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