THE LAST IROC

On the heels of the demise of the IROC-Z badge is evidence of the resurrection of the Z/28. The Z/28 designation is displayed prominently on the dash over the IROC-Z identification. Chevrolet hasn’t made a big deal over this, but apparently the ’90 IROC-Z is a transitional model. It seems that a marketing decision has been made to reestablish the Z/28 as the high-performance Camaro. Of course, any Chevrolet high-performance fan would tell you that no matter what the marketing guys decide, the IROC-Z has always been a Z/28 to them. Maybe after two deaths and subsequent resurrections of the Z/28 badge, Chevrolet will finally see the error of its ways.

Speculation aside, the ’90 IROC-Z (or Z/28 IROC-Z) is every bit the performer we’ve come to expect from the IROC-Z Camaros since they first stuffed the Corvette 350 in l000 of the cars back in 1987. The ’90: IROC-Z TPI 350 still comes with roller lifters, dual-catalyst exhaust, Multec fuel injectors – and the big motor is only available backed-up with the 700-R4 4-speed with overdrive automatic. A 3.23 rear gear combined with an overdrive gives a final ratio of 2.26 that yields mellow cruising at interstate speeds.

If you’re the shifty type, the only way you can get a 5-speed Camaro is to step over to the Tuned Port Injected, 5.0-liter IROC-Z or, if you’re budget-conscious, the RS with its throttle-body injected (TBI) 305. Though the power and acceleration stats are down from the IROC-Z figures on the stock RS package, there are ways to make the RS respond. (See “F-Notes,” HRM October through December ’89).

The ’90 IROC-Z is, for the most part, a replay of last year’s machine. There are a few changes, however. To begin with, Chevy found five extra horsepower by eliminating the mass airflow sensor (MAF) and switching to lighter aluminum pistons. The IROC-Z TPI 350 is now rated at 245 hp at 4400 rpm with 345 lbs.-ft. of torque at 3200 rpm. The torque output is the same as last year’s.

The difference between the mass-airflow-sensor system and the speed-density system is basic. With a mass-airflow sensor, the amount of air entering the engine is measured directly, and the computer calculates fuel and timing requirements for optimum engine performance. With the speed-density system, the fuel and timing values for the engine are inferred by monitoring several engine parameters, including inlet air temperature, load (as read by the manifold absolute pressure sensor), engine speed, and coolant temperature, as well as the speed of the vehicle.

The speed-density system is also less expensive than a mass-airflow system and, because of a change in the program, doesn’t require the use of a cold-start injector, which helps control cold-start emissions, a major source of air pollution. One other advantage is the elimination of one more expensive component to replace when if fails, which reduces maintenance costs. Ironically, at a time when Chevy eliminates the MAF, Ford has added MAF to what was a speed-density system on the 5.0-liter H.O. Mustang engine.

In terms of driveability, the ’90 IROC-Z is at the top of the class. The 350/700-R4 drivetrain combination thrusts you through traffic with smooth precision. However, some concern was voiced over the effect the switch from a mass-airflow system to the speed-density system might have on throttle response. One staffer felt the MAF-controlled ’89 IROC-Z delivered a more crisp response, but this is purely a subjective viewpoint. The ’90 IROC is still a muscular 14-second car that approaches 100 mph through the eyes, and yet will weather the rigors of a stop-‘n’-go commute with no complaint. And when you come up against a twisty stretch of road, the supremely capable suspension and 16-inch Z-rated Goodyear Eagles grab the pavement and won’t let go until you do – which is what you expect from a car named after a racing series.

The car sticks best on smooth roads, and does so tenaciously. You’ve really got to push the car hard to find the limits of adhesion. When you get there, you’ll find the usual bias toward understeer that the factories design into production vehicles, even high-performance cars. Transitional responses are smooth and linear. The shock valving, spring rates, and anti-roll-bar diameters are well blended and matched to the grip of Goodyear P245/50ZR16 Eagles mounted on 16×8 aluminum wheels. The car is stable and responds quickly to steering commands, a fact that is greatly appreciated when going hard into a corner and finding that the road surface is rougher than the rear suspension likes – and it skips into intermittent oversteer. This is not a criticism of the handling; it is merely recognizing the limitations of a solid rear axle suspension. As for the brakes, they are very, very good. The big ventilated discs at all four corners put the contact patches to work smoothly, with excellent pedal feel. Brake dive is minimal, as is acceleration squat.

Of course, there are some compromises that come with a car with such capabilities. Most notable is the ride. Experience with the current F-body chassis and its strut-type front suspension and solid rear axle is that you have to limit suspension travel to achieve the awesome handling characteristics of the IROC-Z. That leads to a rather stiff ride. And with the stiff ride comes those annoying interior rattles and squeaks as the car ages. That may or may not be a concern to you, and it points to one of the major difficulties that suspension engineers face when designing a suspension, particularly a high-performance suspension: user preference.

Most hot rodders, we suspect, would prefer a firm suspension with good feedback over a soft and vague one. Still, living with that all the time can get annoying. And given the IROC-Z’s broad appeal, we wonder why they don’t offer, at least as an option, cockpit-adjustable shock valving, as is available on other vehicles. The technology is proven, and would add greatly to the enjoyable drive that the ’90 IROC-Z provides.

Another effort aimed at optimizing the IROC’s handling potential is the little-known fact that the front and rear wheels are not interchangeable. In the case of the IROC, the offset of the wheels front to rear is different, even though the tire size remains the same. The change was made to equalize track width front to rear and to stabilize handling. Obviously this makes rotating the tires more difficult and costly. But sometimes that is what it takes to make a car handle as well as this one does.

Inside the IROC is the roomy and handsome interior we have come to expect from the Camaro. However, two changes stand out. The first, a new instrument layout that features semicircular gauges with high-visibility yellow graphics on black and smoothed side panels, replaces the old-style circular instrument arrangement with sharp-edged side panels. The oil-pressure gauge is now part of a four-gauge cluster between the speedo and the tach instead of being contained in the tach. Second is the addition of the driver’s-side air bag, which required a new steering wheel. The styling updates fit in with the performance image of the IROC-Z, and the ’90 Z cockpit is livable, comfortable, and secure enough to allow you to drive the car hard with confidence.