Tässä pelican linkin water vapor cooling juttua.
How hot is too hot? What can you do about it? To answer the “How hot is too hot” question, Chuck hit it squarely on the head. Up to a point it insidiously eats away at your wallet until time for a rebuild. Above some temperature there is fairly immediate failure. An air cooled VW Type I in a ’72 transporter well demonstrated that. A 911 is a far better and stronger engine but why tempt fate at extreme temperatures somewhere in the red area? The reality is that high temperatures accelerate wear and stress parts of the engine. You have to ask yourself how much effort (expense) you are willing to go to in order to reduce that stress and what an acceptable level of stress is. Clearly you can install cooling that is beyond the point of diminishing returns. On the other hand the 911s we are talking about aren’t even close to there. We are asking a lot from our 911s; they are called on to drive around town in traffic with the A/C on, cruse at high speed on desert Interstates, and perform on the race track. With careful balance all if these demands can be reasonably met. What can you do about it? I think the engine cooling fan is the first line of defense because it addresses the heat at the source. The 1.82:1 ratio is simply a combination of Factory 911 parts that Porsche chose not to use except under certain circumstances (930 Turbo.) We can easily see the Factory progression from no front cooler to larger and larger heat dissipation capacity. Our DE track use allows for some novel ideas (Rubbermaid Solution.) For a given 911 you can find the correct set of pieces for the fan up-date. Some 911s only need the fan itself while others need all the parts, including the fan housing. The other issue is having the correct crank pulley for the various Factory and aftermarket A/C installations. This it the reason you haven’t seen an exact list of part numbers. Chuck is exactly correct; all this is for naught if the heat dissipating fins of the heads, cylinders, and oil coolers aren’t perfectly clean. This is regular maintenance. The oil cooler installations and up-rates are clearly the most expensive. There has been much posted about them. The critical issue is to have sufficient heat rejection so the oil, and consequently the engine parts, stay in the acceptable long-life situation they are capable of. My opinion that is 180-210F. The Rubbermaid Solution is only suitable for the situation where you can replenish the water every 30-45 minutes or so. It is an ideal low cost, temporary, very effective solution for track use. I did a quick calculation assuming 5 gallons of water evaporated over 30 minutes. That is absorbing heat energy at the rate of 32 horsepower or 81,000 btu/hr (23.8 kW). I don’t know what the exact heat rejection of a 911 is but this is a big number. For comparison this propane heater, when cranked up to full power, generates 85,000 btu/hr. Envision heat being absorbed at that rate from the fins of a 911’s heads, cylinders, and engine oil cooler. IMAGE 00PropaneHeater.jpg When I use this technique I use a regular 911 VDO windshield washer pump with two 1-ohm, 10-watt resistors in series with the pump motor. This slows it enough to last. I never had a pump failure. The water is stored in a collapsible plastic camping container with any air purged. This allows the pump to pick up the water without any captivation. This is the same technique spacecraft use for handling liquids in high-G and 0-G situations. The water bag is constrained in a Rubbermaid dish pan (hence the name) by eight bungee cords and the pan is fastened to the car. I usually put it at the front of the passenger’s seat and a passenger’s legs go over the bag. The washer nozzle sprays at the hub of the fan pulley and the rotation of the pulley distributes the water into the fan where it is vaporized at high fan speeds. The combination of the 1.82:1 fan and the Rubbermaid Solution is sufficient to keep my 11.5:1 CR 2.8S at 210F on 98F track days with NO front cooler. Best, Grady