From the October 2005 issue of Car and Driver.
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 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.
There are some interior elements that enhance the entertainment. The front buckets, for example, are close to BMW territory in comfort and support, and the grippy steering wheel and nicely sized shift buttons enhance the sense of driver involvement. The head-up display is effective for keeping track of speed without glancing down, and the center-dash info display even includes a g-meter. Unfortunately, the latter will only deliver its readings–accel, decel, and lateral–when the car is stopped. Thus, the driver can only see the peak numbers achieved during his most recent stretch of road, rather than what’s going on in real time. Minch admits the GXP team made the mistake of checking with the liability lawyers before programming the g-meter display.
An intriguing footnote to this front-drive breakthrough is that it won’t be immediately applied to other GM offerings. The 2006 Chevy Impala SS, for example, gets the 5.3-liter V-8, but the package will use equal-size tires all around and won’t have the Bilsteins.
With an as-tested price of $31,135 (base, $29,995), the GXP is on a more or less equal price footing with some compelling competitors-the Acura TL, the BMW 325i, the Infiniti G35. And that adds up to a tough sell. But this is GM, remember, the discount leader. We checked the company’s latest sales ploy-everybody gets the GM employee discount-and came up with a GXP base of $24,696. At that price, this good-looking all-American is tough to resist.
Counterpoint
Here is a car that depicts the difference between soccer and football, between Peter Pan and Terry Bradshaw. The GXP has brute force that can be summoned instantly, a pugnacious snarl, and a chassis that didn’t go to Harvard. Very American, and very likable in a roughhouse, bawdy sort of way. If you wear your baseball cap backward, this Bud’s for you. Look for owners to drive by in the hip-hop position, heads behind the B-pillar. And yet the GXP corners and handles well, isn’t overly teched up like the Euros, looks bad-ass in black, and the price is right. It’s one of the most memorable sporting American sedans I’ve driven in a gawdawful long time. —Steve Spence
Skeptical is not a strong enough word to describe the brain waves circulating after taking in the GXP’s spec sheet: 303 horsepower driving which wheels, you say? And a four-speed automatic? A taller final drive actually makes the GXP civilized and not the supreme burnout machine I expected. This V-8 makes nothing but good noises and lots of torque, but how does gearing a car to go over 100 mph in second gear make sense? Come on, a closely spaced six-speed could have smoked the porky Hemi Charger. Five years ago, I think Pontiac would have been on to something. Nowadays, the similarly priced Charger R/T seems like a no-brainer. —Dave VanderWerp
In the early ’90s, I drove an experimental Chevy Lumina whose front wheels were powered by a Chevy V-8. I found it to be splendid. At long last GM has put that concept on sale with this GXP, and it still works. As it has for decades, the V-8 delivers effortless performance to this front-driver, and the GXP copes gracefully with the power. One reason is front tires that are larger than the rears. This makes sense, given that the front tires must steer and propel the car while carrying 76-percent more weight than the rears do. There’s still a whiff of clumsiness in this large, old platform, but if you crave V-8 power at a reasonable price, the GXP is worth a look. —Csaba Csere
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Source: Reviews - aranddriver.com
