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Performance Profile: 1990-1992 Isuzu Impulse RS

Rare Turbo Terror

– Lead-in –

Often mistaken for its little cousin, the Geo Storm, Isuzu's Impulse RS offers performance enthusiasts a unique mixture of punchy turbo-charged power, tenacious all-wheel drive traction, and a sharply-tuned suspension developed by Lotus. And because these unique cars were sold in limited numbers, if you're clever enough to locate an example, you won't see yourself coming and going anytime soon.

– Impulse History –

Known as the Piazza outside of the U.S., the first generation Impulse – chassis code PJ – was sold throughout the mid and late 1980s. Best known for their slickly styled exterior, penned by the famous design house Ital Design, PJ cars offer a torquey four-cylinder engine coupled to a fairly conventional rear-wheel drive chassis. This setup was similar to other small sporty coupes of the time, like the Nissan 200SX, Mitsubishi Starion, and Toyota Supra.

Starting off with a clean-sheet design, the second generation Impulse – PI chassis code – were first offered to U.S. consumers in 1990, and were only available through the end of 1992. Since Isuzu was partly owned by General Motors, the PI chassis was also offered under GM's entry-level brand, Geo. Sold as the Geo Storm, the GM offering was nearly identical to the base-model Impulse, known as the XS.

Not wanting to relinquish its best tools to big brother, Isuzu kept the Impulse RS to itself, and to a very small group of buyers. Built on the same basic chassis as the Storm and Impulse XS, the RS did away with the less inspired naturally aspirated powerplant and conventional front-wheel drive arrangement, in favor of a higher-output turbo engine and sophisticated four-wheel drive platform. Performance is impressive, even by today’s standards. When spooled-up, the 160hp engine hustles the 2,700 pound RS through 60mph in just 7 seconds, without any wasted wheelspin or naughty torque steer.

– Spotter's Guide –

Impulse RS Turbos can be difficult to distinguish from their lesser Impulse XS and Geo Storm siblings, because both cars feature the same rounded egg-shaped greenhouse sitting atop a squat mid-section. Unlike the Storm, which features a pointy nose and rounded jelly-bean rear end, the Impulse has a much more purposeful look, with an aggressively blunt front end treatment, and chopped rear deck, accentuated by a sporty body kit.

Up front, a prominent air intake opening is mounted below the front bumper, which also houses two small rectangular fog lights. Above the body-colored rub strip, four round headlights are partially concealed by a thin panel that flips upward, revealing the top half of the lights when in use. A deeply dished egg-crate grill, with body color surrounds, fits between the headlights.

Body colored rear view mirrors and body molding set the RS apart from lesser Impulse offerings, as does the oversized hood vent. Shaped like half a NACA duct, the scoop is recessed into the hood, and provides a direct path of fresh, cool air for the top mount intercooler. A simple body colored spoiler runs along the sides and across the back of the rear hatch. And if the subtle RS clues weren't enough, Isuzu provided a very prominent decal running across the back fascia proclaiming "ALL WHEEL DRIVE / INTERCOOLED TURBO."

Two different wheel designs were offered on the RS, with the most popular featuring a flush 6-spoke design with satin finish. Some cars came equipped with wild looking turbine-styled wheels that incorporate two overlaying 3-spoke patterns, with one set of spokes painted body color. All Impulse models proudly displayed the "handling by LOTUS" badge at the trailing edge of the front fenders.

On the inside, all Impulse models feature a very prominent instrument pod that protrudes out towards the driver. A full array of instruments are provided, including a tachometer, speedometer, oil and water pressure, fuel, and voltmeter gauges, including the obligatory boost gauge for the RS. All regularly used switchgear are located on the sides of the instrument pod, in easy reach of the drivers hands.

Front seats have ample bolstering – good thing considering the Impulse’s cornering grip – and feature either red or blue cloth inserts with a bright, aggressive pattern. As with most small coupes, the rear seat is cramped, especially for taller passengers.

– Technical Specs –

At the heart of the Impulse RS is a potent 1.6L twin cam 16 -valve engine, code-named the 4XE-1-WT. Designed with plenty of high-revving turbo-spinning capability, Isuzu was still able to offer a brawny mid-range punch that makes for stout day-to-day driving. Power is rated at 160hp at 6,600rpm, while torque is rated at a strong 150 lb.-ft at 4,800rpm.

The 4XE-1-WT engine uses a rigid cast-iron cylinder block that features an aluminum alloy cross-flow cylinder head. Pistons measure 80.0mm in bore and have a stroke of 79.0mm, giving this potent little package 1,588cc of displacement. The compression ratio is set at 8.5:1, requiring that top grade unleaded fuel is used.

A small top-mount air-to-air intercooler receives fresh air from the hood mounted NACA duct, and features very short turbo piping , helping to minimize lag and heat gain. An XXX water-cooled turbo is mounted at the front of the engine and spins up 8.5 pounds of boost inside an aluminum intake manifold. A multi-point electronic fuel injection system is employed, with individual intake-port-mounted injectors. The stock exhaust manifold is manufactured of cast-iron, as is the constricted turbo flange that connects to a single 2-inch diameter downpipe.

Inside the aluminum-cased transmission lie five forward gears, each with a conventional single cone synchronizer. Gear ratios are set at 3.909 for first, 2.150 for second, 1.448 for third, 1.027 for fourth, and 0.829 for fifth gear. The final drive is a rather short 4.117. The all-wheel drive system employs a viscous coupling center and rear differential, with the torque split 43% to the front, and 57% to the rear.

For secure stopping power, Impulse RS models employ four-wheel disc brakes, with optional anti-lock control. Front discs are ventilated and measure 9.7-inches in diameter, while the solid rear brake discs measure 10.1-inches in diameter. Both ends are grabbed by floating single-piston calipers.

A power-assisted rack-and-pinion steering system features a tight 16:1 ratio, giving quick responses and a tight turning radius. The fully independent suspension incorporates MacPherson struts in the front, with coil springs, lower control arms, and a 19mm anti-roll bar. In the back, Chapman-style struts are used with coil springs, twin lateral links, a trailing link, and a 18mm anti-roll bar.

The cast aluminum road wheels measure 15x6 inches and feature a standard 4x100mm bolt pattern. Off the showroom, Impulse RSs came shod with all-season performance rated 205/50-15 Bridgestone RE93 tires. This combination was good enough to give the RS class-leading skidpad numbers for the time.

– Hot Rod Tricks –

Since we're on the subject of rolling stock, Impulse RS owners will improve their car’s already considerable road holding manners by upgrading to wider wheels and tires. Thanks to the standard 4x100mm bolt pattern, numerous aftermarket wheels are available, in 15, 16, 17 and even 18-inch diameters. The most popular upgrade choices include a 16x7-inch wheel, wrapped with a 215/45-16 performance tire.

For autocrossing and road-racing enthusiasts, 15x7-inch wheels with 225/50-15 or 225/45-15 "R" compound tires provide a good balance of increased grip without adversely affecting the overall gearing of the car. When venturing off-road, Impulse owners can take advantage of the numerous choices available in rally tire sizes, including the XX/XX-15, that provides a good balance of width and traction.

Finding upgraded suspension tuning parts can be frustrating, especially when owners seek to replace their stock struts. OEM struts from the dealer don't have the proper valving for spirited use, and are quite expensive. KYB offers its non-adjustable GR2 as a replacement, which provides owners a good upgrade for street use.

For serious enthusiasts, inserts designed for the Volkswagen A1 chassis can be modified to fit the stock strut housing. In addition, inserts from the front of a 1st generation Toyota MR2 can be modified to fit in the rear strut housings on an RS. Both option gives autocross and roadracers a choice if they want to specify custom valving.

Several manufacturers offer lowering spring kits for the RS, including Eibach, Intrax, and Suspension Techniques. Stormstylz – a good source for performance Impulse parts in the U.S. – offers a comprehensive coil-over kit designed to fit over stock strut bodies or KYB GR2 struts. In addition to threaded aluminum collars and aluminum spring perches, this kit uses standard 2.5-inch ID coil springs with 450# front and 350# rear spring rates. Eibach manufacturers camber correction kits that will be needed if the Impulse is lowered more than an inch.

For crisper handling, a complete set of replacement suspension bushings is available through Energy Suspensions. For improved braking performance, a set of replacement high-performance pads can be installed. Manufacturers like EBC, Hawk, and PBR/Axxis offers different compounds for both street and track use.

For Impulse RS owners seeking elevated power outputs, a bit of detective work and a willingness to fabricate custom setups is in order. As with most turbo engines, building safe horsepower is a matter of improving air flow both in and out of the engine, and supporting elevated boost levels with proper fuel management. Stock engine internals have shown to be durable enough to support upwards of 20psi of turbo boost on 92 octane pump gas, giving close to 300hp, with significant gains in mid-range pull. Even higher outputs can be expected with higher boost and fuel grade levels.

A good place to start making more power is with an improved intake and exhaust setup, including the turbo outlet, downpipe, and pre-cat. Most local exhaust shops can fabricate a custom exhaust with any specified pipe diameter, although 2.5-inch works best for everything from mild street engines to high horsepower machines. For serious power junkies, a custom manufactured equal length tubular exhaust manifold will yield impressive gains in top end power.

A popular turbo upgrade is to adapt a hybrid T3/T4 setup, designed to give stronger push at higher revs than the stock turbo. In order to keep the engine happy under increased boost levels, higher flow injectors - 550cc proving popular - and a higher flow fuel pump should be used. Popular fuel management systems, like the Haltec, can be used to help tune fuel delivery for serious hi-horsepower engines.

To harness all the power, a stouter clutch setup is required, and Clutch Specialties offers several different versions, including a 6-puck full metallic clutch with uprated pressure plate. Bolting this to a new Fidanza 9-pound aluminum flywheel will help the engine to rev quicker.

– Conclusion –

Finding an Isuzu Impulse RS Turbo can be difficult, but when you do find a good example, you’ll be rewarded. It’s not everday you can find a truly unique car that offers incredible turbo-tuning ability, and has a capable chassis to harness all that extra urge. And thanks to a few select – but trustworthy – sources of information, even non-Isuzu experts will be able to enjoy everything that an Impulse RS offers.

– Owner Profile –

Kip Anderson is considered by many to be the authority when it comes to tuning and developing Impulse RS turbos. And up until very recently, when he upgraded to a new Subaru Impreza Sti, his 1991 Impulse has proven to be a test bed for countless modifications aimed at increasing its fun factor, and its competitiveness on the autocross circuit.

Kip is a regular competitor at Emerald Empire Sports Car Club autocross events, typically held in Eugene, Oregon. Just before he sold his car, Kip’s machine was pushing out close to 400hp on race gas, and is renown for its ability to squirt off tight corners, all four wheels scrabbling for traction. At the heart of Kip’s machine is a fully built engine, featuring JE Pistons, balanced rods, and ported cylinder head.

A custom fabricated equal length tubular exhaust manifold bolts to a hybrid T3/T4 turbo featuring a 0.46 A/R compressor, running upwards of 20 pounds of boost on pump gas, and 28 pounds of boost with race gas. A Haltech E6GM engine management computer, together with 550cc fuel injectors and a Walbro 190 L/hr fuel pump, help to supply the required fuel to keep the engine happy at those elevated boost levels.

Other performance improvements include a custom built 2.5-inch exhaust, a TurboXS H-34 blow-off valve, and an Unorthodox Racing underdrive pulley. A Mueller Fabrications aluminum flywheel helps to speed up the engines high revving capability, while the Clutch Specialties 6-puck full metallic clutch and uprated pressure plate help to harness all that power.

To help lighten his RS, Kip removed the air conditioning, cruise control, front bumper supports and fog lights, the entire sound system and speaker enclosures, ABS hydraulic unit and controller, and replaced the door panels with custom fabricated units. He also converted the power steering over to a manual setup, and installed a large two-row aluminum radiator, that helps to deal with the extra heat generated by the elevated horsepower. Total vehicle weight is down to around 2,500 pounds, just over 200 pounds less than stock.

Being a serious autocrosser, Kip’s Impulse is also outfitted with a serious suspension, that includes KYB GR2 front struts and Tokico Illumina adjustable rear struts, both modified to accept an APC coil-over kit, with 500 lb/in springs. Urethane bushings are installed on the stock anti-roll bars, and the front bar’s end links have been modified for greater stiffness. A four-point bolt-in roll bar helps to stiffen the chassis, as does a custom built rear strut tower brace.

Axxis Metal Master front brake pads helps to increase braking power, and according to Kip, virtually eliminate fade on the street. A sharp looking set of 17x7-inch Konig Villain wheels are wrapped with 215/40-17 Yokohama A-520 performance tires.

Even though Kip has recently sold his ride, he is still an active participant in the Isuzu tuning world. In fact, if it weren’t for his efforts, the largest and best on-line Isuzu Impulse forum – Isuzone – wouldn’t be up and running. Kip’s continued commitment to keeping these rare and fast machines running will continue to help hundreds of other owners to enjoy their Impulse RS Turbos, almost as much as Kip has enjoyed his car.

– Resources –

Stormstylz:

Forums: forum

Kip Anderson:

Isuzu-mods:

IsuzuWeb:

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