What oil temp is too hot?

By Clinton HulleyInformative
Oil Temp
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When building a race car or just modifying an engine there is one thing that sometimes gets forgotten about. That is oil temp. When engines get designed for cars they have a certain oil temp in mind that they want to run. It is usually around the 100 Deg C mark, some times a bit higher. With the airflow over the sump and the cooling system cooling the block and head that is sufficient to do the job.


When you get to modified engines you are pushing the limits. There is usually n factory safety factor that so if it is a really hot day or if you sit in bumper to bumper traffic the cooling system can cope with what you through at it. The more power you make the window gets smaller and smaller. In modern cars, the cooling systems have big safety factors as a lot of the engines get designed for higher power output but for the market, purpose power gets reduced.

So what is too hot and why?

Well like I said earlier engine oil temps should not really go past 100 Deg C mark. 105 Deg C is an absolute max. The reason for this is the hotter the oil gets the thinner it gets. If the oil gets too thin then it starts affecting your oil pressure. The engine oil pump has a pressure relief valve in it that helps to a certain point. The pump delivers more oil than the engine need and dumps the rest via the relief valve. The relief valve is opened via oil pressure. When the oil gets to thin the relief valve dumps less and less oil to try and maintain the max oil pressure the engine needs.


This is only at the higher RPM, at the lower RPM the relief valve stays closed as the oil pressure is to low for the regulating value. This means the hotter the oil gets past the 100 Deg C mark the less the oil pressure is going to be. This is where the bad thing start. The oil gets thin so the oil pressure drop, because the pressure drops every liner bearing start to wear. This means every liner bearing will be putting extra heat into the oil. So from here, it will be downhill. The oil temp will escalate past the 130 Deg C mark. At 130 Deg C, the oil will break down losing its ability to lube the engine.

If the oil temp goes past the 130 Deg C mark the oil is damaged, it needs to be replaced. As the engine wear will just escalate from there.

Will other oils handle the above 100 deg C temps

There is a lot of oils on the market but the rule of thumb with all oils is try to stay below the 105 Dec C mark. Although oils can run higher temps, they all break down around the 130 Deg C mark. You need to have some headroom. If something does go wrong, or you have an extremely hot day you have a 25 to 30 Deg C window you can run into.

If your oil temp was sitting at 128 deg C as a standard oil temp and you get a hot day you will go past the 130 deg C mark. The oil will break down and the engine will start to wear. At some stage, the engine will grenade itself.

How must I check the oil temps

If you build a heavily modified car it is always a good idea to have an oil temp gauge in the car. This way you can monitor your oil temps and see if you need to alter your oil cooling system. If you measure you will know. The cost of an oil temp gauge is a lot cheaper than rebuilding a blown engine.

How must I cool my oil

Well, the best way is to fit an air to the oil cooler. The oil gets pumped through the air to oil cooler and back into the engine. The oil that goes into the bearings is a lot cooler than the oil in the sump or oil tank. So this will limit the temp of the oil in the sump or the tank. If you still run higher than 105 Deg C than you need a bigger oil cooler. With turbo applications, it is always a good idea to run a bigger oil cooler as the turbo(s) tend to put a lot of temp into your oil in your sump.


Oil temps with racing

In certain forms of racing like stock cars, oil temps are usually an issue and people don’t know about it until it is too late. They run one gear around the ovals usually 2 or 3 gear. Its issue is they run at high revs most of the time. Usually at that revs cars will do higher speeds which means more air over the engine. The lack of airflow causes the oil temps and cooling system to suffer. These high oil temps will cause an engine to fail prematurely.

Conclusion

From experience with engines, I have found that if you get a system to be in control then that part will not be the cause the engine fails. If all systems are in control the engine will just last. As soon as something goes out of spec then that is where the issues start.


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