The age old question, “How hot is ok?” comes up many times in conversations, tech calls, forums, and groups. Being that factory ratings are not always the best for longevity or real world use there are many things that have to be taken into consideration when deciding fluid operating ranges for an engine or transmission. Here are some of the main things: 

• Fluid type
• System
• Heat cycle duration
• Environment 
• Condition of the mechanical components

COOLING

The 6.7 Powerstroke, as an example, has primary and secondary cooling systems which operate at completely different temperature ranges. Primary stock is around 195°F whereas the secondary system is around 100°F. Therefore, saying coolant temp should be around 200°F in this application does not work so well. 

Although coolant temps are regulated by the thermostat and operate accordingly, knowing your base temperatures are key to keeping them in check. That means knowing your thermostat temperatures are key. Also, it’s recommended to maintenance and check your coolant with a hydrometer every few months to make sure the freeze point ratio is correct. Remember, water cools better than coolant.

For lower temperatures, you can add theSPE low temp, high flow thermostat. For more information regarding it and how it worksCLICK HERE. 

ENGINE OIL TEMPERATURES

Step one is to know your oil. This is going to make all the difference so you know when your oil breaks down and when it’s additives and lubricity are no longer functioning. Mineral based oils, for example Rotella T4 and similar CJ4 rated oils, will carry a lower maximum temperature for operation in comparison to a full synthetic oil. Mineral based max temps should be kept under 250°F. 

At 250°F the oil begins to degrade very quickly causing many issues. Most CK4 17+ synthetic grade oils will withstand much higher temps, some manufacturers actually claim close to 300°F. These are very basic temperature figures based on general lab testing. 

Looking at this from a real world perspective there are a few things to also consider:

This matters because of local heat differences. For example, if your oil temp sensor is reading 220°F and it is located in or near the pan, the oil in the turbo center section will be 15 to 20 degrees hotter due to the localized heat from exhaust temperature being so close to it. 

Making sure that during operation oil temps reach around 178°Fto burn off condensation that builds up over time. If too much moisture is in the oil that can also affect its ability to do its job.

If proper maintenance isn't done and oil life is degraded its ability to withstand temperatures will be much worse. So, if you see high temperatures frequently tightening up your maintenance schedule would be smart. You should also consider running synthetic.

Since coolant and oil temperatures go hand and hand, you can see benefits to oil longevity from theSPE low temp, high flow thermostat. 

TRANSMISSION TEMPERATURES

Very similar to engine oil most of the above applies with only some differences. ATF is much thinner and similar to the consistency of hydraulic fluid. In the 6.7 Powerstroke SP/ LV fluid, recommended by Ford,  are synthetic oils. These oils will withstand higher heat. The localized heat in the 6R and 10R transmission is usually worse at the converter but once the torque converter clutch is locked there is usually not a measurable change. These transmissions spend a lot of their life with the converter locked. The nature of the LV synthetic fluids make maintenance less of a factor being they normally don't need to be serviced until 100k miles. Although, if abuse is high it's always recommended to do it sooner. Early maintenance is always cheap insurance. 

Customers running theSPE 6r140 Proprietary Solenoid Body will see a 50°F drop in transmission temps due to the patented thermoreg control valve that drops the transmission operating temperature by 50 deg F. This helps with transmission fluid breakdown.

CONCLUSION

As always remember maintenance is key to the health of your engine and transmission. So before deciding what temps you're ok with, make sure your fluids are ok with them first.