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On 4 Sep 2002, at 14:45, Jaranson, John (J.W.) wrote: > I had always assumed that we were concerned about cooling air leaking out of > the holes for the air conditioner and such. But....I am pretty convinced that > the sheetmetal housing that guards the belts is a negative pressure zone. Absolutely, it's the hot air that gets in which may be a problem. > So the REAL concern is about letting in some hotter air from the engine > compartment. Assume that the hole for the timing marks is 1" in diameter and > the small end of the bellows is about a 6" by 12" ellipse. The relative areas > of these two are 0.8 in2 and 56 in2. The timing mark hole adds about 1.5% to > the total cooling opening area. Not much in my opening. If you compare this > to the openings in the rear fenders (56 slots that are about 6 by 1/2" for a > total of 168 in2) the percentage is even smaller. Closer to a 1/2%. Throw in > a couple of more holes for the A/C belt and you have maybe got a few percent of > your total cooling intake that is pulling in hotter air from the engine > compartment. You really have to measure the conductance of the total passage from outside to the fan to get a handle on this. Even the plenum in the back of the body has a less than infinite conductance, and the ellipse and louvers are in series, so their impedances add. The simple 1" hole is a simple flow impedance and it leads directly to the "outside world" so there are no other series impedances to hot air leaking in there. In the end, I think you've underestimated the effect. The other thing to remember is that we would tend to use the AC only when the temp is high, so at the very moment that the engine cooling sees its max stress, we add more heat load as well as 1-2 more HP of load on the engine, which causes extra heat load of its own. If AC were something which we just used in the winter, we wouldn't even be having this conversation. - ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA ******************************* ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/