A Detailed Look At The KohR Mustang GT4 Road Racer

The GT4 is amazing here. It really is unlike any other previous-generation racing Mustang that I’ve driven. — Jack Roush Jr., KohR Motorsports

By now we’re used to Ford Performance offering excellent, off-the-shelf racing Mustangs. Years of Cobra Jets for the drag strip and long line of road racing Mustangs have seen to that. Yes, you could start with a regular street Mustang and buy the same parts piecemeal and pretty much end up with the same thing, but it would cost more, take precious time and stand little chance of turning out so well.

“It wasn’t but two years ago we could build cars on our own [buy a Mustang and modify it into a winning package],” Dean Martin, of KohR Motorsports, said of the Mustang GT4 racer his team campaigns in IMSA’s Continental Tire SportsCar Challenge. “But now it’s all GT4 spec and really requires the factory support for the development. Ford has been very active in support of the program.”

Thanks to much improved ABS, traction control, aerodynamics and the nearly hands-free paddle-shift transmission the 2017 Mustang GT4 has proven much easier to drive than its GT350R predecessors. That’s especially helpful in the longer Continental Tire series enduros. Aerodynamic sophistication improved immensely when the 2017 Mustang GT4 replaced last year’s GT350R racers. Taking lessons learned on the Ford GT wing Ford engineers designed an all-new tea tray for the Mustang GT4. The front splitter, hood venting, diveplanes and rear diffuser are all new and unique to the GT4 as well.

Jack Roush Jr., KohR’s other headline driver put it this way after setting the pole at Virginia International Raceway, “The GT4 is amazing here. It really is unlike any other previous-generation racing Mustang that I’ve driven. It’s very well engineered, much more advanced technology. It’s a true production race car built out of a production chassis.”

Simply put, the GT4 Mustang swims in a larger, deeper, and more sophisticated pond than earlier GT350S/R and yesterday’s bolt-together technology doesn’t cut it anymore. The result is one of the most complex, integrated Mustangs ever. It’s also a car that’s shifted the balance of performance away from the drivers and onto the engineers, as we’ll see.

Physical To Digital

Hardware has been the hallmark of the road-racing Mustangs leading up to the GT4. The 350S and R-models especially were really modern iterations of the good old basic Trans Am cars of the Parnelli Jones era. Everything that didn’t matter was taken off and all the noisy moving parts were heavy-duty stuff. There was a clutch pedal and a shifter where you’d expect it on transmission tunnel, plus an eager-revving, heat-generating race engine that would just as soon hit 8,000 rpm than pull at 3,500 rpm.

Aerodynamic aids were a few tacked-on bits and generally these cars are a bit loose, moving around some as the driver relies on mechanical grip from the simple suspension. These cars were — and are — tough, fundamental machines that put on a good show. Mainly bereft of electronic aids these V-8, rear-drive pony cars placed a premium on driver skill because it’s so easy to punish the rear tires with a little too much throttle, or over-cook the braking and grind the front tires on corner entry.

Several drivers have wheeled KohR Motorsport GT4 Mustangs during the 2017 season, but the Dean Martin and Jack Roush Jr. have anchored the driving squad. Both are fine drivers with extensive Mustang experience dating back almost two decades in Martin’s case. Dean parlayed his mechanical-engineering degree into a job with Roush Industries, then as a Vehicle Dynamics Development Engineer — the guys behind the wheel who tune chassis — for Ford. His last Blue Oval gig was with the now-defunct Special Vehicle Team where he met fellow engineer Larry Rehagen. The pair moonlighted at Rehagen Racing before Dean went out on his own with KohR Motorsports.

Plenty of skill and seat time went into consistently going fast enough while simultaneously conserving these cars through a race so you’d have some tire left for what really mattered — the final few laps.

That’s changed. Specifically, the GT4 Mustang wears an aero package of splitters, vents, dive planes and a large rear wing that’s taken the twitch out of higher speed corners in favor of much improved stability. Derived via Ford’s Computer Fluid Dynamics modeling and proven in expensive full-scale wind tunnels, drivers such as Dean say the aero package has significantly buttoned down the GT4’s persona. There’s a lot less hunting and jiggling in this year’s road-racing Mustang, replaced by reassuring solidity through sweepers and newfound precision in medium speed corners.

Another plus: GT4 Mustangs sport Dynamic Suspension Spool Valve shocks. Developed by Multimatic Motorsports, the same company that builds the GT4’s for Ford, the DSSV dampers replace traditional shock valves—built from stacks of perforated shims — with spool valves featuring laser-etched orifices. Rebound and compression dampening each get their own valve, and the shapes and thus the precision of the dampening are much more precise than traditional shock’s and must be computer selected via Multimatic software.

Much trial and error testing at the track is thus avoided, but if you don’t have access to Multimatic’s software or parts, either at the track or during the car’s development, then you’re out of tuning luck. This DSSV precision is also more predictable than a traditional shock’s action, and it is more repeatable over time. That means the computer modeling concerning the shock and all it affects — which is just about everything in the chassis from the suspension to the frame to the aero package to ride height — becomes more critical to get just right.

Watson Racing builds the GT4 Mustang’s roll cage; it’s about the only fabricated part on the car Multimatic didn’t form. Quality safety gear is used throughout. Besides its obvious crash benefits there’re anti-fatigue plusses to a comfortable, well-fitting driving position. That’s important when races are 12 hours long. There’s no need for a dashboard in a modern road racing sedan such as the GT4 Mustang, but it tricks us all into thinking the GT4 is more of a hopped up street car than it really is. All needed instrumentation is offered by the MoTeC display.

Naturally, the spring rates, ride heights, and overall body movement are tightened up in response to the aero package and shock capability. Tire construction and compounding is also new this year, contributing mightily to the chassis precision as well. Put together, all this chassis finesse puts more emphasis on selecting just the right chassis tune. And, as any racer will tell you, if the chassis is not right there’s no hope of doing well.

With the GT4 Mustang getting the chassis right involves more computer computations than educated feedback from the driver than ever before.

Crossbred Power

What powers the GT4 is thoroughly modern, but it isn’t exactly an off-the-shelf powerplant.

In its press materials Ford says, “The Mustang GT4 is equipped with the next generation, naturally aspirated 5.2-liter V8 racing engine specifically tuned for GT4 competition. The race engine was engineered, validated and built by Ford Performance in partnership with Roush Yates Engines.”

Actually, what all the verbal tip-toeing is about is Ford obviously wants to promote the Voodoo as seen in the street legal GT350 and GT350R, but given the practicalities of complying with rulebooks and winning races the GT4 engine is a Voodoo with retro-modifications, most notably a more typical 90-degree crankshaft in lieu of the production Voodoo’s celebrated 180-degree piece.

Powering the Mustang GT4 is a Ford Performance 5.2-liter engine fitted with a traditional cross-plane crankshaft and a Mustang GT intake manifold.

2017 Mustang GT4 Specs


• 5.2-liter dry-sump V-8 with cross-plane crankshaft

• Stainless steel tuned Ford Performance Parts exhaust

• Assembled by Roush Yates Engines


• Seam-welded and mass-optimized production Mustang unibody fitted with FIA-specification roll cage by Watson Engineering

• FIA-standard race seat

• Five-point harness and lateral restraints

• ATL fuel bladder

• FIA/IMSA-compliant fire suppression system


• Holinger close-ratio, paddle-shift six-speed gearbox

• Ford Performance Parts adjustable-ramp, limited-slip differential


• Bosch MS6.1 ECU with engine calibration optimized for endurance racing

• Custom MIL-spec, route-optimized wiring harness

• MoTeC Power Distribution Module

• Onboard MoTeC datalogger


• Brembo six-piston front and rear calipers with full-floating, slotted rotors

• Bosch ABS controller, race-tuned


• Adjustable coilover MacPherson struts with race-specific bushings, ball joints and Multimatic DSSV adjustable cartridge-type dampers

• Ford Mustang rear multi-link IRS with Multimatic-designed lower control arms, race-specific bushings, race-specific balljoints, and Multimatic DSSV adjustable dampers

• Blade-adjustable front and rear anti-roll bars


• 18×11-inch Forgeline forged multi-spoke alloy wheels

Furthermore, what minimal power loss the smoother-running 90-degree crank induces is academic. At 5.2-liters the Ford V-8 is already bigger than competition, and with its multiplicity of camshafts and excellently breathing cylinder heads it makes too much power given the slightest tuning. Only a maximum of 450 rear-wheel horsepower is desired by the sanctioning body to produce competitive parity, so the engine is reigned far in from its potential.

In fact, tractability was the goal with the GT4 engine. Thus, with the 180-degree crank gone there was no need for the tuned Voodoo intake manifold so it is replaced by the 2011-2014 production Coyote intake minus its variable runner control. Dean reports that with this broadened the torque curve, in conjunction with some rather trick engine management software, the GT4 mill meets its power goal by 5,000 rpm. The engine readily revs to 8,000 rpm for the ability to avoid shifts between tightly spaced corners, but the power output is dead flat from 5,000 to 8,000 rpm.

“It just comes up and levels out,” he said. “…It gives the car some flexibility. You’re not wringing its neck all the time; if you short shift it’s still making the power… [it’s] stable through corners, not a sharp on/off throttle, no big power loss through the shift. It’s a really nice piece.”

Behind that ‘really nice piece’ is another game changer, no less than a six-speed sequential, paddle-shift gearbox from Holinger, an Australian racing gearbox manufacturer. No more taking hands off the wheel to row a manual, the Holinger box, engaged by a Sachs racing clutch, allows full-throttle upshifts and blips the throttle on the downshifts. Apparently durable and with easily changed gears, the Holinger offers yet more tuning possibilities because each gear ratio is easy enough to tailor to either track demands or driver preference.

Impressively large Brembo six-piston brakes get all the attention, but it’s the smooth, sophisticated ABS that drivers notice on the Mustang GT4. Once a somewhat clumsy driver’s aid, invoking the GT4’s race-tuned ABS is simply how it’s done nowadays.

Digital Consistency

And then there’s the big one: digital engine and transmission management plus ABS and traction control. Last year’s GT350R combo relied on the driver to sense and control wheelspin, incipient lock-up under braking, yaw angle and change of rate of yaw angle and lateral grip among other things, but all those duties — except for sensing maximum lateral grip — have largely been taken over by computers and the engineers writing their software.

This is the sort of technology that makes fair drivers good and good drivers great, but doesn’t really let the great drivers run away and hide because the driving chops have been rather equalized among the better drivers. And in a series such as the Continental Tire SportsCar Challenge, with its need for a large number of drivers to man a full grid of cars during races that last around the clock, these powerful driver’s aids mean more consistent driving. The advanced amateurs that are the main customers for GT4 Mustangs are usually great drivers, according to Dean, but they just don’t get the seat time a full-time pro driver does so the advanced software helps them with consistency.

Modern drivers spend much time fiddling with knobs and switches on the steering wheel because those knobs mainly control how the car handles. KohR’s Dean Martin admits he’s still learning to “touch-type” the GT4 wheel saying he only has the radio button consigned to memory so far. Controls include headlight flashers for signaling slower traffic, a three-position fuel map, data-acquisition selection (practice, warm-up and race settings); master alarm reset, fuel capacity reset (hold down during refueling) and pit speed limiter (60 kph).

And just how sophisticated is the software? It’s a tuxedo cocktail reception at the Metropolitan Museum of Art compared to last year’s bowling-league beer bash. Previously drivers used the brakes as hard as they could without invoking the ABS because the ABS was choppy and abrupt enough to slightly upset the car. But this year’s velvety Bosch-based ABS means simply simply hammering the brakes into ABS mode and letting the braking computer hold the tires on the edge of adhesion achieve the fastest times.

More Bosch electronics found in the GT4 Mustang’s traction control. Legal in the Continental series, Bosch’s race-quality traction control — this is not the same production-car nanny blanket used to keep commuters from spinning tires in snow banks and speed-addled youths from looping their musclecars into the center median — is powerfully sensitive to racing conditions. Its nuanced feel for traction and ability to select several methods of stabilizing car control, from throttle to one-wheel brake intervention, means it can do things normal humans can’t.

Winning Ways

Previously Multimatic, Ford’s main contractor on the GT350S, GT350R, and GT4, has run its own team in domestic sports car racing, but elected to not compete against their North American customer teams and concentrate on European races in 2017. This automatically made KohR Motorsport the top-dog Ford team in the Continental Tire SportsCar Challenge; it also allowed Multimatic’s two ace drivers, Scott Maxwell and Jade Buford to make a single appearance in a second KohR Mustang at Sebring or cycle through the Martin/Roush car occasionally.

Avoiding the electrical problem that plagued Dean Martin and Jack Roush Jr. in the KohR #59 entry, the Maxwell/Buford pairing took their #60 to the Sebring winner’s circle (where we captured our photos before the #59 car received its menacing wrap). This was just one race after the season-opening round at Daytona and a real boost for both Ford and KohR, but then there were mysterious problems with the #59 car.

If it weren’t for wiring-based teething issues early in the season the KohR Motorsports Mustang GT4 could be leading rather than chasing a Porsche for the Continental Tire SportsCar Championship right now. A scant nine points separates the two marques going into the final race in the constructor’s championship.

“It was hard to spot, it wasn’t always there,” Dean said. “We’d go through the data and you’d see you were running on the battery, and so we thought we were failing alternators.” But once the wiring was sorted out and the rear axle made dependable the KohR team and its GT4 Mustang have been an increasing threat to its exotic European competition. Wins came at Sebring, Watkins Glen and Road America; backed by a second at Daytona.

With one race left in the Continental series the team has put Ford a close second behind the more numerously represented Porsche and ahead of McLaren in the manufacturer’s championship.

Ford — via KohR — can still just beat Porsche for the championship if Ford wins at the season-ending Petit Le Mans at Road Atlanta this weekend. It’s a task Dean Martin takes seriously. He removed himself from the driver’s at Laguna Seca strictly to concentrate on his team management duties. His driving chores thus went gave Scott Maxwell, which we expect will hold true at Road Atlanta as well.

And no matter how 2017 finishes, with the #59 car now well sorted 2018 could well be KohR’s and Ford’s year. Look for them running mainly under the Roush Performance name next year as KohR Motorsports makes the best of every advantage it has.

Photo gallery


About the author

Tom Wilson

Infatuated by things that make noise and go fast, Tom has been writing about cars and airplanes for over 35 years. So far that’s meant a decade editing Super Ford magazine, plus long associations with Road & Track, MSN Autos and more lately Kitplanes magazine. It’s also meant some SCCA racing and a lot of fun sampling everything from Trans Am cars to F1 chassis as part of “work.” Besides the racing hobby Tom enjoys flying his biplane, plinking tin cans and messing around with telescopes.
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