Spoon Sports Acura RL Endurance Racecar
The world's only endurance racing luxury sedan endures its longest day at the 25 Hours of Thunderhill
By Josh Jacquot Email
Admit it, the Acura RL isn't the first car you'd choose to go endurance racing. Packed with technologies like Honda's Super-Handling All-Wheel-Drive, a manually shiftable five-speed automatic tranny, and enough electronic controls to pilot a cruise missile, the RL is one of the world's finest premium sport sedans. But is it an endurance racecar?
Spoon Sports thinks so.
Spoon, a Japanese tuning company which specializes in making Hondas unusually fast, has a history building some fairly impressive road-racing machines. Owner Tatsuru Ichisima clearly believes that endurance racing is a true test of a car's mettle. His cars have won titles in various classes at the 24 Hours of Nürburgring, the Tsukuba 9 Hours Endurance, and, most recently, the NASA-sanctioned 25 Hours of Thunderhill — the longest endurance road race in the U.S.
Developing a monster
The Spoon RL was developed specifically to compete in the Thunderhill event. In fact, according to crew chief Alan Sensier, it was built to test the durability of the RL's powertrain — something Spoon does periodically in partnership with Honda. And if you're going to test powertrain durability, where better to do it than the extreme conditions of endurance racing?
This logic, according to Sensier, is what led Spoon to choose an RL. Its automatic transmission, electromagnetic clutches and planetary gearsets have never seen the kind of abuse they would in the development process for the event, let alone the event itself.
The assembly process was fairly unique. Spoon acquired a bare chassis from Honda to use as the foundation. First, the chassis was seam welded, which joins sections of the unibody with a full weld rather than the relatively weak spot welding used by most manufacturers during conventional assembly. A traditional six-point mild steel roll cage was welded into the RL's chassis with the main hoop located well behind the B-pillars to increase rigidity.
Powertrain, suspension, electrical and other critical systems came from a second car. But Spoon knew that at 4,012 pounds, a fully equipped RL wouldn't survive 25 laps let alone 25 hours on a racetrack, so many components were left out during the build. Extra weight on an endurance racing car translates to extra load on all of its components. When racing for 25 hours, every extra ounce that must be accelerated and decelerated causes unnecessary wear.
The diet began in the obvious places — the entire air-conditioning and heating systems were excluded. The interior is void of sound-deadening material and insulation. Unneeded brackets and accessories were never installed. Even the trunk hinge was lightened. The result, according to Sensier, is a race-ready weight of about 2,700 pounds.
Heat management
The biggest trick to racing a car with an automatic transmission is managing heat. This is especially true in the RL since its drivetrain uses multiple oil-lubricated gear housings which must stay in a relatively small temperature range to function properly. Stand-alone cooling systems were built for the RL's transmission, transfer case and rear differential. Each one uses a separate pump to circulate fluid through a heat exchanger. The heat exchangers for the transmission and transfer case are mounted behind custom holes cut in the front bumper. The rear differential's heat exchanger is mounted in the airstream just behind the left-rear suspension.
The exclusion of the air-conditioning condenser helped increase flow through the radiator at speed. Remarkably, Spoon deemed the RL's stock radiator up to the task of cooling the engine — a testament to the stock Honda design.
Another challenge was the RL's braking system. Normally, in a manual-transmission car, small amounts of engine braking are introduced at each corner when a driver downshifts, which marginally reduces load on the brake system.
In the RL, however, when the driver takes his foot off the throttle there's no engine braking to help slow the car. That burden is absorbed completely by the stock rotors and calipers. So cooling them was made a priority. Spoon designed a complex ducting system which routes cool air directly onto the car's stock 12.6-inch front rotors. The system was proven during testing in Japan to reduce on-track rotor temperatures by up to 100 degrees.
Thankfully, the factory RL comes with some fairly substantial four-piston calipers up front which house Endless pads. The rear four-piston calipers are also stock. The final addition to the brake system? A secret motorcycle brake fluid which Sensier says has a very high heat capacity.
Powertrain
The RL is an unusual racecar which is being asked to complete an unusually long, difficult race. So it seems strangely appropriate that its engine is unusually stock. In fact, the only hardware changes that were made were primarily designed to add durability rather than power.
Spoon started by fully balancing the 3.5-liter single-overhead-cam mill then adding a larger throttle body and baffled oil pan. Intake air is routed through custom ducting to the stock airbox where it's filtered by an aftermarket drop-in air cleaner.
The radiator plumbing was sanitized by removing the heater circuit and many of the stock rubber hoses were replaced with aluminum tubing to save weight and increase efficiency.
Most of the engine modifications focused on the fuel system. A 32-gallon ATL fuel cell is fitted in the trunk. Three Bosch 200-liter-per-hour pumps feed a surge tank and ultimately move fuel to the engine through braided lines.
The ECU was recalibrated to account for the lack of accessories like air conditioning, and its fuel and ignition maps were tuned to be slightly more aggressive. One hundred octane fuel is used to increase the margin of safety by reducing the chances of detonation. Spoon estimates the lightly modified J35 makes 316 hp, up marginally from the RL's stock 290-hp rating.
Super suspension
With double-wishbone suspension in front and a multilink setup out back, the RL is designed to make few handling compromises right out of the box. More impressive is the fact that most control arms and both subframe assemblies are built from aluminum giving Spoon a capable, lightweight platform from which to start.
Each corner received a Spoon coil-over adjustable for rebound and compression damping. The coil-overs also have adjustable spring-perch height which allows Spoon to precisely dial in the RL's ride height. Spring rates were set during development to match the damper's performance, according to Sensier. Antiroll bars and bushings are stock as are all pickup points for the control arms.
Because the car competes in the unlimited ES class, it's allowed to run slicks. Spoon selected 245/40-18 Yokohama Advan racing slicks mounted on lightweight forged Volk Racing TE37 wheels for the job.
The cockpit
The Spoon RL's interior is a far cry from its days as a climate-controlled, leather-clad transportation lounge. In fact, the Barcalounger driver seat is replaced with a Spoon Sports carbon-shell racing bucket. A Takata five-point harness is tasked with keeping the driver locked in position. In place of the stock steering wheel, which once housed buttons to control stereo and cruise-control functions, is a leather-wrapped Momo wheel with a singular purpose: keeping the car on the track.
There are also two Defi gauges which display oil pressure and coolant temperature housed above a carbon-fiber switchplate with switches for the ignition, fuel pumps and the fire suppression system.
Knock, knock
After more than a year of development in Japan, a trip across the ocean, multiple photo shoots and plenty of work Stateside, the RL's moment of truth arrived when the green flag dropped on December 3, 2005. It didn't last long.
Within the first few hours of the race, the RL hit a piece of debris on the track, bending a wheel and damaging its wiring harness. Even thorough inspection on the back side of the pit wall didn't help the crew find or diagnose the electrical problem. The car still ran, but only in the conservative "limp" mode designed to get it to a dealership in the presence of a potentially terminal problem. Unable to find the problem, the crew sent the car back out. Turns out, driving in limp mode at racing speeds is a terminal problem.
After a short time, the RL returned to pit lane with the dreaded knock, knock, knock sound of connecting rods pounding ruthlessly on the crank journals. And that, as they say, is all she wrote. The RL finished the 25 Hours of Thunderhill in 54th place.
Return to glory?
A new engine will likely be installed in the RL before it returns to Japan. But the question of whether it will race in the U.S. again before returning home remains unanswered. Sensier says Spoon is considering racing the car in the 24 Hours of Nürburgring.
The real question is whether Spoon's lightened, strengthened, highly engineered sedan can compete with lighter, more powerful cars in the proving ground of 24-hour competition. Given Spoon's history and Honda's reputation for durability we know where our money will be. Only time will tell.
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