Photo Credit: Subaru

Subaru’s factory-backed driver and madman Mark Higgins broke the Isle of Man record this year for four-wheeled vehicles not once but twice in one week. The lovely folks over at Subaru had me out to the Isle of Man to take a close look at the car. I can tell you they didn’t just build a car—they built a weapon.

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A couple of years ago Subaru, in a multi-year sponsorship deal with the Isle of Man TT, took a very stock-ish STi and with Higgins at the wheel broke the lap record (set by Rover back in the ‘90s) with a time of 19:26 at 116.47 mph.

Now in 2016, Subaru’s current sponsorship deal is coming to a close, so the company wanted to put the record for cars out of easy reach for any future manufacturer that might dare try usurp them.

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Enter the TT Challenge STi.

Photo by author.

This car lapped the 37.7-mile Snaefell Mountain Course in a wickedly impressive 17:35.1. It may have been designed with the simple goal of setting a record around the Isle of Man, but Subaru ended up building what has to be one of the fastest road-covering cars on the planet.

The secret to its speed is that its tech comes from the world of rallying, and there’s a good reason why Subaru went in that direction even for this paved-roads-only racer.

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The TT Challenge car starts life as a box standard Subaru WRX STi but that all changes as soon as it rolls through the doors of the legendary Prodrive race shop. Yup that Prodrive, the one that since its founding in 1984 has won four Le Mans 24 Hours class wins, three FIA WEC titles, three British Touring Car Championships and the Le Mans Series.

Prodrive is probably best known to us across the pond for the six World Rally Championships they have won during their 19-year association with Subaru. The most famous of those is arguably the late Colin McRae’s 1995 win in the RAC which crowned him Britain’s first ever World Rally champion.

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Given this long and very successful relationship I have to imagine that it was a pretty easy decision for the suits over at Subaru to pick up the phone and dial the boys at Prodrive when it came time to build a car to challenge the record.

A cursory look at the TT Challenge STi reveals a car that dips heavily into the Subaru/Prodrive parts bin. With Prodrive’s past success in WRC it’s understandable that they would start off with a tarmac rally car as opposed to a a more road-racing biased machine.

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Spend more time with the thing and you can see the rally roots go much deeper. From the ECU boxes mounted in the drivers footwell (in rally racing you have a codriver, so you need to mount the boxes that normally would be there in a road racing car) to the H-pattern semi-automatic paddle shifter (more on that in a sec) the rally roots run deep in this one.

Photo by author.
Photo by author.

Power output is 600 horsepower. Torque is nearly even at 590 lb-ft from a production based (that means stock block and heads) 2.0- liter turbocharged boxer four cylinder engine out of the STi. It’s the crankshaft, rods, pistons, gaskets, cams radiator/intercooler and Garrett turbocharger that are all custom motorsports bits. One of the hardest things to get this engine to do was rev up to 9,000 RPM. Prodrive normally builds engines like this to redline at about 6,700.

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That six-speed semi automatic gearbox developed by Prodrive with help from transmission gurus Xtrac is hydraulically actuated, but unlike standard motorsports transmissions that are based around a sequential layout where you engage one gear and the next sequentially in a line, this transmission is based on an H-pattern box. This is a far more complicated layout, as it requires two pneumatic actuators to select the requested gear. You need one to move the shift fork over and the other to move it up. Getting this sequence dialed is far more difficult than the already temperamental single pneumatic sequential actuator.

The reason for using this more complex setup is that an H pattern box allows you to select first gear far more quickly than a sequential, as with a sequential you have to go down through each gear which could take five to 10 seconds. That might not be hugely useful at the Isle of Man, but it’s massively valuable on a rally stage, recovering from a spin.

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Continuing the rally theme is a long list of parts lifted straight off WRC designs. A hydraulic active center differential works in conjunction with limited slip diffs on both ends of the car. Custom motorsports suspension with WRC-based links and four-way adjustable dampers and massive six piston AP Racing brake calipers on vast 355 mm disks.

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One area where this car goes off the rally reservation is the rubber. Where as most tarmac rally cars use specific Pirelli or Michelin tarmac rally tires (or tyres if you’re British, and like pissing off the rest of the world that can spell things correctly), this car uses medium compound 245/ 18 Dunlop tires lifted straight from the British Touring Car Championship mounted to Speedline rims.

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But there are specialized concessions even with those tires, too.

There was talk of using the Dunlop soft rubber but the consensus was that the loads placed on the car over the course of the circuit would be so consistently high that the softs might not hold up for the entire lap. Also, as me and my team have discovered after using these tires for a few years now in the BTCC, the soft Dunlops usually only work in a very small temperature window and the Isle of Man circuit is infamous for having widely varying weather from one side of the island-encircling course to the other.

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The last bit to making a race car quick is the aerodynamics. The aero on Subaru’s TT car is fairly straightforward and typical of any rally build. It has mainly stock bodywork excepting the deep front bumper and fender flares. But there’s one notable exception, specially optimized for this notoriously high speed circuit. Prodrive added an active rear wing to the car, which can lift and stall out, substantially reducing drag. It’s similar to the DRS wings in F1.

The Subaru TT wing is activated by a button on the driver’s steering wheel, but its use is also monitored and controlled by one of several onboard systems made by McLaren (yes, that McLaren). These systems won’t allow the wing to deploy if they detect that the car is in a state of yaw and steering angle—anything that would represent the driver is changing direction and needing all the rear downforce the car can produce. Those same systems also won’t allow the wing to stay in the open position for too long after the driver hits the brakes.

All of this technology has gone into making the fastest road car to ever go around the Isle of Man. There’s some interesting additions from the world of road racing, but it’s the rally tech at the car’s heart that made it so devastatingly fast on the rough, narrow roads of the Isle of Man course.

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It may well be one of the fastest point-to-point cars on the planet.