My first race in the Thunder Roadster was everything I wanted it to be except for one thing: oil temps. As I raced through the field during the Aussie Pursuit race, I was closely monitoring my oil temperature gauge as the temperature crept to 290 degrees. Flirting dangerously close to being pinged at 300.
Fortunately, I was able to ‘manage’ these temps with a bit of short shifting to finish the race. However, the Barber Motorsports event would be one month away and likely 10 degrees hotter. So it was time to dig into the Thunder Roadster’s cooling system to get the oil temps under control.
Understanding the Current Cooling Setup (Water Cooled Head Jacket)
My Thunder Roadster is equipped with a Yamaha FJ1200/1250 engine that was initially designed for air cooling. This engine, however, has been fitted with a head cooling jacket. The jacket is an aluminum box that encases the original cooling fins around the engine. An electric pump then circulates coolant through this jacket and a traditional radiator. Effectively creating an external heat sink around the head to transfer engine heat to water.
Unlike a modern liquid-cooled motor that has coolant passages inside the motor, this design only pulls heat from the outside. This works well in the Thunder Roadster to stabilize head temps because there is not a lot of airflow inside the engine compartment.
However, this setup still relies on the engine oil to do a bulk of the cooling. As evidenced by the 290-degree oil temperature during the long race at Nashville Superspeedway. Another interesting data point is that the coolant temperature peaked at 140 degrees and remained at this level. That had me scratching my head until I understood how the cooling jacket works.
Cooling Solutions for the Thunder Roadster
The goal is to get oil temps to 250 and below under consistent lapping in the hot Southeast summer. I have five solutions in mind:
- Relocate the oil cooler from the engine compartment to the front of the radiator.
- Increase oil line size from -8AN to -10AN or -12AN to improve heat exchange.
- Convert the oil cooler to a liquid-to-liquid laminar flow unit. Shedding the heat into the underutilized coolant loop.
- Run two coolers for additional heat exchange.
- And if all else fails, chemical cooling or water/methanol injection.
That is my solution path, and I am starting with the simplest first, relocating the oil cooler to the front.
‘Factory’ Yamaha Powered Thunder Roadster Oil Cooler Placement (2 ways)
Before I jump into my solution, let’s look at how the Thunder Roadster oil coolers are typically placed. Initially, Thunder Roadster oil coolers are located on the intake side of the engine compartment behind the front right wheel with a small puller fan for airflow. The front ductwork of the car is then used to direct airflow over the engine fins for air cooling. This initial configuration had a reputation for catastrophic engine failure.
Which brings up the two solutions I have seen.
Cars without the water jacket run multiple oil coolers in front of the engine (up to 3) that are fed through ductwork. Anecdotally, I have been told these setups stabilize the oil at 240 degrees. Front ductwork is also used to route air over the engine.
And then there are cars like mine with a water jacket to thermally stabilize the head and combustion chamber while retaining the original oil cooler.
So why can’t I have the best of both worlds with a thermally stable head and a large, optimally placed oil cooler to stabilize the entire system?
Relocating the Oil Cooler
That is what I decided to do: relocate the original oil cooler to the front of the car and in front of the radiator. This would ensure fresh and forced ambient air instead of the fan-drawn engine compartment air in the current configuration.
To do this, I removed the radiator shroud and welded two tabs to the square tube in front of the radiator. I positioned these so that the oil cooler would bolt to the brackets and hold the majority of its weight.
To support the top of the oil cooler, I used a radiator fan mounting kit and ensured proper spacers to maintain a 1/4 gap between the oil cooler and the radiator. This would ensure the fins of each system do not rub against each other, but it is also close enough that the radiator fan pulls strongly through the oil cooler.
I also ditched the 13-inch puller fan and replaced it with an oversized 16-inch fan from the Super Touring 6 Miata.
Extended lines were then made out of -8AN PTFE hose. Although -8AN is small, I stuck with that sizing as that is the size every Thunder Roadster oil cooling loop seems to run. (More on that later. Link)
Finally, I reinstalled ducting make minor adjustments to pass the new lines through.
Results
Temperatures at Barber would be about 5 degrees hotter than the Nashville event. However, the sun would be relentless with the racing surface being close to 140 degrees. The oil temps stabilized at 265 to 270. Making the oil cooler relocation a great success. Bringing the temp down at least 20 degrees, but still not where I want it.
I also noticed that the engine heat soaks pretty noticeably at these temps, with lap 2 being my fastest lap in each race. The data tells the story, as you can see, lap times and top speed degrade as the engine heats up to the tune of over 1 second.
And finally, an unintended consequence of relocating the oil cooler in front of the radiator is thermal load stacking. The coolant loop temperature increased by 20 degrees from 140 to 160. This is the result of the radiant heat of the oil cooler heating the radiator, and also the oil cooler heating the air before it enters the radiator. This was something I had not accounted for. Fortunately, there is a ton of thermal overhead in the coolant loop.
What’s Next
Moving the oil cooler in front of the radiator in the Yamaha FJ1200/1250 powered Thunder Roadster was a massive step in the right direction. However, with a target of sub-250-degree oil temps, there is still efficiency to be found. I still can’t help but wonder if the restrictive -8AN line sizes are a bottleneck. Would switching to -10AN or even -12AN improve flow through the system and improve oil cooling performance? Or should I rethink this system completely and switch to a laminar flow oil-to-coolant exchanger and take advantage of the massive thermal overhead of the radiator?
