The Ultimate Subaru WRX STI
When it comes to motorsports, the Isle of Man (IOM) punches well above its weight class. Geographically small and sparsely populated, the 221-square-mile plot of earth quietly nestled between Ireland and England is, in some ways, understandably low-key. For two weeks each year, though, a spotlight in the form of one of the world’s most storied motorcycle races illuminates the isle’s rolling hills, narrow streets and towering cliffs.
The Tourist Trophy (TT) race doubles Man’s population, floods the nation with capital and strings together one of the most arduous road courses ever conceived. For over a century, motorcycle riders have risked life and limb howling along a 37.7-mile loop for competitive glory and the viewing pleasure of spectators.
In 2011, Subaru and Isle of Man native Mark Higgins disrupted this two-wheeled ecosystem by sending a lightly modified WRX STI around the course in just under 20 minutes, breaking the 21-year vehicle lap record. Just three years later, the Manxman (local lingo for an Isle of Man inhabitant) and Subaru returned with a new 2015 WRX STI to shed another 41 seconds from the record.
This year, Subaru decided to raise the bar. Placing confidence once again in Higgins, the automaker partnered with UK motorsport engineering group Prodrive and Subaru Tecnica International to develop a racecar tailor-made to the IOM TT circuit.
Mark Higgins racing the 2016 TT Challenge WRX STI on the Isle of Man course.
Challenge Car, Accepted
The standard Subaru WRX STI is a remarkable piece of engineering: 305 hp and 290 lb-ft of torque power all four wheels via a 6-speed manual transmission. The 2.5-liter turbocharged 4-cylinder engine wastes not an ounce of power, thanks to the Subaru front and rear limited-slip differentials and a Driver Controlled Center Differential (DCCD). Combined with Brembo brakes and a sport-tuned suspension, the stock 2016 WRX STI inhales all types of road surfaces.
At the Isle of Man, however, the WRX STI was destined to be pushed beyond its original limits. Enter Prodrive. In mere months, Prodrive transformed a stock WRX STI into a purpose-built racecar that could handle the TT’s 200-plus corners, elevation changes and geographically compromised road surfaces.
|Subaru Lap Data
|2011 Isle of Man TT
|2014 Isle of Man TT
|2016 Isle of Man TT
Interior of the 2016 IOM TT Challenge WRX STI.
The differences between a standard WRX STI and the 2016 TT Challenge Car begin under the hood. While the production engine block remains, every other powertrain component has been uniquely formed. Prodrive developed a tailored crankshaft, intercooler, rods, pistons, gaskets, cams and other internals, with a resulting 2.0 liters of displacement. A larger diameter turbocharger from Garrett® was then added to deliver torque between 4,250 and 8,500 rpm. This setup allows Higgins to keep his foot planted for longer without worrying that thrust will unsettle the car midcorner.
Subaru WRX STI engine block with uniquely formed components.
Mated to the 600-hp motor is Prodrive’s Xtrac 6-speed semi-automatic sequential gearbox. While most drivers prefer a pair of steering wheel-mounted paddle shifters, Higgins requested a single paddle. In addition to the transmission’s 24-millisecond gear changes, the H-pattern can offer more efficient shifting than linear layouts.
Though the IOM car shares the bones of the WRX STI, its suspension geometry has more in common with a WRC rally car. Four-way adjustable dampers, links, struts, sway bars, cross members and other components are designed to soak up the IOM’s punishing roads while providing the stiffness required for cornering stability. Aiding the suspension’s struggle for traction is a hydraulic active center differential and beefier front and rear limited-slip differentials. Rounding out the racecar’s grip-hoarding hardware is a set of 6-piston AP Racing aluminum brake calipers and 355 mm two piece discs.
While the thunder of the Challenge Car’s straight-pipe exhaust hints at its modified menace, its inflated bodywork is equally imposing. A stripped-down cabin, lightweight doors and fenders, and other lightweight materials make it 812 pounds lighter than a standard WRX STI. Downforce is amplified thanks to a jutting, wooden front splitter; hydraulic, driver-controlled rear wing; large air inlets on the hood; and air channels along the body. Speaking of that rear wing, Higgins articulates its movement via a steering wheel-mounted button, increasing or decreasing its pitch in fractions of a second. On straights, the wing is lowered to reduce drag, but under hard braking or cornering, raising the spoiler increases traction. Finally, Prodrive installed a high-strength steel FIA-spec safety cage.
On an uncharacteristically warm June 6, Higgins strapped into the work of mechanical art and obliterated his own record. In just 17 minutes and 35 seconds, he lapped the circuit at an average speed of 126.9 mph. To put that figure in context, the fastest-ever lap by a motorcycle on this course is a mere 31 seconds quicker.
Despite confidence that, given cooler temperatures, the car could tour the island even faster, a coupling of weather delays and course delays prohibited Subaru from further attempts this year. Prodrive says tweaking the car’s chassis, suspension, aerodynamics and transitioning to dry sump engine lubrication will yield quicker lap times. As for Higgins, rain, shine, midday or midnight − he just wants another shot.
||2016 IOM TT Challenge WRX STI
||2017 Subaru WRX STI
||800 Nm/590 lb-ft
||393 Nm/290 lb-ft
||1,175 kg/2,590 lb
||1,543 kg/3,402 lb
||2.0-liter 4-cylinder with Garrett® turbocharger and custom internals
||2.5-liter turbocharged boxer 4-cylinder
||6-speed semi-automatic sequential gearbox by Prodrive
||6-speed close-ratio manual
||WRC-inspired motorsport geometry with 4-way adjustable dampers
||4-wheel independent STI suspension with inverted dampers
||Permanent all-wheel drive
||Permanent all-wheel drive with Active Torque Vectoring
||6-piston custom AP Racing aluminum calipers on 355 mm two piece discs
||Super Sport ABS Brembo Performance,4-piston front, 2-piston rear
||Dunlop 245/18 racing slicks, Bespoke wheels
||18-inch BBS aluminum wheels wrapped in 245/40/R18 summer performance tires
||FIA-spec safety cage, bespoke bodywork, actuated rear wing
||High-strength steel and aluminum construction, fixed aerodynamics
*Click on the plus signs in the chart to view the 2017 Subaru WRX STI stats
Watch the WRX STI flat out at IOM.
Q&A with Mark Higgins and Prodrive
Mark Higgins is an Isle of Man native (Manxman) who has been racing since age 9. Among his long list of professional accomplishments, Higgins has won the British Rally Championship three times (1997, 2005, 2006) and has shown marvelous car control on-screen in the three most recent James Bond films (Quantum of Solace, Skyfall and Spectre).
Subaru has worked with Higgins since 2011, when he set the automaker’s first IOM TT lap record. His knowledge of the course and his skill behind the wheel has been crucial to dialing in this year’s Challenge Car and setting consistent records. In the time I spent with him, Higgins appeared completely at ease; whether he was strapped into a racecar or watching Bangers racing with his family, his countenance always reflected an inner calm and joy.
Despite driving Isle of Man roads all his life, Higgins has only lapped the closed circuit about a dozen times, including each of his record attempts. While most racetracks can be driven throughout the year, the IOM course is trafficked by residents 98 percent of the time. The fact that Higgins can tackle the circuit at full speed with almost zero traditional practice is a feat by itself.
How would you compare the driving experience of the Challenge Car to the mostly stock WRX STI you drove in 2014?
Higgins: It was very different, as this year’s car was specifically built for one job, the stability was very good because of the aero, and chassis gave great confidence. Also the braking and sequential gearbox gave a totally different feeling.
Can you single out a few of the Challenge Car’s attributes that were most impressive?
Higgins: The positive feel of the car and how little you needed to steer it – you can almost think the car around the corner – and the car felt it could go faster. It was very precise, which is always a good feeling rather than scaring yourself! It also got up to full speed quickly.
How do you mentally prepare before hitting the TT course?
Higgins: It’s very difficult because you often don’t have a set time for your run, and there is a lot of time spent waiting. The only time to really be able to focus hard is when the helmet is on and the door is shut. Thankfully, we get a 3-mile warm-up before the flying start to relax and prepare for the run.
What are the most challenging, and most enjoyable, sections of the course?
Higgins: There are a few key points of the circuit I call ballsy corners; they’re the ones you want to take completely flat but have to build up to it. In the six years of attempting the course, I have only had eight flat-out laps in three different cars, so it’s very hard to know the safe limit on every corner, as the track changes every time you go out, including weather temp and grip levels. Wind is also a major factor around the 37.7-mile course.
One of my favorite sections is from Ballaugh to Ramsey; it’s bumpy and has some great corners. I'm still not a great fan of the bottom of Bray Hill after 2011.
What do you enjoy most about working with Subaru?
Higgins: I have spent a lot of my career with Subaru in Production World Championship, British Rally Championship and many wins with their cars. For me, the reliability and strength of the cars and working with a brand that has so much history in 4-wheel drive rallying. Subaru USA has been fantastic on the TT project and gave me the dream car for this year.
What do you enjoy most about working with Prodrive?
Higgins: I have had the pleasure to work with them before and they are total professionals in every respect. Details win events and championships, and every box is always checked. Our sport is about confidence, and Prodrive gives me full confidence when I get into their cars.
Prodrive is a UK-based manufacturing and tuning house that has designed and developed vehicles across a range of competitive segments, fueled by the best and brightest engineers. In addition to building carbon composite parts for a number of automakers, its motorsport expertise, especially related to the World Rally Championship, has earned the company global recognition.
Prodrive has worked with Subaru since 1990, establishing the Subaru World Rally Team and leading the team to three drivers’ and three manufacturers’ WRC wins. To say the two entities work well together is an understatement. Subaru relies on Prodrive’s engineering expertise and, in turn, Prodrive relies on the build quality and platform potential of Subaru. Prodrive was able to convert a stock WRX STI into the Challenge Car within a matter of months, customizing WRC and military-grade parts specifically for the IOM TT course.
I was personally convinced of Prodrive’s attention to detail approximately 30 seconds into an engineer’s one-hour download on pressure rings. It’s clear their work is as much a labor of love as it is an art form.
What was the most complicated part of the Challenge Car’s build?
Prodrive: Without a doubt the engine. Although the standard WRX STI is a superb starting point, we had to double the power. The duty on the engine is around 75 percent full throttle, which is an incredible value for such a long period of time. Engine revs and boost create their own challenges.
How long did the design, development and production process take?
Prodrive: The whole process from agreeing on terms to completion was 10 months; for a project of this nature you would normally allow two years. We had to use a lot of our resources internally to meet our targets and carry over elements. Albeit substantially updated from our World Championship-winning Subaru cars from the late 2000s, this was the only way we could meet our goals.
How did the characteristics of this circuit play into the Challenge Car’s development?
Prodrive: The TT course is a formidable lap. Incredibly high-speed corners, high vertical loads from jumps and compressions, wide-ranging grip levels from very abrasive to shiny blacktop were just a few of the considerations we had to take into account when coming up with the design brief. It certainly was a challenge for the Dunlop slick tires we took to World Touring Car Championship. We had to be very careful with tire loads and had to devise our own pressure settings from the nitrogen-filled pre-warmed tires that were used.
What are some of the strengths of the stock WRX STI that make the finished Challenge Car so great?
Prodrive: The standard car chassis balance is superb. With that we broadly mirrored the kinematics. The WRX STI road car really is at home on fast, sweeping A roads found in the UK and we used the same layout.
Also, the center differential control strategies within the standard car are very complex, providing stable handling and braking that we expanded and adapted slightly to suit the higher grip levels and speeds encountered using the TT Challenge Car.
What about your relationship with Subaru is unique to the automaker?
Prodrive: Our heartbeat is the same as the fantastic engine note of the boxer engine. The foundations of our building are built on the years of WRC wins with Subaru. You can find a huge amount of hardware still painted in the famous blue and gold color scheme dotted around the company. Our trophy cabinets are bulging with success delivered by Colin McRae, Carlos Sainz, Richard Burns and Petter Solberg. A huge proportion of the current workforce fondly remembers the WRC successes. It certainly was an emotional moment to run the TT engine on the dyno for the first time; there was a crowd of people around as it was running flat chat.
In what way(s) could the Challenge Car still improve to afford a higher average speed and lower lap time?
Prodrive: Firstly, there is a bit more to come from Mark. [Not much! He is a talented guy and I doubt if anyone knows the island better than he does – maybe his dad, but he prefers to drive his Forester nowadays!] The TT riders this year were very fortunate to get good weather and, as they enter a number of bikes in differing categories, they got 15 or so free practice laps in total. Mark did one standing start lap, then was straight into the run. It was an incredible feat, however. With one or two more laps and only small changes I firmly believe a 131+ mph lap was achievable. After that, post event we studied data and stripped the engine. All looked perfect, so we know we can push the superbly strong engine further. Six-hundred-fifty bhp is a realistic target. This will come from more boost and a 9000-rpm rev limit. We used 8500 this year. Better aero in the shape of a diffuser and slightly lower drag will ensure we make a good step.