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I spent the day on my dyno dialing in the NSX. No catastrophic failure, I didn’t get the cancer, no dangerous conditions ruining the car—just 600hp on a Dynojet. The car wanted a lot more, but I got nervous about head lift and wanted to see how this power level performed on the street. 600hp in an NSX is a handful. Sidenote: those of you pushing higher on street cars are wild, lol.
And this is still with the radiator plumbed backward. Speaking of which—if you're arguing against top-to-bottom radiator flow, take a look at almost every production car, especially race cars. This design isn’t just a convention; it’s based on physics and practical engineering. Challenging this concept is a bold stance and requires some serious mental gymnastics to justify as a superior method.
When hot coolant enters the bottom side of a radiator instead of the top, it disrupts the natural convection and efficiency of the cooling system. Radiators are designed to use gravity and thermal dynamics—hot fluid rises, and cooler fluid sinks. If hot coolant enters at the bottom, it forces the cooling process to work against this principle, potentially leading to uneven heat dissipation. The hottest coolant may not get enough time to transfer heat properly before being recirculated, reducing overall cooling efficiency. This can also cause air pockets to form in the system, leading to hot spots and potential overheating. Additionally, since most thermostats and temperature sensors are positioned to read coolant leaving the engine, improper flow direction can cause inaccurate temperature readings, leading to poor thermostat regulation. In extreme cases, this incorrect flow can create localized boiling within the engine, leading to cavitation, coolant breakdown, and long-term engine damage.
Are there times that hot in bottom is done? Sure. Does it work. Yes. But for most cars and race applications, top-in is simpler, more reliable, and thermodynamically sound.
The new NSX (NC1) is plumbed in this top down way…is it wrong?
What I’m suggesting isn’t some wild experiment—it’s just how most cars already handle cooling. The NSX’s unique setup is what made me notice it in the first place that it was plumbed oddly. The diagram Honcho posted is what I should have included in my original post to clear up the confusion, so I’m using it here to clarify:
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- Point A is my proposed new thermostat location.
- Point B should remain open with my modified gutted housing. As Honcho pointed out, making this wider than the OEM thermostat could even have additional flow benefits.
- Fan trigger should be based on fluid exiting the block…not post cooled in the thermostat housing. The low fan speed temp sensor is what should be the fan reference for the entire system.
Why This Is Better:
- OEM design creates the highest temp gradient in the system, forcing the coldest fluid to mix with the hottest at the thermostat, which acts as the gate at this turbulent point. This results in inefficient thermostat control, with slow response times and poor temperature regulation.
- OEM places the high fan speed trigger at the coldest point in the system, which doesn’t make sense. Triggering the fan based on radiator-cooled coolant leads to over-cooling during engine warm-up or under load, reducing overall system efficiency.
- OEM design results in the least uniform temperature distribution across the system. My new approach ensures the thermostat sees consistent engine-out temperatures, resulting in smoother temperature transitions that help prevent thermal stress, improving engine longevity.
- OEM has more restricted flow due to less efficient routing. By using a high-flow thermostat in my new setup, coolant flow is improved, which increases the system's cooling capacity—especially crucial when pushing more power or under high-load conditions.
- My setup makes servicing the thermostat easier. While not a huge deal, having direct access without disassembling the cooling system or dealing with complex routing saves time and reduces the potential for mistakes during maintenance.
- This is simpler—two temperature reference sensors instead of three. Fewer sensors means fewer failure points, less complexity, and more reliable readings, which contributes to overall system reliability.
- The OEM radiator is plumbed backward! The reverse flow doesn’t optimize the cooling process and causes inefficient temperature dissipation. My setup follows the natural top-to-bottom coolant flow, improving heat transfer and cooling efficiency.
At the end of the day, this isn’t some radical idea, I am not suggesting to fill up your radiator with bananas and only drive backwards—it’s just applying fundamental cooling principles that are standard on almost every vehicle.
I’m not proposing that everyone should rush to make this modification. The OEM cooling system is perfectly capable of maintaining proper coolant temperatures, and this isn’t a necessary upgrade. However, for the reasons mentioned above, I believe this setup offers improvements that are even more pronounced on FI standalone cars . What’s interesting is that I may be the only one who has run an NSX both ways—OEM and modified—and can confirm that, at least on my car, this works better. Yet, some may still argue against it from behind a keyboard.
