Pelican Parts Forums - valve guide question

Quote:

Originally Posted by Flieger (Post 4283051)

Horizontally-opposed, air-cooled piston engines have a hard time cooling the cylinder heads because the valves and cams are in the way of cooling fins and air.

Exhaust valves are on the bottom of the Porsche engine, further away from cool air, suffering from reduced flow and pre-warming of the air from the top of the engine.

Exhaust valves on a horizontally-opposed Porsche engine have a harder time with oil leaks also because the oil is pulled by gravity towards them.

Porsche experimented with metallurgy for the valve guides. As they ran engines hotter for emissions reasons (or more power creating more heat), the metal started to show more wear. They went to harder manganeze bronze (from phosphor-bronze) and this may be a reason why SCs have valve guide issues but Carreras are better.

Furthermore, the valve stems seals can either be Viton or Teflon. Some say Teflon seals too well, not allowing enough oil and librication to the valve stem (but the seals don't leak ;-)) The seal can complicate the valve guide metal performance due to lubrication dynamics.

Give the exhaust valve guides a break, they have a tough life. A little oil now and then is all they ask...:-)

GOOD EXPLANATION.
There are other good tid-bits above.

An often neglected explanation is basic engine cooling.

Porsche made some slight efforts by increasing the oil cooling with more effective front coolers and eventually fans. Note that they also removed the calibrations from the oil temperature gauge and installed lower pressure oil pressure warning light switches.

The basic cooling is with the engine fan. In fact Porsche went the wrong direction with engine cooling. Starting with the farcical 5-blade fan used with the California emissions cars in ’75 to the smaller fan in ’78 and slower turning fans in ’80, Porsche ended up with a down-rated air flow fan with the curved blade 964 fan.

Why would they be so stupid?

The answer seems to be European (particularly Swiss) noise regulations. From the mid-‘70s there was a constant effort to make the engines quieter. A side ‘benefit’ was they ran hotter and the emissions went down. This was particularly true with the advent of catalytic converters in ’78.

What is the result? Engines that run too hot. Valve guides that prematurely wear (exacerbated by ‘better’ valve stem seals). Cylinder studs that pull out of the case or break in half. Lower performance.

Fortunately we live under no such constraint. The easiest fix is to use the 11-blade, 245 mm diameter fan turning 1.82 times the crank speed. Porsche used this starting with the 1975 930 Turbo but chose to NOT properly cool all the other production cars. For track only use, the “Rubbermaid Solution” keeps engines cool.

Best,
Grady