which reminds me: why can't air cooled eng have 4 valves ...

What is the low down on this. I had read that in order to get to 4 valves per cylinder, Porsche had to dump the air cooled model. What is the reason for this? Or am I misremembering the statement. Can't for the life of me imagine how 4 valves per cylinder would impact an air cooled car versus water...


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Kurt B
1984 911 Carrera Cabriolet
75 914 1.8

Heat transfer is much more efficiently done with water, also every tolerance in the engine is much more easily controlled.

Kurt,

There just isn't enough room for adequate fins and air passages to dissipate the heat from a highly thermally stressed engine at high revs in the 8-9000 rpm arena. It could probably done for a low-output engine, but what would be the point?

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Warren Hall
1973 911S Targa

There was an article in 911 & Porsche World about a 4valve conversion for air cooled 911's. The company claimed a 30% plus power increase with these heads alone on a 3.6 RS. As you may expect, the heads were very, very expensive (well into 5 figures). If anyone is really interested, I'll dig the issue up for more info.

The Japanese have been building high-output 4-valve motorcycle engines for decades, but they have two advantages: smaller bores (less area that can heat up) and ram air cooling direct onto the cylinder heads. It is possible to build 4-valve 911 cylinder heads (as done by one German tuning company), but there is specific a problem: cooling the cylinder head between the 4 valves. There is not enough air from the blower to reliably do the job. It would be very un-cool for Porsche's reputation if the 911 cylinder heads start to crack between the valves.
The main reasons for the demise of the air-cooled engine is cost and emissions. The water cooled engines are sooooo much cheaper to build and the 4-valve heads also run cleaner.
Regards
Stef

The Advantec four-valve heads are shown on p. 193 of the 2nd ed. of Bruce Anderson's Porsche 911 Performance Handbook. I don't recall him having anything to say anything about them, though.

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Warren Hall
1973 911S Targa

The way I understand it, it wasn't until the water cool heads that they could get rid of that much more heat. As you raise the RPM red line you build more heat in the motor, I think they rev the GT-3R motor to 9400 RPM.

I saw a head from a GT3-R in my motor wizards milling machine. He was doing a valve job,valve guide replacment work on the water cooled head. They are a neat deal, triplex head, all three combustion chambers and 12 valves in one long head as opposed to one cylinder one head. They are CNC produced, the two ports come together at the intake/ exhaust surfaces and form a figure eight looking port. These things are built or rev high.

The bototm end uses titianuim rods,bolts, and nuts. The rod caps are torgued to a setting (don't rember) then the wrench is turned another 90 degrees. Won' get to hear the think run, he is only doing the machine work part of the rebuild.

Randy Jones
1971 911

Maybe Porsche did not want to invest the time and money into a 4 valve head for the aircooled motor since they knew they'd be phasing it out for a decade. After all, it's remained pretty constant over 35 years.