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Registered
Join Date: Dec 1999
Location: Novato, CA
Posts: 4,740
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Why Heat Kills Top End
Found some jewels of info that explains why Porsche had to go the water cooled route:
http://www.liquidcooledairpower.com/lc-topendheat.shtml Joe |
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02-01-2002, 10:17 AM
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Very informative article.
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02-01-2002, 10:57 AM
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Information Junky
Join Date: Mar 2001
Location: an island, upper left coast, USA
Posts: 73,167
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hmmm.
Sodium filled valves and ceramic exhaust ports give some great thermal management. . . as Porsche engineers have shown us. Hans Nader & Max VonBeancounter are behind the water. (read environ / manufacturing concerns) |
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02-01-2002, 11:32 AM
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Registered
Join Date: Apr 2001
Location: New Orleans, LA
Posts: 88
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other than maybe in the 75-77 model years, I was unaware that heat was prematurely killing 911 top ends. They seem to last just as long or longer than top ends on water cooled cars.
My air cooled 911 top end outlasted my water cooled BMW top end by many, many miles. |
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02-01-2002, 11:41 AM
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Information Junky
Join Date: Mar 2001
Location: an island, upper left coast, USA
Posts: 73,167
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Good Point
Pumping a brine solution thru a bi-metalic block is just asking for trouble. I'll take the "thermal fatigue problem" over the corrosion problem. But then I'm bitter about these water pumpers today; having to replace a Saab ('94) radiator because of a low-life plastic. It's all about producing a car to last the warrenty period; and no longer.
Last edited by island911; 02-01-2002 at 11:56 AM.. |
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02-01-2002, 11:53 AM
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Registered
Join Date: May 2000
Location: Los Alamos, NM, USA
Posts: 6,044
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Sodium cooled valves may have been "exotic" in the 40's or 50's but International Harvestor was using them in farm equipment in the 1960's. On the farm we had a cotton picker powered by a little water cooled IH V-8 with sodium cooled valves/valve rotators. It out lasted everything else on the machine. The picking heads were a mechanical marvel/nightmare though.
The metallurgical explanations in the essay are a little simplistic; I suspect work hardening has little to do with the failures of alumimum engine heads. It is more likely an issue of thermal stress/thermal fatigue. I thought Porsche went to water cooling with the advent of production four valve heads. There was no longer the space left for sufficient finning to transfer the heat away plus there was more heat.  Jim
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02-01-2002, 12:44 PM
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Senior Member
Join Date: Nov 2000
Location: Stuttgart FRG
Posts: 2,307
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Hello
Yes thats right the 4 Valve technology pushed to watercooling. Some 4 V air cooled heads where produced using a alcohol filled heat pipe system to transfer the heat around the centersparkplug to the outside. But watercooled heads where simpler ( cheaper to produce ) and more reliable. Also some other issues like noise and interiour heating wher to expensive to be managed aircooled for the nowerdays new car buyers. However i think CMW has 4V heads for Porsches. Not to forget the idear to use a full ceramic head with ceramic valves. Grüsse |
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02-02-2002, 01:40 AM
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Registered
Join Date: Oct 2001
Location: Grand Forks, ND
Posts: 290
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having overhauled an aviation engine(O-300-D) coking is not something you what to find in the oil pan, the operating temps were on the mark to, he didn't talk about shock cooling, when you're in cruise flight and about to descend to your destination people had the habit of cutting the power to idle and diving at the ground, aluminum heads don't like a going from a cozzy 400deg to 180deg in 15 seconds, they crack with about a $1200 bill to fix it, food for thought
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02-02-2002, 02:41 AM
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1977 Porsche 911 S 2.7 CIS 
