Due to the high combustion chamber pressures in supercharged and turbocharged
engines, higher octane fuels such a E85 ethanol, are advantageous because they
help prevent pre-ignition. Pre-ignition can cause serious internal damage to the
engine.
Many aftermarket tuners of extremely high-performance engines have switched to
running on E85 ethanol blends to take advantage of the 105 octane rating of the
biofuel. Premium pump gasoline in the US is typically 91 octane. Given GM's desire
to make the ZR1 Corvette one of the fastest cars in the world as well as the automaker’s
promotion of E85, it would seem an ideal candidate for running on ethanol.
But Winegarden explained that GM's goals for the LS9 included both performance
and all around drivability at low speeds. To achieve it, a dual pressure fuel
system was developed. The high fuel flow rates required by the LS9 at full throttle
require a fuel flow rate of 58g/s. The available injectors to achieve the full
power flow rates have insufficient dynamic range to provide good control at low
speeds.
The dual pressure system has a separate ECU that runs the fuel pump at one of
two pressures depending on operating conditions. At large throttle openings and
higher speeds the pump runs at 588.4bar. At more moderate speeds the pressure
is reduced to 245bar. This system provides good low speed control without starving
the engine at maximum performance levels.
The high octane of ethanol allows a boosted engine to be run at much higher pressures
without pre-ignition. However, the lower energy density of ethanol requires higher
fuel flow rates to maintain the power output. In order to take advantage of ethanol's
advantages, the necessary fuel system changes would have sacrificed more low speed
drivability than the Corvette engineers were willing to give up.
Winegarden does not rule out the use of E85 in future versions of the LS9. Before
that can happen more development will have to be done on the fuel system to provide
the necessary dynamic range.
However, it’s a safe bet at least a few aftermarket E85-fueled ZR1s will be hitting
the strip soon after the vehicle reaches the market in summer 2008.
