The topic of horsepower in the Subaru BRZ and its twin, the Toyota GR86, often sparks debate among car enthusiasts. Specifically, discrepancies between manufacturer-claimed crank horsepower (chp) and real-world wheel horsepower (whp) figures observed on dyno tests raise questions about the true output of these sports cars. This discussion becomes even more intriguing when considering how engine temperature might influence dyno results.
To delve deeper into this, I consulted with a nuclear engineer, whose expertise in fluid dynamics provides a unique perspective on engine efficiency and performance. His initial question was straightforward: where would the supposed efficiency gains come from that would bridge the gap between claimed and dyno-measured horsepower? After explaining the common theories suggesting improved fluid dynamics or component efficiency, his response was that such gains would likely be insignificant, certainly not enough to account for the percentage difference seen between manufacturer figures and dyno readings.
This led us to consider the possibility of dyno inaccuracies or inflated readings. While some might argue that dynos are inherently flawed, my engineer brother pointed out that for many other vehicles, the claimed chp closely aligns with dyno-measured whp, within a reasonable margin of error. However, the BRZ/GR86, along with the Subaru WRX VB generation, consistently fall short, showing a more significant power loss than expected based on typical drivetrain efficiency. As a comparative example, he highlighted the BMW A90 Supra with its 6-cylinder engine, advertised at 382chp, which routinely dynos around 390whp. This over-delivery suggests potential under-rating by the manufacturer, a known characteristic of BMW, rather than a dyno anomaly.
The conversation then shifted to the peculiar phenomenon reported by some BRZ/GR86 owners: the car seemingly making more power as it gets hotter during a dyno session. His reaction was immediate skepticism. He questioned whether the cars were reaching proper operating temperature before dyno testing. From a physics standpoint, he emphasized that a car should ideally produce less power as engine, transmission, and differential oils reach their optimal operating temperatures after extended driving – typically around 30 minutes on the road. Increased heat generally leads to decreased efficiency, not the other way around.
Supporting this real-world perspective, anecdotal evidence from track days and regular driving, such as observations from @latapx and others in online communities, often fails to corroborate the inflated horsepower figures suggested by some dyno results. The cars simply don’t consistently perform in the real world as if they possess significantly more horsepower than advertised.
Considering these points, my brother proposed a compelling hypothesis: if dyno results show increasing power output as the car gets hotter or further into a dyno run, it’s plausible that the Engine Control Unit (ECU) is programmed with specific parameters to create this artificial situation. This wouldn’t be due to genuine efficiency gains from temperature increases, but rather a deliberate calibration within the ECU. This theory offers a potential explanation for the dyno discrepancies and the apparent disconnect between dyno numbers and real-world performance experienced by BRZ and GR86 drivers.
