... 80 F and 100% RH -- isn't that where Lane lives??
The funny thing is, cool temperatures do not affect the engine temperature as much as rain does.
The engine gets up to heat on a cool day, but not a rainy day.
You lost me at RH. Isn't that something to do with blood LOL!
And THAT, my friends, is why I hired Engineers to know this stuff, rather than becoming one and having to learn how to manipulate all that math, myself - That math always gave me headaches. Nothing against Engineers (notice that I capitalize that out of utmost respect) because I would never have had a totally interesting job without them, but they never give a simple answer (although I am sure that Kelly believes that his answer was really "simple", and it was!)
So, to paraphrase (at my own peril and no offense intended, Kelly, but I always had to translate "Engineering Speak" from my guys and gals for the suits.....)
"Hot, moist air will cool air-cooled metal cylinders 10% better than cold/cool, dry air and THAT is why Bob Carley loses cockpit heat when it's raining out in cold, damp Canada in November. His engine is running cooler and supplying less exhaust heat, because it is getting a constant supply of water vapor blown onto the cylinders by the fan."
Or, it's just damp and clammy out and "chillin' ya to da bone!" as my granmudder used to say.
So think of it like this: The rain is really water vapor by the time it gets to the cylinders, and it mixes with the ambient air to increase the relative humidity (RH) of the cooling air. The rate of evaporative cooling is driven by the relative humidity (RH) to a greater degree than by temperature. But, in fact, the two are inter-related. As the temperature of air is increased (by the cylinders), it can absorb more liquid and, therefore, the relative humidity is decreased. Lower relative humidity promotes faster drying which, as you remove the humidity from the cylinders, it takes heat with it. (I have been accused, in the past, of getting this Bass-Ackwards, but I'll let Kelly sort that out).
And for the suits: It has something to do with the evaporative effects of water interacting with a hot surface, and the transfer of thermal energy to convert water to a vapor, but let's not go there, just accept it.....for now. (Magic happens there, and it doesn't cost you anything - that's all you need to understand.) { Lots of head nodding from the suits.... }
El Frazoo posted:Quick look would show that the diff of air properties (all at 100C ) between 0%RH and 100%RH, would have the heat transfer over the fins be about 25% more effective at 100% than 0%. So, all else equal, moist air is a better HT fluid than dry air. I should add that the effect is greater the higher the bulk temperature of the air, with little difference at 0 C, where of course there is a whole lot less water vapor per cubic ft of air than at 100 C. This would be absolute best possible case, and assumes the air is either all dry throughout or all wet throughout. Clearly of you take 80 degree air at 100% RH and run it through your fan and cylinders, it will emerge hotter, and hence at a lower RH. The 25% augmentation number quoted assumes RH is 100% throughout, which it could never really be, absent any water addition -- which there is not. In the real world, and I'm just estimating here, the relatively humid ambient air will work better than the relatively drier ambient air, but likely the net effect would be less than 10%. what really counts for air cooled engines, as you all know, is the ambient air temperature (duh!!) and the density, i.e., altitude, with hot, high air being the worst.
Yeah, TMI . . .
You speak my language, Kelly.
In layman’s terms: RH is Relative Humidity, a number which is determined by the total water content in the air as a percentage of what would be needed for saturation (all the water air can hold) at a given temperature.
Hotter air has the capacity to hold a lot more water, so a given amount of water in the air will have a higher RH number at a lower temperature, which is why dew forms on surfaces on a cooler (but humid) summer morning.
Everybody talks about 100° and 100% humidity, but even very, very humid climates like Charleston will have RH numbers under 50% in the heat of the day.
Anyhow, in order for the moisture to have more than negligible effect removing cylinder head heat in an air-cooled engine, it would really need to be a liquid boiling off into a vapor, which requires an enormous amount of heat to accomplish (because of something called the latent heat of evaporation). Even 99% RH isn’t going to do much.
But spraying water on the heads would be a very bad idea. One could never spray all sides equally, so there would be hot spots and cold spots in the same casting, which would crack the heads, almost for sure.
So back to the original question, I think it would be much more effective to spray a remote oil cooler with water. Oil is also a cooling medium in an air-cooled engine, and large quantities of cool oil circulating through the internals can only help. It’d take significantly less water to fo it, and have significantly less risk of doing something bad.
Running 10.6:1 C/R, it’s something I’ve given some thought to.
So might not rain splashing onto Bob Carley's front-mounted oil coolers have a similar effect?
Gordon Nichols posted:So might not rain splashing onto Bob Carley's front-mounted oil coolers have a similar effect?
Absolutely.
Also, the moisture in the air would acts as a poor-man's water injection, which slows the progression of the flame front in a hot cylinder and reduces the tendency of pre-ignition. My car runs fabulously when there is cool, moist air entering the carbs, as does yours, his, and anybody else using carburetors.
Yes, my car runs great in cooler air - a nice evening run is a lot of fun - even with fuel injection. All of my past cars have done the same. I would assume that's the whole idea behind cold air intake setups.
But, getting back to my issue, the engine thermostat shuts off oil to the two oil coolers below 180, and sends the oil forward beyond 180, to try to maintain a constant temperature. (But even with two oil coolers, I need to turn on the oil cooler fans a lot of times in hot weather, even when moving briskly).
Back to my issue with a heavy rain, the problem is that the engine doesn't even get up to 180, so it's not an issue of the oil coolers cooling the oil too much in a heavy rain.
The oil doesn't head out to the coolers, as the engine can't reach normal operating temperature to send the oil forward, so that is not the problem.
The engine stays below operating temperature, and hence, not much heat gets to the passenger compartment.
I'm thinking the really moist air gets sucked into that massive axial fan and cools the cylinders pretty effectively. 911s don't have shroud flaps and a thermostat, which would explain the cool running, at least partially.
... plus water is spraying up from the bottom.
Amazing, the knowledge base of this group! I learn something new every time I visit this site. 
With my aircooled IM the fall crisp weather was always the time the engine seemed to be just purring purfectly 
MusbJim wrote: "Amazing, the knowledge base of this group! I learn something new every time I visit this site."
That's true. And some (maybe a small percentage) of what you learn is actually useable in every day life! It's all great stuff, though, for sitting around the bonfire with a beer (maybe several) and some friends, watching the stars above the desert and then just dropping something like, "Hey! Did you know why relative humidity decreases with higher ambient temperatures?" And then you watch as the other bonfire-ers start to glaze their eyes over and sip more beer, while their upward gaze intensifies.....
Hmmm . . . Not sure Gordon really got the message. He still wants to get some sort of evaporative effect out of "moist" air at high RH. Once it's vapor, the heat of fusion has been spent elsewhere, and is not available to you. I have assumed no patent liquid water enters the fan and cylinder fin plus oil cooler fin arrangement. And cool moist (high RH) air is not going to display as much effect as hot moist air at same RH. I tried to explain that the augmentation effect on heat transfer properties of the air is nearly non existent at cooler temperatures. My quick look comparo was done at 100C, where the data I have shows the largest change in properties.
Water splashing on oil cooler fins is a whole other story. Here much augmentation vs not raining would be expected. And of course, road water splashing on the engine case, which is also finned as you must have noticed, would be a big effect too. If you can get the water to boil off of the heated surface, you have entered a whole new realm for heat transfer, orders of magnitude above just plain air, regardless of RH.
Another item: mention was made of using water to cool an air cooled engine. Hmmm, WTF is that? Not really sure. Aside from a closed system with a radiator (air-to-liquid heat exchanger) if you lived in the desert where temps get to be very high (100 F +) and RH gets to be really low (10% or less), You could spray a very fine mist of water into the air stream just ahead of the fan. This water would evaporate at the local dew point thus cooling the air to that temperature. Could be 60F -- I'll have to look that up. Out west, they call such arrangements used to air condition homes swamp coolers. The air that results is much cooler, and also much higher RH.
Don't know if you can still buy canvas bags to hold water that you would hang out the window of your car when traveling down rte 66 in New Mexico. Used to be on every car out west. The bags would allow a little water to seep out through the weave, said water would evaporate in the hot dry air, and the bag, hence the water inside, would be cooled to close to the dew point. Its like the claim that Tucson sure is hot, but it's a DRY heat, meaning that your sweat readily evaporates, cooling your skin considerably, so you do not feel so hot. You skin will feel the dew point, mostly, not the actual air temperature.
And of course these systems are open loop -- they use up water. Not sure you could or would want to carry enough water in your Speedster to use this method to keep engine cool. And that canvas bag is going to need filling up regularly.
And lastly, for those who want to drive their air cooled Speedsters in cold, damp and rainy Canada, I really have no answer for you. Wear more clothing. Swap in a Suby, eh?
So take the concept of the canvass bag full of water (and on Model Ts and As it was just hung in front of the radiator to cool the engine) and use the same concept in a wetted mesh material, formed into a cylinder, that hung outside of the car to force air through it and into the car’s cockpit and that became the “Swamp Coolers” that were all the rage on VWs in the ‘60’s that were “Po-Boy” car air conditioners. Look ‘em up!
No shyt??!! How much water to carry?? Not going to work worth a plug nichol over here on the least coast, as humidity is almost always too high.
PS, said I would look it up: at 100F ambient air temp, dew points are:
80% RH -- 93F 50% 78F 20% RH -- 52F 10% -- RH 34F
El Frazoo posted:Another item: mention was made of using water to cool an air cooled engine. Hmmm, WTF is that? Not really sure. Aside from a closed system with a radiator (air-to-liquid heat exchanger) if you lived in the desert where temps get to be very high (100 F +) and RH gets to be really low (10% or less), You could spray a very fine mist of water into the air stream just ahead of the fan. This water would evaporate at the local dew point thus cooling the air to that temperature. Could be 60F -- I'll have to look that up. Out west, they call such arrangements used to air condition homes swamp coolers. The air that results is much cooler, and also much higher RH.
Don't know if you can still buy canvas bags to hold water that you would hang out the window of your car when traveling down rte 66 in New Mexico. Used to be on every car out west. The bags would allow a little water to seep out through the weave, said water would evaporate in the hot dry air, and the bag, hence the water inside, would be cooled to close to the dew point. Its like the claim that Tucson sure is hot, but it's a DRY heat, meaning that your sweat readily evaporates, cooling your skin considerably, so you do not feel so hot. You skin will feel the dew point, mostly, not the actual air temperature.
And of course these systems are open loop -- they use up water. Not sure you could or would want to carry enough water in your Speedster to use this method to keep engine cool. And that canvas bag is going to need filling up regularly.
And lastly, for those who want to drive their air cooled Speedsters in cold, damp and rainy Canada, I really have no answer for you. Wear more clothing. Swap in a Suby, eh?
Cold, damp and rainy Canada? You must be thinking about the left coast. Up here in the greater Toronto area (GTA), we're suffering through 100+ F with the humidity for weeks and need rain badly; June was a scorcher like it was in much of the US East coast. Right now it's a relatively cool 90F and the humidity has mercifully dropped. Our cottage lake is down a good 6-8" from normal this time of year.
No cold, damp or rainy in sight 
.... actually calling for rain next weekend so hopefully.