AI Online


Rotary Car Revival

Mazda’s RX-8 four-door sports car packs innovation, refinement and engineering daring into a very convincing package.

Technology marches at such a forced pace that engines seldom get a second chance to prove their mettle. When Studebaker ceased flat-head engine production in 1960, they were gone for good. Straight eights and inline threes also went the way of the buggy whip. So the second coming of the rotary engine under the hood of Mazda’s new RX-8 is truly remarkable. The combination of a unique engine with an unprecedented bodystyle represents more creative thinking than the auto industry has seen since the Tucker Torpedo.

The rotary bounced back because a hard core of Mazda enthusiasts was convinced its merits deserve one more play on the automotive stage. To its everlasting credit, Felix Wankel’s creation is fundamentally simpler, smoother and more compact than 4- or 6- cylinder engines with comparable power. But, as is the case with all mechanical devices, there are negatives. Past rotaries demonstrated questionable reliability, high fuel consumption and difficulty meeting emissions standards.

For the Mazda RX-8 to be successful, the rotary had to clean up its act AND compete favorably against some of the best piston powerhouses on the market. Eight years ago, just as the third generation RX-7 began migrating into the history book, Mazda engineers presented a thoroughly reconfigured engine at the Tokyo Motor Show. Their major breakthrough was relocating the exhaust ports from a peripheral to a lateral position. Forty years ago, when Mazda began developing the rotary under NSU license rights, exhaust outlets were oriented radially to avoid coking — burned-oil deposits in rotor sealing grooves. At the start of Mazda’s rotary revival program, engineers solved that concern with improved seals and drastic reductions in the amount of oil injected to lubricate the seals. This paved the way to side exhaust ports, precisely the breakthrough the rotary needed to achieve major gains in power, fuel efficiency and emissions.

While piston engine designers struggle to pack in more valves and larger fluid passages, their rotary counterparts enjoy ample freedom determining the size and position of their induction and exhaust ports. To create a normally aspirated new-generation rotary that beat the outgoing turbo version on all counts, Mazda engineers doubled the size of the exhaust ports and increased intake area by nearly 30 percent. Positioning the port array (two or three intakes and two exhausts per rotor) to eliminate overlap improved idle stability, dramatically reducing both emissions and fuel consumption. (As in a piston engine, overlap is the short circuit by which spent gasses invade the fuel-air charge.) To take maximum advantage of the new-found volumetric efficiency, the mass of the rotors was trimmed by 11 percent and the flywheel was lightened by 20 percent. Elaborate resonance-tuned intake passages with computer-controlled shut-off valves were devised to force feed the rotating combustion chambers throughout their expanded rpm range. On the exhaust side, piping was made as large and as straight as possible to minimize flow restriction. Secondary air delivered by an electric pump after a cold start was added to speed warm-up of the RX-8’s catalytic converter. While they were at it, Mazda engineers gave their RENESIS (the rotary engine’s genesis) powerplant new fuel injectors providing better atomization, longer-lasting iridiumtipped spark plugs, a drive-by-wire throttle, a mass-air-flow sensor (replacing the previous speed-density means of fuel-air-ratio control), double-loop exhaust-oxygen sensing and a much smarter 32-bit powertrain control computer. The payout is a power peak raised by 2,000 rpm (to 250 hp at 8,500 rpm), a 90 percent reduction in exhaust emissions and EPA combined fuel economy figures over 20 mpg. Two engine configurations are offered. The “standard” edition, with two intake ports per rotor and a 7,500 rpm redline for use with a four-speed automatic, delivers 210 hp and 164 lbs-ft of torque at 5,000 rpm. The “highpower” version inhales through three intake ports per rotor, winds to 9,000 rpm through a 6-speed manual transmission and produces 250 hp at 8,500 rpm and 159 lbs-ft of torque at 5,500 rpm.

Top-of-the-line RX-8 has leather seats, a Bose sound system and other accessories.
After such brilliant work by the engine department, RX-8 program manager Noboru Katabuchi was duly inspired to create an equally remarkable car to carry the reborn rotary engine. Recalling the origin of this new species, the 30-year Mazda veteran reports, “What we needed to succeed the RX-7 was one car suitable for three diverse markets— Japan, Europe, and the U.S. In 1998, we began considering various possibilities with ideas solicited from our advanced-engineering branches in those three locations. Our goal was a four-seat RX-7 with all the driving pleasure that concept implies. Within a year, our idea how to achieve this was accepted and the program was approved for production.” Asked how this radically different vehicle fits into the grand Ford Motor Co. scheme of things, Katabuchi-san is quick to point out, “While Mazda’s front-drive products are integrated with Ford’s model plans, the new RX-8 platform is exclusive to Mazda. We intend to use this foundation for other sporty cars in the future. The platform was engineered with broad capabilities to yield an RX-7 derivative, a convertible bodystyle, and, if necessary, cars powered by conventional [non-rotary] engines.”

Mixing sedan and sports car genes to create the first true crossover car is risky business. Instead of the desired stallion with extra seating, there’s a distinct possibility of ending up with a donkey. The key to the RX- 8’s success literally hinges on its rear door design, a system Mazda calls “freestyle.” Notes Katabuchi, “We needed four doors to provide more convenient access to the rear seats than is possible in a conventional 2+2 coupe. But using conventional door hinges and a B-pillar would have stretched overall length by 200mm (7.9-inches). Taking that approach, the basic proportions depart from the sports car range and begin looking like a four-door sedan, which we definitely didn’t want. So we chose the pillarless arrangement and selected the freestyle name because the RX-8’s rear seat is so readily accessible.”

Mazda’s pillarless design allows for easy access to the back seat.

While interlocking rear doors are common practice in pick-up trucks and a feature Saturn has used for a few years in its coupe, Mazda has truly advanced the state of the closure art with the RX-8. Instead of tip-toeing past rising safety and occupant protection expectations with a barely-legal design, Katabuchi’s team aimed for the rafters with five-star capabilities in every impact-related area including lateral collisions. In place of a fixed B-pillar, there’s a moveable one cleverly integrated within the forward portion of the rear door. A steel tube that latches into the roof and the side sill is supplemented by three “catcher pins,” one located in the front door, two in the rear door, which engage sockets built into the RX-8’s sills. A large reinforcement plate forms the root of the vertical tube and spreads concentrated side-impact loads over a one-foot-long section of the sill. Wrapping around this stout heart of high-strength steel is an aluminum skin used to give the rear doors a light, easyto- open feel. Canting the hinge axis 10- degrees from vertical also reduces the effort needed to open the door from inside the vehicle. A swing angle of 80 degrees (versus 67 for the front doors) gets the short, thick rear doors out of the way for efficient ingress and egress. Seat-mounted side air bags help guard front occupants’ torsos and side curtains provide head protection for all four passengers.

The RX-8’s RENESIS powerplant (rear view left, front view right) has better performance, fuel consumption and emissions than its predecessor.  

Achieving the rigidity needed to support a sports car’s aggressive moves with gaping holes in the side areas was no small feat. Key structural features are a tall backbone through the center of the interior, deep-section sills, three crossmembers to stiffen the transmission tunnel and single-piece aper- ture panels stamped from tailored blanks. A roof-area crossmember is nitrided (bonding titanium and nitrogen to the parent steel after stamping) to raise its tensile strength to 145,000 lbs/sq.-in. That step trims the weight of this high-altitude part by a significant 2.2 pounds. In the interests of road noise suppression, mode-control depressions tuned to vibrate out of phase with the surrounding panel are stamped into the RX- 8’s floor pan. Thanks to extensive use of advanced analytic tools, the overall results are impressive. In spite of a competitive 3,000-pound curb weight in fully optioned trim, the RX-8 is 1.7 times stiffer in bending and twice as stiff in torsion than its revered RX-7 predecessor.

For the Mazda RX-8 to be successful, the rotary had to clean up its act AND compete favorably against some of the best piston powerhouses on the market.

Stretching the wheelbase by 10.8 inches (to 106.3-inches) in order to squeeze in rear passengers is contrary to sports car fundamentals but Katabuchi’s team overcame that adversity with creative engineering. The rotary engine’s compact size allowed it to be moved down 1.6 inches and rearward 2.4 inches in comparison to the RX-7’s driveline layout. Positioning the plastic fuel tank ahead of the rear axle and dispensing with the spare wheel and tire also helped concentrate mass within the wheelbase. Only four mounts are needed to support the engine, transmission and differential thanks to a stiff boxsection steel “powerplant frame” that rigidly ties the tail of the gearbox to the nose of the final-drive unit. To eliminate the cost, weight and complexity of a two-piece driveshaft, manual-transmission-equipped RX-8s employ an exotic single-piece composite design consisting of steel, plastic and carbonfiber- reinforced components.

Virtually all the suspension components are forged aluminum to save weight. A rigidly- mounted fabricated-steel front subframe supports the engine mounts, suspension control arms and an electric-power-assisted rack-and-pinion steering gear. A direct-acting electric motor was chosen over hydraulic assist to diminish parasitic losses. Suspension arm lengths are substantially increased over those used in the RX-7 to provide very gradual geometric changes as the wheels move through their full range of travel. Canting the coil-spring and mono-tube damper units in a sea leg fashion improves motion control over uneven road surfaces. The welded-steel rear subframe, which supports a five-link independent suspension system and the differential, is rubber isolated through six mounts. A designed-in offset between the rear suspension’s reaction axis and its steering axis yields a desirable toe-in correction during hard cornering. To remove lag from steering commands, bushings operate in a preloaded condition created by a substantial offset between the centerline of the rear suspension’s load path and the orientation of its coil-damper units.

Thanks to sharp-pencil design and sweat over the details, chassis engineers were able to trim the RX-7’s yaw-axis moment of inertia by a significant five percent. Weight distribution is 52/48-percent front to rear, not exactly BMW grade but none the less a respectable achievement.

Two wheel and tire packages are available: the sport suspension (standard with manual transmission, an $1,800 option with the automatic) includes 225/45WR-18 radials mounted on 8.0-inch wide aluminum wheels while the standard suspension rolls on 225/55VR-16 tires surrounding 7.5 inch wheels. The single-piston floating-caliper braking system also varies according to the package selected. Sport customers receive 12.7×0.94-inch front rotors while standardsuspension customers get 11.0×0.94-inch rotors. Rear discs are 11.9×0.71-inch for all RX-8s. Fade resistance is enhanced by incorporating more internal ventilation ribs than the RX-7 used.

The RX-8’s structure has a tall backbone, deep-section sills, three cross members and a single-piece aperature panel stamped from tailored blanks.
Life in the sports car lane has changed drastically since the 1990s so Mazda had to search its soul to elevate craftsmanship and quality while lowering prices in order to gain the fancy of a broader market base. Considering the fact the RX-7 left the playing field with a window sticker over $38,000, the entry level RX-8 at $25,700 is a genuine bargain. (The rival Nissan 350Z is upstream by $1,109.) At the opposite end of the price spectrum, the more-powerful manual-shift edition loaded with a $2,000 navigation system and a $3,900 grand touring package (Bose sound system, leather seats, xenon headlamps, and other accessories) rings the cash register up to a reasonable $33,100. That’s an aggressive price strategy, but the RX-8’s true claim to fame is its daring attempt to offer four passengers comfortable access to the sports car kingdom. There are shortcomings, to be sure—almost no trunk room, styling that loses its way through the middle of the car, the need to open three doors to exit one rear-seat passenger on the curb side — but Mazda deserves full credit for demonstrating the courage of its convictions.

Drastically different cars seldom appear in any cutthroat-competitive age. When one arrives crammed full of clever engineering AND a special powerplant under its hood, it’s time to pop corks and howl toasts.

Previous posts

Next posts

Sat. February 4th, 2023

Share this post