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Oil Temperatures: How Hot is too Hot?

September 27, 2010 | Troubleshooting & Repair

By Barry Lee Brisco

The 356 engine is often described as "air-cooled", but a more accurate phrase would be "oil-cooled". After all, cars with radiators are "water-cooled" even though ultimately that water is cooled by the air.

With that in mind, it's easy to understand why 356 owners are concerned about their oil temperatures during hard running on hot days. If the oil gets too hot, engine damage can occur. But how hot is "too hot"?

Starting in 1950 (Ludvigsen, Excellence Was Expected) or 1951 (Johnson, Authenticity Guide) an oil temperature gauge was standard equipment. Up until mid-1957, the various gauge types over the years — first MotoMeter, then Stork, and finally VDO — had numbered scales that ended at 250F / 121C (MotoMeter, shown at right) or 280F / 138C (Stork, shown below right). At around the time of the T1 / T2 change in mid-1957 the numbered temperature scale used in the VDO "combi" gauge—oil temp and fuel level—was dropped and a simpler scale was used (photo at bottom of page). A thick green bar at the low end of the scale indicated the "warm up" zone: many 356ers recommend avoiding spirited driving until the needle is out of this zone. At the top end of the scale was a narrow green bar (to indicate the end of the "safe" zone) and a narrower red bar to warn the driver of potential engine overheating.

So why did Porsche take the temperature numbers off the gauge? Perhaps they got tired of anxious owners asking "How hot is too hot?". There is no simple answer to this question even when the cars, and engines, were new and behaved in a consistent and predictable manner. Certainly it is best to keep the needle out of the red zone: anything below that point is okay, assuming that the gauge is working correctly.

The good news is that modern oils (conventional or synthetic formulations) can safely handle hotter temperatures than the oils the factory had to choose from 50 years ago. And synthetic oils tolerate significantly higher temperatures than conventional oils, by at least 10 – 20 degrees.

Keep in mind that on mid-1957 engines and later, the gauge measures the oil temperature after it goes through the cooler, which means that the sump temperature is probably at least 20 degrees hotter (earlier engines had the temp sender located in the sump). A "dipstick" style thermometer inserted in place of the standard oil dipstick will verify that the sump temperature is hotter than the gauge reading (assuming both intruments are calibrated correctly!).

Notes on Oil Temp from from 1958 Christophorus Magazine

"The oils of well known firms...are so far advanced today that even at a temperature up to 320 the oil film in the bores does not break down and you can still consider the lubricating ability to be sufficient... The maximum oil temperature that our motors can be expected to sustain without serious damage is 280...Only since July 1957, have Porsche motors done away with the camshaft drive gears of plastic.  Now they are made of light metal.  The plastic wheels are somewhat more susceptible to temperature than the light metal ones and when you drive a long period between 250 and 280 degrees the life span of the plastic cogs sinks and you must not be surprised if one gives out some day...The performance of a motor decreases at such high temperatures.  An engine usually reaches its horsepower maximum at 175-195.  At a greater motor heat the filling degree is lessened — warm air needs more room and that decreases the performance."

[Thanks to John Wilhoit for bringing that to my attention.]

Note that the mid-point of the VDO gauge is about 212F (100C), the boiling point of water. I suspect this was done deliberately. That is the point at which any water in the oil will start to boil off (other volatile hydrocarbon impurites begin to boil off at slightly lower temperatures). Many 356 owners do not run their cars very often, and when they do it may not be long enough to get the temperature up to the mid-point of the scale. It's a good idea to periodically take the car out for a hard run and get the engine nice and hot to boil off any water in the oil, which tends to accumulate in cars that sit for extended periods. The 356 was meant to be driven, and driven hard. It can be difficult for those who live in cold climates and flat terrain to get the oil temperature over 200F in the winter. A steep hill helps, as does turning on the heater, which reduces the fresh air flow over the cylinders.

You can protect against engine overheating by using good quality oil (synthetics have the edge here) and changing it regularly, making sure you have a clean and properly working oil cooler, having the proper fan belt tension so there is no slipping and slowing down of the fan, keeping the timing set correctly, and ensuring that your oil gauge is accurate (send the gauge and sending unit to North Hollywood Speedometer, or Palo Alto Speedometer to be calibrated). If all those things are in order, then don't worry about it: just get out and drive!