Callaway SuperNatural Impala SS; Chevrolet Impala
customized by Callaway Cars; Evaluation by Don Sherman (Motor
Trend, 5/96)
Highway champions aren't born; they must be crafted piece by
piece. At least, that's the accepted theory in the automotive
aftermarket, where the end of the factory's assembly line is
merely a starting point for greater things to come. A prime
example of this is the SuperNatural Impala SS by Callaway Cars in
Old Lyme, Connecticut. Now in its final year of production,
Chevrolet's big LT1-powered rear-drive Impala is a quick,
entertaining ride for a select few customers who appreciate
classic American engineering served with plenty of elbowroom. But
from Reeves Callaway's perspective, Chevy's hulking sedan is a
Z28 with a stretch wheelbase and surplus doors. In other words,
the Impala SS is an excellent candidate for SuperNatural
treatment.
Callaway's SuperNatural technique is a fancy name for the
same sort of modifications that hot rodders have bestowed upon
the small-block Chevy V-8 for 40 years: reshaped combustion
chambers, polished intake and exhaust ports, larger valves,
wilder cam timing, and less-restrictive intake and exhaust
plumbing. Of course, Callaway has added a few modern touches to
the process. Instead of tediously grinding and polishing critical
cylinder head surfaces by hand, the job is done in a $ 385,000
automated milling machines. Custom software for the engine
control computer is another sophistication far beyond the means
of your average shade tree tuner.
In addition to giving the engine deep breathing lessons,
Callaway stretches piston displacement by means of a 0.030-inch
overbore and a forged crankshaft with a longer stroke. Forged
rods and pistons are loaded into the resulting 383-cubic-inch
(6.3-liter) block to ensure longevity in the hands of the
criminally lead-footed.
Each SuperNatural engine comes with a dyno sheet. The one
attached to our test car revealed a torque curve as flat as
Kansas and a horsepower plot that raced straight from just over
200 horses at 2750 rpm to 404 horsepower at the 5750-rpm redline.
Callaway's claim is a 50 percent power increase over a stock
5.7-liter LT1 throughout the rev range.
During the '60s, car manufacturers routinely sold lusty
400-horsepower coupes and convertibles without fretting over
support systems such as the suspension and brakes. Thankfully
that approach started to change about 20 years ago, when domestic
auto makers began to recognize the importance of handling and
stopping. To rein in the herd of horses under the Impala's hood,
Callaway upgraded the front brake hardware to a full Brembo
system, which includes 13.1-inch vented and cross-drilled rotors,
four-piston aluminum calipers, and Pagid metallic pads. (These
brakes were originally developed for use on the Ferrari F40.)
Chassis modifications include four Koni adjustable shock
absorbers, Eibach coil springs that drop ride height a half-inch
while stiffening wheel rates slightly, five-spoke Forgeline
aluminum wheels, and big-boot BFGoodrich Comp T/A ZR.
tires--275/40ZR17 in front and 315/35ZR17 in back. (The hot '96
Corvette Grand Sport wears the exact same wheel and tire sizes.)
No project car would be complete without a few cosmetic
touches. To correct a factory oversight on the pre-'96 Impalas,
there's a small VDO tachometer added to the instrument panel. A
pair of molded carbon-fiber fuel-injector covers dress up the
engine bay left naked by the installation of Callaway's
low-restriction "Honker" intake duct, which disposes of a large
molded-plastic resonance chamber. The final add-on is a flying
Callaway family crest badge on the hood to inform the curious
that this is not your father's four-door. Actually, the I.D.
badge is a mere detail because the SuperNatural SS machine
radiates such excesses of bravado that whole neighborhoods go on
red alert at the mere thump of its basso exhaust. Broad
shoulders, nasty black paint, and fat-boy tires are prima facie
evidence of evil intent.
The initial tap of the throttle jolts the driver into a zesty
frame of mind.Classic V-8 reverberations rumble through the
2.5-inch stainless-steel exhaust system, but that doesn't fully
prepare the driver when throttle plates are cracked to explore
the SuperNatural dimension.
Steering and braking responses are equally forthright. This
Impala perfectly impersonates a Corvette in four-door clothing.
Over minor road imperfections, the Koni shocks act as if
they're frozen solid, so ride motions are occasionally jiggly and
nervous feeling. That would ordinarily convert to dire harshness
over major bumps, but the Impala has a full frame to intercede
between potholes and pants' eats, so the firm suspension settings
work just fine.
When asked to veer from the straight and narrow, this car
breaks crisply with no discernible body roll. Pitched into a
high-speed sweeper it responds with swift accuracy, despite
(at,214.1 inches long) being only slightly smaller than many
islands of the Indonesian archipelago. The SuperNatural SS,
however, demonstrated its best behavior when we wired our test
equipment to it. It thundered out of the hole with wide rear
tires alight and clicked off two firm redline upshifts running
down the quarter mile. The dash to 60 mph took only 5.9 seconds,
1.4 seconds quicker than stock. The quarter-mile ticket reads
14.0 seconds at 100.3 mph, beating the standard model by 1.6
seconds and 10.2 mph.
This Callaway-modified Chevy wriggled through the slalom with
an agility that belies its two-ton-plus curb weight. The 63.6-mph
average speed through the cones is 1.0 mph better than stock, but
slippery leather seats, numb steering, and haphazard rear-axle
location make you wish that Callaway engineers had focused a
little more of their expertise on handling gains.
Despite wider wheels and tires, there was no measured
improvement in stopping distance, but it's a sure bet that this
modified edition will stand up to repeated hard use with
significantly less brake fade than you'd find in a stock Impala
SS.
As is the case with any aftermarket effort, there are a few
shortcomings to report. The super wide front tires need constant
help from the driver to keep the car in one lane on the highway.
When the back seat is occupied, the rear tires occasionally brush
the fenders over bumpy roads. Fortunately these are minor
irritations that do little to diminish the day-in-day-out driving
enjoyment that's baked into this overachiever.
The fact that Callaway's modifications cost more than the
core car means there won't be a long line of customers begging
for GM's assembly plant to work overtime cranking out Impalas for
Supernatural conversion. Nevertheless, as a demonstration of what
can be done with conscientious re-engineering, the Callaway
Supernatural SS is a shining example of the classic all-American
muscle machine.
RELATED ARTICLE: TECH DATA
Callaway SuperNatural Impala SS
GENERAL/POWERTRAIN
Body style 4-door, 5-passenger Vehicle configuration Front
engine, rear drive Airbag Driver Engine configuration V-8, OHV, 2
valves/cylinder Engine displacement, ci/cc 383/6271 Horsepower,
hp @ rpm, SAE net 404 @ 5750 Torque
lb-ft @ rpm, SAE net 412 @ 4500 Transmission 4-speed
automatic Axle ratio 3.23:1
DIMENSIONS
Wheelbase, in./mm 115.9/2944 Length, in./mm 214.1/5438 Base
curb weight, lb 4160 Weight distribution, f/r, % 55/45 Fuel
capacity, gal. 23.0 Fuel economy, EPA city/hwy., mpg 16/24
CHASSIS
Suspension, f/r Control arms, coil springs/live
axle, coil springs Steering Recirculating ball, power assist
Brakes, f/r Vented discs/vented discs, ABS Wheels, f/r
17x9.5/17x11.0,
Forged Line forged aluminum Tires, f/r BF Goodrich Comp T/A
ZR,
275/40ZR17/315/35ZR17
PERFORMANCE
Acceleration, 0-60, sec 5.9 Quarter mile, sec/mph 14.0/100.3
Braking, 60-0, ft 115 Slalom, 600-ft, mph 63.6
PRICE
Base price $ 24,405 Callaway modifications $ 25,345 Price as
tested $ 49,750