http://www.caranddriver.com/article.asp?se...article_id=9964
Pontiac Grand Prix GXP
Pontiac rewrites the front-drive-performance rulebook.
BY TONY SWAN
PHOTOGRAPHY BY AARON KILEY
October 2005
Highs: Mellow V-8 rumble, plentiful V-8 torque, excellent road manners.
Lows: Hints of torque steer, hefty curb weight, high steering effort at low speed.
The Verdict: A cool idea that would have been even cooler a decade earlier.
The obvious part of the formula is obviously far from new: Cram a big ol' V-8 in there, make the car go faster. Detroit has been doing this since the '60s. But what may not be so obvious is that there's a big asterisk to the formula when you start applying it to a front-wheel-drive car. The footnote reads something like this: "Put enough power through a front-drive system, and the driver will find himself turning right or left when he was planning on straight ahead."
It's called torque steer, and it's the major limiting factor in front-drive performance cars. Despite various engineering advances, the problem persists in cars such as Acura's otherwise superb TL, which sends 270 horsepower through a six-speed manual transmission to the front wheels via a helical limited-slip differential. But in the Grand Prix GXP, with more horsepower (303 at 5600 rpm) and a lot more torque (323 pound-feet at 4400 rpm), torque steer is not a serious issue. There are hints—a little tugging when the driver cracks the throttle at low speed—but no real wrestling.
How'd they do that? By adopting a measure no one else has ever put into production. More in a minute. But first, another front-engine, front-drive problem, one that's even more chronic than torque steer. With a design that puts all the heavy powertrain hardware up front, front-drive cars invariably have a pronounced forward weight bias, 64/36 percent in this case. As a consequence, the front wheels carry more than their fair share of the car's mass, diluting the ability of the tires to transmit steering inputs. Worse, the front tires are also required to transmit power to the pavement, and all things being equal, the poor things just can't handle their multiple assignments as well as the front tires of rear-drive cars. The result is understeer. The faster the driver herds the car into a turn, the more it wants to go straight.
Pontiac's solution to these two inherent front-drive directional control problems—understeer and torque steer—is unique. Instead of four tires of equal size, the GXP has a lot more rubber up front than at the rear: Bridgestone Potenza RE050As, 255/45-18 front, 225/50-18 rear.
"We wanted a car to run with BMWs," says program engineering manager Phil Minch. "But we were limited by the W-car architecture, in other words, by front-wheel drive.
"The rear end never lets go when you have the same size tires all around. So we put our computer guys on it, and they came back with a recommendation for a smaller rear tire, to give the car better balance."
This is a radical departure from conventional wisdom, and the idea proved out in initial testing. But there was a nasty side effect: Increasing the contact patch at the front amplified torque steer. However, after experimenting with a number of different tires from a variety of manufacturers, Minch and company decided the problem lay in the tire's construction-the way the plies were wrapped-and not the footprint. With sufficient application of power, the tire sidewalls distort, thus affecting directional stability.
Bridgestone, the supplier of choice, was initially reluctant to accept this theory, but when the GXP team achieved improved results using an off-the-shelf tire from another maker, the Bridgestone people got to work and developed a tire that delivered the desired performance.
Other elements of the GXP package include Bilstein monotube front struts—a first for a front-drive GM car, according to Minch—and forged aluminum 18-inch wheels (8.0-inch-wide front, 7.0 rear), a stouter rear anti-roll bar, and a 0.4-inch reduction in static ride height versus the old GTP Competition Group.
Still another challenge was fitting the 5.3-liter V-8 into an engine bay originally conceived for a transverse V-6. Although GM has flirted with this idea in the past—our man Csere drove a Chevy Lumina mule with V-8 power more than 10 years ago—it wasn't as easy as simply greasing the thing up and cramming it in there. The powertrain troops had to develop a tidier version of the 5.3, achieved by creating a unique edition of the block with a shorter crank, a single-belt accessory drive, and a starter mounted on the transmission rather than on the engine block.
The net of the redesign was a reduction in overall length of "about an inch," according to Minch, which was enough.
The transplant also required mods to the 4T65-E four-speed automatic to handle the extra torque and a three-point engine-mount system designed to damp the V-8's torque rotation at full throttle.
Pared down, the 5.3 V-8 met the assembly parameters—it installs from beneath—and provided an extra payoff at the scales. The all-aluminum V-8 is actually lighter than the supercharged iron-block 3.8-liter from the old GTP.
But as you'd expect, the big payoff is in the realm of acceleration gratification. Tramp on the gas, and the GXP rumbles to 60 mph in 5.7 seconds (0.9 second quicker than the GTP we tested in July 2003). That time would have been second quickest in the all-star sedan field we've assembled in this issue ("$35,000 Sports Sedans,"), and the GXP's quarter-mile time—14.3 seconds at 98 mph—would have ranked with the best of that bunch, even though this Poncho weighs in at 3632 pounds.
More important, though, the GXP's abundant torque makes it a formidable player in the stoplight wars. Super-size helpings of thrust are only a toe tap away. Some other sporty sedans can match this car's test-track accel numbers, but none of them can match its massive punch in urban close combat, nor can any of them hope to upstage the mellow mutter of its V-8 exhaust note. In this sense, the GXP is an appealingly American expression of the sports car disguised as family sedan.
But how does it stack up in terms of Pontiac's BMW objective? Let's be clear. This ain't a BMW. It's not as agile as the sports-sedan pacesetters from Bavaria, and even though the unique tire stagger puts the GXP's responses much closer to neutral, the Pontiac's defining trait is still mild understeer. The four-speed TAPshift manumatic is better than some we've experienced, leaving upshift decisions totally in the hands of the driver, but the transmission offers only four speeds to play with. The engine's torque band is so broad, and the transmission's up- and downshifting so prompt in full automatic mode, that the driver can achieve pretty much the same levels of haste by simply putting the lever in D and leaving it there.
That said, the GXP is not without some appealing traits. If it's not quite BMW eager in transient response, it's not too far off the curve, and if the GM Magnasteer II system is artificially heavy at low speeds, it's quick (2.4 turns lock to lock) and accurate, with effort that lightens as velocities climb. The GXP turned in a ho-hum 0.82 g on the skidpad, but real-world grip feels better than that, and as Minch and his cohorts hoped, a driver can induce a little oversteer. And braking performance—174 feet from 70 mph, and zero fade—is on par for this class. The net is a forgiving and capable four-door, arguably the most entertaining sedan Pontiac has ever offered.
PONTIAC GRAND PRIX GXP
Vehicle type: front-engine, front-wheel-drive, 5-passenger, 4-door sedan
Price as tested: $31,135
Price and option breakdown: base Pontiac Grand Prix GXP (includes $660 freight), $29,995; Leather Trim package (includes heated front seats), $665; XM satellite radio, $325; remote starter, $150
Major standard accessories: power windows, driver seat, locks, and sunroof; remote locking; A/C; cruise control; tilting steering wheel; rear defroster
Sound system: Pontiac AM-FM-satellite radio/CD player, 6 speakers
ENGINE
Type: V-8, aluminum block and heads
Bore x stroke: 3.78 x 3.62 in, 96.0 x 92.0mm
Displacement: 325 cu in, 5327cc
Compression ratio: 10.0:1
Fuel-delivery system: port injection
Valve gear: pushrods, 2 valves per cylinder, hydraulic lifters
Power (SAE net): 303 bhp @ 5600 rpm
Torque (SAE net): 323 lb-ft @ 4400 rpm
Redline: 6000 rpm
DRIVETRAIN
Transmission: 4-speed automatic with manumatic shifting
Final-drive ratio: 2.93:1
Gear - Ratio - Mph/1000 rpm - Max test speed
I - 2.92 - 9.1 - 54 mph (6000 rpm)
II - 1.56 - 16.9 - 101 mph (6000 rpm)
III - 1.00 - 26.5 - 143 mph (5400 rpm)
IV - 0.70 - 37.3 - 137 mph (3650 rpm)
DIMENSIONS
Wheelbase: 110.5 in
Track, front/rear: 63.5/61.7 in
Length/width/height: 198.3/73.8/55.8 in
Ground clearance: 5.9 in
Drag area, Cd (0.36) x frontal area (24.2 sq ft): 8.7 sq ft
Curb weight: 3632 lb
Weight distribution, F/R: 63.8/36.2%
Curb weight per horsepower: 12.0 lb
Fuel capacity: 17.0 gal
CHASSIS/BODY
Type: unit construction with a rubber-isolated subframe
Body material: welded steel stampings
INTERIOR
SAE volume, front seat: 55 cu ft
rear seat: 43 cu ft
luggage: 16 cu ft
Front-seat adjustments: fore-and-aft, seatback angle; driver only: front height, rear height, lumbar support
Restraint systems, front: manual 3-point belts, driver and passenger front and side airbags
rear: manual 3-point belts
SUSPENSION
Front: ind, strut located by a control arm, coil springs, anti-roll bar
Rear: ind, strut located by 1 trailing link and 2 lateral links, coil springs, anti-roll bar
STEERING
Type: rack-and-pinion with variable hydraulic power assist
Steering ratio: 13.2:1
Turns lock-to-lock: 2.4
Turning circle curb-to-curb: 38.0 ft
BRAKES
Type: hydraulic with vacuum power assist and anti-lock control
Front: 12.7 x 1.3-in vented and cross-drilled disc
Rear: 12.0 x 1.0-in vented and cross-drilled disc
WHEELS AND TIRES
Wheel size: F: 8.0 x 18 in, R: 7.0 x 18 in
Wheel type: forged aluminum
Tires: Bridgestone Potenza RE050A; F: P255/45R-18 99W, R: P225/50R-18 94W
Test inflation pressures, F/R: 30/30 psi
Spare: none
C/D TEST RESULTS
ACCELERATION: Seconds
Zero to 30 mph: 2.1
40 mph: 3.0
50 mph: 4.2
60 mph: 5.7
70 mph: 7.5
80 mph: 9.5
90 mph: 11.9
100 mph: 14.8
110 mph: 19.0
120 mph: 24.3
130 mph: 30.9
Street start, 5-60 mph: 6.0
Top-gear acceleration, 30-50 mph: 2.6
50-70 mph: 3.8
Standing 1/4-mile: 14.3 sec @ 98 mph
Top speed (governor limited): 143 mph
BRAKING
70-0 mph @ impending lockup: 174 ft
HANDLING
Roadholding, 300-ft-dia skidpad: 0.82 g
Understeer: minimal moderate excessive
FUEL ECONOMY
EPA city driving: 18 mpg
EPA highway driving: 27 mpg
C/D-observed: 14 mpg
INTERIOR SOUND LEVEL
Idle: 46 dBA
Full-throttle acceleration: 75 dBA
70-mph cruising: 71 dBA
