Although the Pikes Peak International Hillclimb and race events where GT3 finds itself in a regular race season calendar are both forms of motorsport, they have almost nothing in common. As such, taking a GT3 race car designed to compete in circuit racing events on permanent tracks worldwide and adapt it for a sprint hill climb event up a very high mountain in Colorado, USA, is no easy feat. That is the challenge that Bentley Racing took on for its attempt at breaking the road car based vehicle record up the 12.42-mile trek up Pikes Peak Highway with 156 turns that begins at 9,390 feet and finishes at the 14,115-foot summit.
The primary challenge of Pikes Peak for all competitors running internal combustion power is developing and tuning the engine to cope with the altitude, both in producing power at the base as well as the summit while also keeping temperatures under control. The engine in Bentley’s Pikes Peak challenger is a development of the race-proven GT3 4.0-litre twin-turbo V8. Available power is around 550 bhp (BoP dependent) in GT3 trim with a boost curve enforced by the sanctioning body to keep the car competitive with its competition on the track.
For the Pikes Peak challenge, there are no such limits in engine performance or power delivery, and for the hill climb, Bentley set its sights on extracting the most it could from the base GT3 powerplant. ‘We wanted to target about 800 to 850 horsepower at altitude,’ explains David Argent, Technical manager – Motorsport Engineering at Bentley Motors. ‘The main driver in terms of power output was targeting turbo housings and turbo sizing to work at altitude and not at sea level.’
Selecting the target output figure was driven in part by the driver, Rhys Millen, a veteran competitor on the Pikes Peak International Hillclimb and his knowledge from the history of the cars that he’s run there. With Bentley’s knowledge of the behaviour of the engine and Rhy’s experience, the team calculated 850bhp at altitude to be sufficient to power the car to the record and be drivable on the run.
‘At a certain point, you will get traction limited,’ highlights Argent. ‘You can’t just stick a thousand horsepower and 1400 newtons to torque through a rear-wheel-drive because you won’t be able to use it. We looked at suggested figures from Rhys and some simple simulations with inside Bentley to suggest what power figures we should target.’
With the GT3 engine running anywhere from 525-550bhp up to around 600bhp (depending on it’s the series teams entered it in), it is a reasonable delta of about 300 horsepower to find for the Pikes Peak version. Additionally, that’s not comparing apples with apples because the engine needs to provide 850bhp at a high altitude where there is as much as 30% less air density than at sea level.
‘Making sure we get enough fuel into the engine to produce the additional power was a primary development target,’ explained Argent. ‘We sourced appropriate injectors and fuel rails. The injector size and flow were based on the power figure we want to achieve at the bottom of the run at 9,390 ft above sea level. We added port injection to work with the direct injection that already existed on the engine to achieve the volume of fuel we needed to generate the power we wanted and had to work out how we could achieve the power figure in controlling both injector types.
‘We worked in partnership with M-Sport on this engine, and it was Nigel Arnfield, who’s designed the rallycross, WRC and GT3 engines from the company which specified the injectors we used, their location and control.
Port injection and direct injection typically desire very different intake designs, certainly towards the combustion chamber. Bentley was fortunate that it already understood the airflow through the common intake manifold on this engine. Following CFD studies, no changes had to be made to work with the additional port injection system other than locating the port injectors.
Bentley has been working with Esso’s renewable racing fuel for this project – a blend developed from bio content with an ethanol base equivalent to an E85. The characteristics of the fuel meant the engine required little work in calibration to operate on it, despite it being a more volatile mixture than the traditional race gasoline used in the GT3. There are no additives to the fuel blend to aid oxygenation of the combustion fluids to compensate for the reducing are density as the car climbs Pikes Peak.
With the increase of power and chaos in the fuel and air mixture, Bentley had to be careful to ensure that they didn’t reach the knock limits of the mix before the desired ignition timing. Although the ethanol content within the Esso so bio racing fuel blend offered some evaporation and therefore cooling of the charge mixture before reaching the combustion chamber, the team also implemented a water injection to aid the charge mixture cooling further and reduce the risk of knock. The water injection was located pre-intercooler and significantly brings down the charge temperatures. ‘We tried going posting to cooler as well to try and add a little more water in there to keep the not level down a little bit further, notes Argent. ‘However, we got to a point where you saturate the mixture, and you start losing performance from it.’
‘We had to get more air into the engine to coincide with fuel, so we also went for bigger turbochargers with appropriately sized compressors and turbines,’ says Argent. ‘Consequently, we have turbos with a large compressor and a significantly small turbine to compress that less dense air and get the least turbo lag possible. Another significant design criterion was reducing the turbo lag as much as possible. There are so many first geat hairpin corners, so any lag will cause us to drop seconds throughout the whole run to the summit.’
The engine map works from a boost target, as per the GT3 version of it, and throughout the run up the Pikes Peak Hill Climb, the wastegate controller compensates for the percentage of lost air density. There’s a boost target set in the controller and a barometric pressure sensor looking at the density to feed into the wastegate adjustment to achieve that desired boost target.
‘One of the first things we found out when we were out at the Pikes Peak is that the wastegate control we selected when we’re at sea level wouldn’t allow us to meet those boost targets even with the wastegates fully shut,’ notes Argent. ‘We did some optimisation to get the right size turbo housing to allow us to have the response and meet our boost targets. We had to go to an external wastegate because the internal wastegate wasn’t big enough to dump what we needed to control the boost.’ The ratio of compression from atmospheric pressure to turbo pressure is peaking at 3.7.
Knowing that running at high boost pressures will increase cylinder pressures meant that Bentley had to address the reciprocating and rotating assemblies inside the engine. Working alongside M-Sport and taking their experience on other programs that run a lot higher boost than the Bentley GT3, the team has the respective partners to supply the parts to suit. M-Sport sourced pistons and con rods to cope with the high combustion pressures.
Bentley has been working with Akrapovic exhaust systems for several different projects that they have done in the past. This Pikes Peak version of the Bentley Continental GT3 race car is one of the most power-dense powertrains the company has worked on and required some special attention in areas. ‘We provided them all the data from what we were expecting to produce and the turbo sizings we were going to use, and they made recommendations from their side on what sizings we should go to so as well how we ended up with our manifold and exhaust designs on it, says Argent. ‘When you pushing something this hard, you need to make sure we get the right level of part on the car.’
Bentley found no issues with the setup during the limited testing they could do before or the entire run as far as the reliability of all of the systems is concerned. Before the event, teams are only allowed to do sections do off the course to test performance. We ran to the top before the official day of the event, but it was only from Devil’s playground up – a two and a half miles stretch before the summit,’ says Argent. ‘Within that period, we had no struggles temperature-wise, knock limit wise or boost pressure wise – we were still looking strong, so we were confident in our setup.’
‘The real engineering challenge in this event is sewing all these little sections of the run together from all your testing and make the best-experienced judgment for race day on how things are going to turn out. One thing it’s taught me in the past is that you can go there and run the course in sections with the car to test theories, but it may not translate. We did that, and the coolant temperatures were acceptable every time we tried, so there was never a concern. But on race day, you get the significant thermal loading when you add all three sectors together. The ‘W’s’ is the worst area because it’s very aggressive on gear shift but very low speed, so you’ve got no air passing over the radiators, and you’re hammering the engine.’
‘We reached 138 degrees water temperature on race day when we did a sighting run in the Continental GT road car, so we’re very mindful of that loading, and we considered that the design of the engine cooling systems. The engine uses Mobil 1 0W30 synthetic racing oil used in the GT3 cars for years.’
The Bentley Continental GT3 Pikes Peak proved to be the fastest racecar running on renewable fuel at this year’s Pikes Peak International Hill Climb yesterday, finishing 2nd in the Time Attack 1 and a stunning 4th overall. The car, driven by the former King of the Mountain Rhys Millen (NZ), competed on a shortened course due to icy conditions at the Pikes Peak summit, crossing the finish line in 6:36.281 despite suffering a late engine issue. The time was good enough to beat all other sustainability-focused entrants, including fully electric competitors.
Snow and ice near the summit meant that organisers repositioned the finish line lower down the mountain at 12,780 ft at Devil’s Playground, cutting out the last third of the normal course, which sadly made Bentley’s Time Attack record-breaking ambitions impossible to realise. Nonetheless, the Continental GT3 Pikes Peak (jointly developed by a collaborative team of Bentley, Fastr, Roger Clark Motorsport, M-Sport and Rhys Millen Racing) set a blistering pace over the first two sectors, entering the third and final sector 12 seconds ahead of its nearest rival. A few corners from the finish, a boost pressure problem meant Millen lost 16 seconds, dropping back to second place in class.
Bentley Continental GT Pikes Peak
For more insight into Bentley’s Pikes Peak Hill Climb Challenger, be sure to pick up Racecar Engineering August 2021 issue available here.