Located in Fife, about half an hour north west of Edinburgh, Knockhill Racing Circuit is Scotland’s national motorsport centre, and hosts rounds of the British Touring Car, British GT, British Formula Three and British Superbike Championships.
This short 1.2669-mile long, 30ft (10m) wide, nine-corner circuit twists drops and climbs and has a 121ft (37m) change in elevation from its lowest to highest points. Uniquely, it has a dispensation for races to be run in both clockwise and anti-clockwise directions, although the former is more the norm.
Because each lap is so short there are no long straights to offer respite for drivers, so you have to concentrate 100% of the time. And because the combination of corners and elevation changes comes at you thick and fast you really have to adapt your driving style to suit the track.
In the usual Porsche format we would follow the pace car, and because of the short circuit we were allotted two eight-lap sessions, with a breather for a chat with the instructor in between.
My pace car driver was Chief instructor, Matthias Hoffsümmer, with whom I have enjoyed many fast sessions over the years. Because of our long acquaintance he really went for it after two familiarisation laps, and according to the video camera timer our two 718 Cayman GT4s were lapping at a consistent 0.58 min.
To put this in perspective the outright lap record for this short and very technical track is 0:41.9 minute, set by two-time British Sprint Championship, Heather Calder in her Gould GR55 during the 2018 season.
A specialist race machine that dominates the British Hill Climb Championship, the GR55 is built around a F3 chassis, and is powered by a 680hp F1 derived 3.5 litre Cosworth V8 engine coupled to a sequential gearbox. And of course it has F3 levels of aerodynamic downforce and grip from its full race slick tyres.
A bit closer to the real world, current BTCC racecars lap around the 0.52 minute mark, which makes the 718 Cayman GT4's inherent capability out of the box on street legal tyres all the more impressive.
On Knockhill’s demanding dropping and climbing twists and turns the GT4 was really in its element. Of all the manufacturers Porsche is the only one who consistently nails the art of electric power steering, and the GT4’s Alcantara covered 360mm helm feels like an extension of your fingers.
The gearshift is similarly intuitive, requiring just firm finger pressure to execute each ratio change with rifle bolt precision. The pedals are perfectly aligned for heel and toe action under firm braking, which I have practised since my Formula Ford days in my ‘20s, so while a button on the console activates the Auto Blip function for tyros I ignored this.
With the damping of the adjustable and semi-solid bushed suspension set to Sport mode the GT4 responds fluidly and progressively to trail braking, its pointy nose diving for apexes like a bloodhound hunting down its prey.
The painted kerbs at Knockill are flat but a few of the corners have what is locally called “sausage rolls” at the extreme inside of each kerb to discourage you from using any more of the tarmac.
With almost no lost motion in the GT4’s responses, I was able to accurately place the inside wheels within what seemed like an inch of these sausage rolls lap after lap to maximise the track width.
Before each of the two fast cresting right handers, which are third gear affairs a light touch of left foot braking settled the GT4’s nose. It is in situations like this that this car shows off its immaculate balance and stability, hugging the chosen line perfectly under extreme suspension and driveline loading. With its comparatively greater rear weight bias a 911 would have been less confidence inspiring under these conditions.
The stock GT4 brakes are 380mm cross-drilled, vented discs all round clamped by six-pot and four-pot callipers front and rear respectively. The GT4s at the event had the PCCB option, which is straight from the GT3, with 410mm discs and six-pot callipers in front, and 390mm discs with four-pot callipers aft.
At the pace we were going we had to use as much track as possible to get the best positioning before turn-in and the best line into the three right hander coming off the short straights into Scotsman, Clarks’s and Taylor’s Hairpin. This meant planting the left wheels on the smooth painted kerbs to ‘extend’ the track width under braking in the last few metres before turning in.
Braking hard on the painted kerbs is cautioned against in racing school and is something we would studiously have avoided in damp conditions, but the superb stability of the GT4 under hard braking allowed us to take this liberty in the dry.
One of the questions I directed at the powertrain engineer was about the tall first, second and third ratios of the original Cayman GT4 of 2015 that do not allow the motor to get on cam quickly enough. This makes the engine feel less snappy than it could be at times given the excellence of its chassis balance. That said it is something you notice more in road driving than on track where you stay in the upper end of the rev band.
The bad news is that Porsche has only beefed up some internal components like the steel synchros to take the extra power and torque of the 4.0-litre motor with no other changes. The good news is that this extra power and torque and the crisper response of the bespoke higher revving engine subjectively eradicate about 75% of the high gearing issue.
The resonance induced by the new cast alloy intake manifold with its small and large resonance flaps charges the airflow throughout the rev band, adding to low and mid-range torque while enhancing high-end power as the engine screams towards its 8,000rpm limiter. That said with the additional backpressure from the particulate filter and other legal constraints, the 420hp output falls short of this engine’s true potential.
You notice this more on track where the top end is not quite as strong as you might expect, but this is relative as the benchmark for comparison is the awesome 500hp+ GT3 motor that peaks at 9,000rpm.
The 4.0 litre flat six used in the 718 Cayman GT4 and 718 Spyder is unique to these two new mid-engine models and is based on the 9A2 Evo motor that powers the 992 Carrera. The main differences are the lack of turbochargers and a displacement increase from 3.0 to 4.0 litres, along with various other modifications required to engender a high-revving naturally aspirated engine with the characteristics required of a car wearing the GT badge.
Displacement is 3,995cc from an oversquare bore x stroke of 102.0 x 81.5mm using bespoke Mahle pistons and 3.0mm shorter (138mm) connecting rods that use the same 22mm diameter gudgeon pin. Compression ratio is up slightly to 13.0:1. Another innovation is the hydraulically actuated roller rockers for the valve train, required by this higher revving derivative of the 9A2 Evo motor.
The result is an output of 420hp at 7,600rpm and 420Nm of torque between 5000-6,800rpm, with the engine developing 105.1hp/litre. The rev limiter kicks in at 8,000rpm.
“A major change in the combustion chamber arrangement compared to the 3.0 litre turbocharged engine in the Carrera sees the spark plug in the centre and the injector offset and aimed slightly towards the exhaust valves to encourage better burn characteristics,” explained engineer Fabian Zink. “Incidentally the one component this engine shares with the 918 Spyder hypercar is the spark plug!”
Direct fuel injection with Piezo injectors has made a significant contribution to both power and emissions. This camshaft driven high-pressure system runs at 200 bar.
Durable and reliable the crystal control system in these injectors is perfect for a high-revving sports car engine as their faster reaction time compared to conventional magnetically activated injectors allows them to accurately squirt fuel four to five times per combustion cycle, and with greater accuracy. Their funnel end shape also obviates the tendency of conventional injectors to occasionally form droplets at their tip that can result in wall wetting and unburnt fuel, which is bad for emissions.
Engineer Joachim Meyer explained that they had to completely revise the electronics in the car since the Continental ECU used before is not compatible with the digitally driven Bosch DI system. “This is not easy as the old and new architecture ‘speak’ a different language for which translation is difficult. The new system is a combination of FlexRay and CAN-BUS architecture,” he said.
The new motor weighs 200.8 kg dry, which is about 3.0kg more than before. While some weight saving was made, this was countered by the particulate filter and the heavier alloy intake manifold that replaces the plastic one. The new saddle shaped exhaust is more efficient however.
Tipping the scales at 1,420kg, the new GT4 weighs 60kg more, the result of the heavier motor, particulate filter, and more standard features like PCM.
But aerodynamically, the new GT4 is streets ahead of its predecessor. The combination of a new front air splitter, completely flat underbody panelling, larger rear air diffuser and a new rear wing conspire to increase overall downforce by a massive 50%.
Thanks to the air curtains that smoothen out airflow through the front wheel arches and the big rear diffuser the drag coefficient is unchanged at 0.34.
The front splitter and rear wing are adjustable and have Standard and Performance settings. In the former the downforce numbers over the axles are 10kg front and 53kg rear at 200km/h. In Performance settings these increase to 16kg and 64kg respectively.
Enthusiasts agree that the outgoing GT4 had a near perfect chassis set-up that could easily handle another 50hp or more with ease. While the new car comes with a 35hp power boost a few small changes had to be made to compensate for the slight increase in weight and the significant increase in aerodynamic downforce.
Thus, while the suspension hardware and the geometry are the same, the latter including 1° 30’ of negative camber at each corner, small adjustments were made to the spring and damper rates.
Just to recap, the Cayman GT4 uses the 991 GT3 front end with H&R springs and Bilstein dampers.
The GT4’s rear suspension is fundamentally different from other Cayman models and features a bespoke larger and stiffer sub-frame with bespoke engine mounts for the 4.0 litre motor, and GT4 specific hub carriers, beefed up driveshafts, larger wheel bearings, motorsport-style forged alloy split lower arms, fully adjustable coil-overs, adjustable tie rods, and a three-position adjustable anti-roll bar with reinforced mounting points.
Like the front end, Rose joints are used throughout, but in a departure from the GT3’s rear end, the GT4’s coil-overs feature a helper spring to improve the low-speed ride.
The pivot points and geometry ensure that the lower wishbones are parallel to the ground at a static ride height 18mm lower than the Cayman GTS and 30mm lower than a stock Cayman.
The chassis also makes full use of the latest Michelin Cup 2 trackday rubber, with Dunlops as an alternative. This means 8.5J and 11.0J x 20-inch wheels wrapped in 245/30ZR20 and 295/30ZR20 rubber. Tyre pressures are 2.0 bar in front and 2.0 bar at the rear.
As the 718 Cayman GT4 is aimed at track day junkies its Nürburgring lap time is important. This is an impressive 7:28 minutes, equalling the time of the 997 GT3 RS 4.0. Porsche also pointed out that this is 12.0 seconds a lap faster than the previous Cayman GT4 of 2015, and 4.0 seconds faster than the Carrera GT of 2004.
While 7:32 minutes was the Carrera GT’s official lap time at its 2004 launch the revised Michelin tyres that debuted along with the 918 Spyder in 2013 chopped 10 seconds off that lap time and dramatically improved stability in normal road driving. This shows just how tyre technology contributes to the handling and grip of cars today, and is also responsible for nearly half of the 718 Cayman GT4’s gain over the 7:40 minute lap set by the previous model.
The stopwatch shows the clear performance gains. Top speed is up from 295km/h to 304km/h while 100km/h is the same 4.4 seconds, the car is slight quicker after that with 160km/h in 9.0 seconds and 200km/h in 13.8 seconds.
The 718 Cayman GT4 is an exceptional car in every respect. It goes hard, has the engaging flat six scream that we have missed since the 718 replaced the 981, and it looks great too.
Considering its reasonable price tag it is tempting to say the 718 Cayman GT4 is a cut price GT3. But its handling balance and character are so different from the 911 that this would be doing the GT4 a grave injustice. The fact is that the 718 Cayman GT4 is a five star sports car in its own right, and one that simply has no peers in its class.