Prodrive Subaru_edited-1Each year in June, the Isle of Man hosts the Tourist Trophy (TT) motorcycle road races on the Mountain Course: 37.73 miles of narrow, closed public roads on which the top competitors set lap speeds in excess of 130 mph on race-spec Superbikes. In this year’s headlining Senior TT, race winner Michael Dunlop lapped in 16 minutes 53.929 seconds at a record average speed of 133.962 mph.

In recent years, event sponsor Subaru of America (SOA) has used TT week to showcase the potential of its WRX STI street cars. In 2011, Isle of Man native (‘Manxman’) Mark Higgins, a multiple British Rally champion and brother of multiple Rally America champion, David, lapped a stock STI with safety modifications in 19 minutes 56.67 seconds, breaking a 21-year-old record set by Tony Pond in a Rover 827 Vitesse (think Sterling). Higgins and SOA came back to this small island in the Irish Sea in 2014, taking another 41 seconds off the time and raising the lap speed from 113 mph to 117.510 mph, again in a stock STI. For 2016, they returned to break the record again with the purpose-built Subaru WRX STI Time Attack.

In 2014, Mark Higgins drove a largely standard Subaru WRX STI to a new TT lap record of 19 minutes 15 seconds (117.510 mph). Photo credit: Subaru of America

Rally roots

The story of this one-off super-Subie begins in 2008. The financial crisis had yet to hit and Subaru still had a works World Rally Championship (WRC) team. The Subaru World Rally Team (SWRT) had been run since late-1989 by Prodrive, the Banbury, UK-based motorsport preparation specialist and engineering consultancy co-founded by Ari Vatanen’s 1981 WRC champion co-driver, David Richards.

SWRT’s final full WRC car was the WRC2008 (known internally at Prodrive as the S14), based on the GE Impreza WRX. Subaru withdrew the team at the end of that season, but all the designs, data and knowledge gained was safely archived.

The Prodrive-run Subaru World Rally Team was very successful in WRC competition with cars such as the WRC2004 (Petter Solberg is seen here on Wales Rally GB). The final SWRT machine was the WRC2008. Photo credits: Wales Rally GB, Subaru

Fast-forward to July 2015. Prodrive had developed the Volkswagen Golf SCRC for the factory FAW-VW Rally Team in the Chinese Rally Championship (CRC) and was on its way to a drivers’ and manufacturers’ championship double. Also competing in the CRC was Subaru driver Mark Higgins. Higgins had known Prodrive’s lead engineer for the Golf program, Richard Thompson, since the two worked together at Nissan Motorsport Europe more than 20 years ago. Higgins told Thompson about the TT program and put Prodrive in touch with SOA. In September, the design and build of the Subaru WRX STI Time Attack began in Banbury, with technical assistance from STI in Japan.

Spiritual successor

Richard Thompson talks of developing a car that was in the spirit of the exercise. Yes, this would be a highly modified, specific race-rally hybrid developed specifically for the TT course – but at its heart, the Time Attack retains its street-car shell and a Boxer engine under the hood. It is not, for example, a silhouette-bodied Le Mans Prototype (think Peugeot 208 T16 Pikes Peak). It is a car that Subaru fans can identify with and old SWRT rally fans can warm to.

Mark Higgins on Glencrutchery Road during his 2016 TT practice lap. Photo credit: Tony Goldsmith/Prodrive

The car had to be developed quickly, and on a limited budget. “You’d normally want about two years to run a program like that for a manufacturer,” says Thompson, “but times change, so you have to use what you know to best effect.”

“From 1990 to 2008, hundreds of millions were spent on developing a product for Subaru with sign-off at a wide range of temperatures, grip levels, and surfaces, and that meant that we had all the archive data to lean on,” he continues. “We won a lot of rallies and championships with those cars and we still had jig bracketry and kinematics and compliance [data] from that period, for example.”

“We had a successful package that we could adapt slightly to the TT. As a basis we took our last smooth-tarmac [Subaru] rally car design. From a customer point of view, that meant that we didn’t need to do thousands of kilometers of testing, which in any case the timescale and budget didn’t allow. We had to shortcut the system.”

Powertrain

Around 75 percent of the TT lap is spent at full throttle. The high average lap speed – almost double the FIA’s 68 mph (110 km/h) average WRC-stage speed recommendation – meant that the engine became the “biggest single factor” in the performance of the Time Attack car, according to Thompson.

Elements of the highly modified WRC2008's Boxer engine and semi-automatic gearbox were tweaked to meet the demands of the fast TT course. The cooling package is largely unchanged. Photo credit: Prodrive

The starting point for the in-house engine team was the S14’s wet-sump Boxer unit, which made mid-300s horsepower and around 502 lb-ft of torque. But with the WRC-mandatory air restrictor not required and the focus now on top-end power and high-rev durability rather than low-end response, some important changes were made. In came a beefier Arrow crankshaft, Arrow conrods, Omega pistons and a different flywheel, as well as a slightly lower compression ratio (10.75:1). The efficiency of a new Garrett turbo is biased towards high engine speeds.

The team wanted to keep the standard, Xtrac/Prodrive semi-auto gearbox from the S14, which offered 20 ms shifts that would not upset the car’s balance if Higgins shifted midway through a long, fast corner. However the gearbox casing limited the ratios the engineers could get into the ’box. Slightly longer drops on 4th, 5th and 6th gears than were typical for the WRC car proved good enough for 180 mph.

Prodrive had the engine hardware defined in late-November 2015, built the engine in December and had it on the dyno in early January 2016 to start the detail tuning work. The resulting super-Boxer puts out 590 lb-ft of torque and 575 hp at 7,750 rpm and 2.7bar of boost, running on knock-reducing Elf 113-octane race fuel.

Chassis and body

Meanwhile, work had begun on the car itself. The shell from a standard WRX STI was completely stripped before a full roll cage and new crossmember points were welded in, roughly doubling torsional stiffness to around 42,000 N/deg to ensure that suspension tuning changes had the desired effect.

The standard WRX STI shell was completely stripped. 700 hours of work were required to fit the roll cage and new crossmembers. Photo credit: Prodrive

Mounting the Boxer 20 mm lower than standard helped bring the center of gravity height down from to ~420 mm (from ~500 mm on the standard car); the motor is also slightly further back to improve weight distribution. Changes to the metalwork were made for aerodynamic and cosmetic reasons, and to accommodate the combined requirements of wider Dunlop British Touring Car tires, lower ride height and bump-friendly amounts of wheel travel.

The TT has its own character and if you went with a pure race car, you’d be a bit short on damper travel – Richard Thompson, Prodrive

The similarities between the 2015 WRX STI and the 2008 car in terms of its chassis architecture meant that Prodrive could use an asphalt-spec S14 as the starting point for the Time Attack’s suspension kinematics. When it comes to tuning, Thompson likens the TT course’s demands to an Irish tarmac rally stage.

“The TT has its own character and if you went with a pure race car, you’d be a bit short on damper travel,” he explains. “We designed a smooth-tarmac WRC damper [supplied by EXE-TC] with slightly more damper travel. The only negative is a slightly more unsprung mass from the longer damper, but packaging-wise it’s not a problem.”

Aerodynamics

Careful thought was given to the aerodynamics on such a fast course. Thompson explains the thinking behind a package that incorporated a 200 N front splitter, 700 N rear wing and a drag-reduction system (DRS) that built on knowledge gained from Prodrive’s involvement with DRS on the McLaren P1. The budget didn’t stretch to a rear diffuser so the car is flat-floored.

Fitting the Time Attack with a DRS is equivalent to a gain of around 40 horsepower, according to Richard Thompson. Photo credit: Prodrive

“If you talk about ratios it’s something like 7 parts drag and 1 part downforce, a bit like Le Mans,” he begins. “At a circuit with a more normal speed profile, you might be looking at a 3:1 or 4:1 ratio. The Time Attack therefore needed to be as slippery as possible, while respecting the airflow to the [WRC-standard] cooling package. Aero for cooling is vitally important; after that you make your base design and trim your front splitter, rear-wing angle of attack and your Gurney to give you the numbers you’re looking for.

“The design was all done in CFD, and by using historical CFD – we did a lot for the radiator and intercooler with the old SWRT program and the manufacturer’s bumper has only evolved slightly, so it was just a tweak here and there to get the program up and running. That was another advantage of SOA going with us – we weren’t starting with a clean sheet of paper.”

Downforce is important for a small portion of the TT course and the car had to be safe at a fastest jump speed of 170 mph (calculated using CFD and rally experience). To achieve this without compromising drag for the rest of the lap, a 10 bar pneumatic, automated DRS system was devised with two actuators and new control software for the car’s TAG-McLaren ECUs. A full-scale wind-tunnel test at MIRA Proving Ground, “to quantify our aero numbers, make sure that the front was in balance to the rear and the DRS was doing what it should,” was conducted on April 18, around a month after the car had completed its initial shakedown at Turweston airfield and some high-speed running at Bruntingthorpe Proving Ground, which boasts a 2-mile, ex-USAF Cold War runway.

The front splitter and rear wing were assessed in the full-size wind tunnel at MIRA before the car was rolled out in its final paint scheme. Photo credit: Prodrive

We made a staggering number of detail changes to get the balance that we wanted without increasing drag. – Richard Thompson, Prodrive

“Initially we’d specified a DRS concept of 0-7-degree angle change; that didn’t prove to be enough to balance the front up so we defined 0-14-degrees instead [a design zero point, not 0-degrees to the horizontal], which wasn’t far from the limit of the system,” notes Thompson. “Even then, we took the decision to put the 10 mm Gurney on there. With a 15 mm one we would have lost top speed; instead we preferred to work hard with the chassis, which we did on our subsequent loops on the Isle of Man. We made a staggering number of detail changes to get the balance that we wanted without increasing drag, just working on ride height, springs, damper clicks, that kind of thing.”

Testing and tuning

Unseasonably cold temperatures and snowfall ruined the team’s late-April test on the Isle of Man. Thompson sat alongside Higgins in the car for less than half a day’s running on a 1.25-mile test road in the south of the island, and on the soft-compound tire rather than the medium-compound rubber that would be used for the runs in June.

“It was so far removed from what we would see in June that it was next to meaningless,” he recalls.” We decided to quickly get to a slightly understeer-y balance that gave Mark some confidence, basically focusing on rear grip, making sure he could really lean on the rear.”

The Time Attack undergoes pre-event testing in the hands of Mark Higgins. Photo credit: Prodrive

By contrast, the TT meet in June enjoyed its best weather in living memory, providing markedly different conditions. Now on the medium tire, Higgins’ track time was still very limited, however. Even in perfect weather conditions, the unpredictable nature of the TT meant that the three timed runs the team had hoped to complete became only two when a fatal bike accident – the fourth of the week – caused the final one to be cancelled.

“When runs are at a premium you make some assumptions,” says Thompson. “We knew the grip levels would be up compared with the April test so when we arrived for the sighting lap on Saturday morning [June 4] we brought the ride height down, stiffened the car up a little bit, fitted the medium tire and went. It was into the unknown to some extent – the medium tire behaved quite differently to the soft, and after that the setup changes, although they were only details, made for quite a balance change.

“That all proved to be a positive step, so the Saturday evening [chassis adjustment] was a trim, and the Monday changes too. We ended up a good step stiffer front and rear on the main springs, with more damper clicks, reduced ride height and a rake change as well – a couple more millimeters down on the rear compared to the front to win a bit more grip. We were just trying to balance the car up and react to ever-increasing grip levels.”

Higgins streaks across the start/finish line during the record-setting run on June 6. Photo credit: Subaru of America

Higgins streaks across the start/finish line during the record-setting run on June 6. Photo credit: Subaru of America

It was on Monday, June 6 when Higgins set the new marker of 17 minutes 35.139 seconds (128.730mph). Thompson is in no doubt that he would have gone faster still had the final run gone ahead, and by the 11 seconds required to break the 130 mph barrier.

Nevertheless, he has already thought about what improvements could be made to the car, should the team return for another shot in the future. The engine would come first – the turbo has “plenty of headroom,” but work would be required to keep the heads on at higher boost. With that might come a revised intercooler/radiator. More aero development could cut drag even further, more testing on the medium tire might yield setup refinements, and shorter dampers would reduce the unsprung mass. But even in its current specification, the Subaru WRX STI Time Attack is a remarkable piece of engineering.