HONDA The Power of Dreams

When developing a car to meet new regulations introduced to the GT500 class of SUPER GT in 2014, Honda chose the second-generation NSX as its base vehicle. While the new regulations stipulated such things as a common monocoque structure designed for cars with a front-engine rear-wheel drive (FR) layout, Honda tweaked the design for its competition car to use the same midship-engine rear-wheel drive (MR) layout as on the mass production NSX.

Manufacturing the advanced development car in 2013, it found cooling around the engine to be an urgent issue. The 2.0-liter inline four-cylinder direct injection turbocharged engine was mounted to the rear bulkhead of the common monocoque, with air intake positioned on the left side and exhaust on the right.

However, when running the advanced development car, Honda identified an issue with the temperature of air being too high when taken in via the side air intake, which was positioned in front of the right rear fender. After some investigation, it found that hot air exhausted from the hood outlet, after passing through the radiator, was flowing into the intake.

Without enough time to implement drastic countermeasures before the opening round of the 2014 season, Honda worked on the shape of the side mirrors to change air flow so that the hot air exhausted from the hood was not directed to the side air intake. This achieved a reduction in air intake temperature of approximately 7 degrees Celsius (°C). After implementing a more fundamental countermeasure in 2015, it raced with an orthodox mirror shape.

CFD simulation results

Intake air from the front bumper passes through the two-part radiator to undergo heat exchange, where it cools down the engine coolant while gaining heat from the hot radiator. As shown in the image, the heated air is exhausted via the hood outlets. The exhausted air then cools down as it undergoes heat exchange with the ambient air (color changes from red to yellow, green, and blue as the temperature drops). However, the CFD simulation shows that the air still has considerable heat in it around the side intake. High intake air temperatures increase the likelihood of engine knocking, so it is naturally preferable to keep intake air temperature as low as possible.

This CFD image shows air flow down the left side of the car. The actual air intake is on the right side. The simulated air flow across the car body shows that hot air exhausted from the hood flows toward the side intake. After verification, it was found that vortexes generated by the side mirrors caused the air to be blown downward. While this phenomenon was understood during the 2013 season, there was not enough time to implement drastic countermeasures before the opening of the 2014 season, so it dealt with the issue by changing the shape of the side mirrors.

Side mirror

The side mirror shape was changed to ensure the flow of hot air exhausted from the hood was not directed to the side air intake. As a result, the temperature of intake air dropped by approximately 7°C.

Honda faced other heat-related issues as well. After the opening of the 2014 season, it became apparent that heat damage was frequently occurring within the engine compartment due to heat generated by the turbo engine and the lack of sufficient air flow for cooling. Heat was deforming the plastic taillight covers and the polycarbonate rear window. Of a more serious nature, the heat was damaging the wire harnesses, and components of the electrical system, including solenoids and sensors, were failing. The drive-by-wire throttle motor was also at risk of switching to safety mode due to temperature increases.

While the advanced development car had also been driven in the middle of summer, there were no real signs on trouble at the time. This was believed to be the result of gaps, including around the cowl seams, that allowed heat to escape due to the fact that because the car was still in development, it was not yet precision-engineered to the level of being able to enter an actual race.

To deal with the problem of heat damage, Honda added slits to the top of the rear window in the fourth round to allow hot air to escape. The rear window material was also changed from polycarbonate to carbon fiber reinforced plastic (CFRP) because of its good thermal resistance. These measures dramatically reduced the temperature within the engine compartment (from 225°C down to 90°C) and eliminated the heat damage problem.

The intercooler, which is used to cool down air pressurized and heated by the turbocharger, was originally mounted in a low position on the right side of the car. In addition to a body-side intake, cooling air was also ducted from the front grille. However, because cooling was insufficient with this setup, the layout was changed in the fourth round to position the intercooler above the engine. Although this raised the center of gravity, the emphasis was on engine performance instead. At the same time, it changed the structure of the system by bringing cooling air from the front grille down two ducts, one on the left and one on the right, with the left duct cooling the left half of the intercooler and the right duct cooling the right half.

Intercooler layout changed from the fourth round of the 2014 season

It was logical to locate the intercooler on the right side of the car because it was used to cool down air compressed and heated by the compressor for the turbocharger, which was also located on the right side of the engine. Cooling air supplied from the side air intake was insufficient, so a duct was used to bring air from the front grille to cool down the intercooler. However, this solution was also insufficient. To improve air flow within the engine compartment, Honda moved the intercooler to a position directly above the engine despite knowing that this would raise the center of gravity. It therefore changed the structure of the system by bringing cooling air from the front grille down two ducts, one on the left and one on the right, with the left duct cooling the left half of the intercooler and the right duct cooling the right half. This twin left/right layout was employed from the fourth round (Sugo round) of the 2014 season.

The intercooler mounting position was changed again for the 2018 specifications. Moved lower again, like the position in early 2014, the intercooler was positioned on the left side (intake side) rather than the right side (exhaust side). The two ducts leading from the front grille remained the same as when the intercooler was positioned above the engine, but this time with the left duct used for the intercooler and the right duct used for air intake. Until and including 2017, air was taken in via the side air intake on the right side of the car, but bringing ducts from the front grille enabled ram pressure (wind pressure when driving) to be used to lessen the load on the turbocharger.

2018 intercooler layout

The intercooler was positioned on the left side of the car in 2018. The right side duct was used exclusively for air intake, enabling ram pressure (wind pressure when driving) to be used.

As a trial, one car only was equipped with a new oil system in the final round of 2017, with official adoption in 2018. With a midship engine layout employed for the NSX CONCEPT-GT (2014–2016) and the NSX-GT (from 2017), the engine oil lubrication and cooling system was mounted compactly on the left side of the engine. Honda decided to move the oil system toward the front to correct the effect of weight distribution toward the rear (removal of the oil system from the left side of the engine then enabled the intercooler to be mounted at the bottom left of the engine).

After solving the heat damage problem, the main focus of development turned to reducing weight and lowering the center of gravity. Honda removed its hybrid system after the 2015 season, so the handicap weight for hybrid cars was also removed. However, the MR handicap remained despite the MR layout not actually providing a performance advantage, with the imposition of a minimum weight of 1,049 kilograms (kg) making Honda’s car 29 kg heavier than the competition. (Minimum weights of 1,034 kg and 1,044 kg were also imposed at various times.)

Honda decided to reduce weight as much as possible and distribute weight in the same manner as an FR layout car, which it felt would enable it to use tires developed for FR layout cars and therefore maximize performance. Lighter cowls introduced in 2018 were part of these efforts to shift weight distribution forward. It developed two cowl types; a lightweight cowl that was lighter than normal cowls and an ultra-lightweight cowl that was lighter again. The latter of these cowls focused on reducing weight at the rear, which was achieved by not only reducing the weight of individual parts, but by also optimizing the structure of members combining multiple parts.

When developing the cowls, loads equivalent to those at maximum speed were simulated to analyze pressure displacement distribution. The cowls were introduced after confirming that there were no problems with strength. However, because of the possibility of damage from unexpected inputs, Honda warned the team attaching and removing the cowls to take additional care during handling.

Simulation analysis results

Simulations were also used when lightweighting the cowl. Pressure displacement distribution was analyzed using CFD to simulate loads equivalent to those at maximum speed.

In terms of vehicle weight, center of gravity, and weight distribution changes from 2014 to 2019, Honda achieved a major reduction in vehicle weight in 2016 with the removal of the hybrid system. It further reduced weight in 2017 with a comprehensive redesign premised on the car not having the hybrid system, and thereafter consistently reduced weight through additional lightweighting measures.

Honda raised the center of gravity significantly in 2015 with a change of mounting position for the intercooler. Implementing a comprehensive redesign in 2017, it was able to return the center of gravity to where it was in 2014, with another change to the intercooler mounting position achieving a significant lowering of the center of gravity in 2018. Through a series of subsequent lightweighting measures, it continued lowering the center of gravity even further.

Honda shifted weight distribution dramatically to the rear when minimum weight changed from 1,090 kg to 1,077 kg in 2015. This was due to the 13 kg weight reduction being taken from the front because there was nothing available in the rear to be lightweighted. The oil system layout was then changed in the final round of the 2017 season, with official adoption in 2018, to dramatically correct the effect of weight distribution to the rear. Further corrections were achieved in 2018 with the introduction of an ultra-lightweight cowl and other measures.

Since 2017, Honda has been actively utilizing a driver-in-the-loop (DIL) driving simulator to achieve more efficient development. The advantage of DIL is that it enables evaluations at the target setting stage using highly accurate driving characteristic predictions. In addition to being used to judge whether development items reflect the targets in the initial stages of development, it is also used in the final stages of development to refine the setup.

The benefit of DIL is that not only professional drivers, but the developers themselves can try out new items, allowing them to experience the effects of their own developments. Thankfully, the resulting deeper understanding of technologies led to the generation of ideas that were not possible prior to use of the DIL driving simulator.

Driving simulator

Since 2017, Honda has been actively utilizing a driver-in-the-loop (DIL) driving simulator to achieve more efficient development.

In 2019, Honda made full use of simulation technology to develop a new front suspension system. During simulation-based investigations, it found that lap times improved as caster angle increased (axial tilt of the kingpin connecting the upper and lower ball joints of the double wishbone suspension).

However, there was a tendency for response to become heavier as caster angle increased, with this and other issues making handling feel worse. Honda was also worried about its ability to guarantee strength because the component layout had to be forced for it to work well. Another concern was whether the change fell within the capabilities of the power steering system, which was another common part, so many matters had to be checked.

Development this time made use of desktop simulations and a driving simulator to confirm the feeling of handling, with exhaustive testing of characteristics but not actually having to manufacture parts. Because parts could also be rearranged on the computer as well, time and costs associated with resetting configurations were also reduced. Experiencing the changes on the simulator helped the developers to understand them as well.

Finally, in addition to employing high caster specifications, Honda offset the pushrod attachment point, which is normally along the kingpin axis, for the 2019 specifications front suspension. This had the effect of changing geometry when steering, thereby reducing the height of the car. Honda was only able to implement such drastic design changes because simulation technology enabled it to conduct scrupulous preliminary investigations. It was able to move to field tests only after setup had already progressed to a certain point, which enabled it to effectively utilize the limited time it had for testing.

Front suspension

Development from the NSX CONCEPT-GT to the NSX-GT actively utilized the latest technologies, including DIL, to effectively implement measures in rapid succession and approach the ideal setup. To summarize, Honda focused on lightweighting, lowering the center of gravity, and optimizing weight distribution as it worked to faithfully tackle areas thought to be fundamental to improving driving performance. The knowledge acquired through development over this period has been applied to subsequent development as well.