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copbait73 copbait73 is offline
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Join Date: Nov 2003
Location: Chesterfield IN
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Porsche TURBO Engine Cooling Issues

Two members are embarking on “Baby” TURBO engine programs. The power densities needed bring up a topic that I find in my thread search is really not discussed as perhaps it should.

I’m a lifelong air-cooled engine guy, 43 years. Starting as teenager in VWs then on to Porsches, add in 15 years of owning and piloting aircraft, now back to Porsches. This problem of properly cooling an air-cooled motor was researched at length before, during and shortly after WWII.

VW had serious head cooling issues in the late 60s which kept me working to pay for school by rebuilding VW motors in the evenings. Then it was common for #3 exhaust valve to be gone at 50K miles of interstate driving.

Porsche’s corporate secrets will probably remain corporate secrets however much of what they know can be glean from the development stages of their air-cooled product.

I’d like to start this thread as a clearing house of ideas, investigations and application.

I will begin (a discussion of the history of water vs. air-cooled motors):

“But as air quality awareness rose in the 1960s, and laws governing exhaust emissions were passed, unleaded gas replaced leaded gas and leaner fuel mixtures became the norm. These reductions in the cooling effects of both the lead and the formerly rich fuel mixture, led to overheating in the air-cooled engines.[citation needed] Valve failures and other engine damage was the result.[citation needed] Volkswagen responded by abandoning their (flat) horizontally opposed air-cooled engines,[citation needed] while Subaru took a different course and chose liquid-cooling for their (flat) engines.”

This brings up something I’ve learned about air-cooled engine powered aircraft. In operation, air-cooled aircraft engines are yes, air-cooled and oil-cooled (like Porsches) but also FUEL cooled (as are modern Jet engines). During a flight they effectively burn off the otherwise detrimental weight of their cooling system. What happens to our Porsche’s cooling when we optimize A/F and spark advance with modern electronics tuned on a dyno?

If you go to an airshow you will see WWII vintage aircraft leaving very visible exhaust trails (RICH mixture) at takeoff and later at full throttle flybys. I’ve heard it several times, “without fixing the stock WUR the engine runs pig rich in mid-range.” This maybe true, and it may be caused from wear, but just maybe Porsche did this for sufficient head cooling at mid range, high load operation?

Another consideration:

Two old NACA documents. Lots of good reading but a highlight for me. 400 degrees cylinder head temperature (CHT) taken at the rear sparkplug flange is considered the limit for this high performance turbocharged, intercooled and supercharged air-cooled motor.

What is the limit and location for the air-cooled 911 motor? Surely Porsche works with a similar limit and location. This should be known to racers by instrumentation on their panels. Racers, can you fill in this critical piece of information to begin our discussion?
Old 08-02-2009, 09:33 AM
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