Porsche: Air Cooled Engine - Thermal Performance

Thermal performance for an air cooled Porsche engine is measured in how well an engine sheds heat. Obviously the cooling fan plays a major role in this heat-shedding process but there's more. Integral to the effectiveness of a Porsche engine's fan forcing ambient air over the crankcase and down through the cylinder fins and oil radiator is the thermal performance of each of those engine areas. Thermal performance meaning the ability of any given engine area to transfer heat away from the engine. Grime build up from oil & debris on an engine's exterior is the enemy---it serves as an insulator that degrades efficient heat transfer. Over time, this insulation can build up to a point where an engine's thermal performance is compromised beyond acceptability. Depending on the driving conditions an engine is subject to, thermal performance decline may or may not be easily noticed. Engines that experience stop-and-go traffic offer greater opportunity to detect thermal performance decline due to sitting repeatedly at idle.

Assuming a properly working temp sensor and temp gauge, a Porsche engine with optimal thermal performance can sit and run at idle (in roughly 80 degrees ambient) indefinitely and never will the temp indicator rise much above the bottom box or 210 dF or 90 dC. A Porsche air cooled engine with compromised thermal performance is indicated by an engine sitting and running at idle with its temp indicator progressively climbing into the red zone box. >> Prior to an indicator entering the red zone box, an engine should be turned off and allowed to cool prior to restarting. Opening the engine compartment lid facilitates cooling. << The faster a temp indicator climbs... the worse an engine's thermal performance is.


There are 2 fundamental benefits gained by preserving an engine's thermal performance:

1. Cooler engine operation preserves engine oil viscosity. Viscosity basically refers to oil's ability to lubricate/protect and so preserve an engine. An engine that consistently runs at a higher temp degrades its oil's viscosity faster and so sooner than a cooler operating engine. To remain on par with a cooler running engine, a hotter engine requires more frequent oil & filter changes. The absence of attending oil & filter changes more frequently in this case translates into premature engine wear. Bottom line: given all things being equal but for thermal performance, a cooler running engine will outlast a hotter counterpart.

2. An engine with poor thermal performance that's caught in stop-and-go traffic is concerning. In this circumstance, a driver should constantly monitor engine temp and consider options to prevent the engine from overheating. For any driver interested in their engine's well being, this is an annoying situation at best. Worse would be to ignore engine temps entirely. Doing so can be an expensive proposition---one best avoided by ALWAYS & CONSISTENTLY scanning gauges for engine status and taking intelligent action to preclude the unwanted.


We'll look at this from 2 perspectives. The first is restoration of thermal performance. The second is preservation.


Porsche's air cooled engines are notorious for leaking oil. Unless cleaned regularly, that oil will collect road debris and create unwanted insulating value over time. Where mild oil & debris build-up exists, degreasing an engine is a restorative process that can return adequate to optimal thermal performance to a Porsche air cooled engine.

In cases where oil-debris build-up has reached significant proportions, degreasing will work only partially (as is evidenced by repeated applications of degreaser failing to cleanse a surface to bear metal.) In such instances, oil-debris has baked onto the engine's surface forming a gelatin insulation and sometimes even a hard crust. Any accumulation of this nature between cylinder fins prevents cooling air (as provided by the cooling fan) from extracting heat from the engine as it is designed to do. Instead of exiting, unwanted heat radiates throughout the cylinder assembly outwards to the heads an inwards to the case... ultimately to the engine's oil. The more heat imposed on oil, the sooner its viscosity degrades.

In extreme cases---where degreasing is ineffective---one has to evaluate how best to clean a given engine. The best option may be to remove the engine from the car---this being required in order to gain necessary access to otherwise inaccessible areas. Compared to using a degreasing agent, more aggressive options are pressure cleaning, dry ice blasting, and media blasting to name a few. Each has advantages and disadvantages according to a given engine's situation. Optimal results are achieved by skinning an engine via media blasting. Skinning refers to cleansing