Well my take on it
Thanks Paul. That's what I suspected. We go through a lot of trouble to seal the baffles, then open up big holes for mag cooling. Just didn't make a lot of sense. Jim (On with baffles...)
Does not have the be a big hole, how about a small 1/4" or 3/8" or 1/2" hole.
Factory planes for decades have had blast tubes on (in) the magneto, fuel pumps, gascolator alternator and even the battery. I suspect there is a operational LIMIT on temps and they found things where running hot during flight test. I would not unalatterly decide against any air blast tubes any where just because you don't want a small hole in the baffles.
Magneto - Cooling air will reduce wear & tear. Hard to put a number on it, but magneto parts, points and even springs often fail from heat related issues. Kent Paser even went so far as to route air into his magnetos. The coil & points of mags do get HOT all on their own! Kent Paser of "Speed w/ Economy" has some info on that.
Fuel Pump - Vapor lock happens even when running Av gas. There are good reasons to shroud and route air to our RV's mechanical fuel pump, especially if you might run Mogas in the future. Some even shield and blast the gascolater. Look at a piper the electric fuel pump & gascolator are in a shielded vented box. I'm just saying people know things.
Don't be too fast to blow off the concept of cooling air to components all together.
Alternator - B&C says it does not need it. Cool for them. They don't have a voltage regulator on board, but than again their fan runs backwards from its intended direction for auto use.
If you're running an Internally Regulated Alternator I recommend a blast tube right on the back towards the voltage regulator fins. The idea is not force feeding air INSIDE the alternator but across the back, keeping
convective hot air from impinging on or being sucked up by the alternator.
PART II of the hot alternator is
"radiant heat". The #1 pipe stack has at least +1300F temp gas flowing through it, a handful of inches away. I recommend a heat shield regardless of brand of alternator. Nothing fancy just a piece of aluminum that keeps the pipe & alternator from having a direct line of sight of each other. Van sells
heat shields but they are a little wimpy. May two together would give you coverage. A shield off the alternator would work as well or better. Clamp the shield off the alternator or exhaust pipe as you prefer. The sun gives earth radiant heat a 100 millions of miles away, it's pretty strong right. Radiation or radiant heat energy moves as waves in the
infrared. Imagine if you where the size of an Ant, hanging on the back of the alternator, looking at that red hot (at least infrared red hot) exhaust pipe. Your little Ant body would be toast; it would be hot. A shield lowers the heat energy (and transfer), is cheap & light weight. You can utilize a shield independent of an air blast tube, but both shield & blast air would be best. Unlike the magneto, battery and to an extent the fuel pump the alternator is working right next to the exhaust. Remember automotive alternators where designed to be high and dry up front on top of a water cooled engine, away from exhaust. Some alternators on cars do have heat shields, typically turboed cars.
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The down side of punching holes in the baffle is more work & weight (although not much work or weight). The biggest negative is may be a tad more cooling drag. Bob or some one claimed they picked up a MPH or two by blocking all their air blast tubes? I don't know about that but I can see you are causing some cooling drag. Air being sucked in and routed around doing cooling is "work" and takes energy. Work is energy which is drag. Frankly your soft seals will probably be leaking more air than 12 air blast tubes. You just never are going to get it sealed perfectly. You don't know what the seals are sealing, in flight at 190 mph, its hidden from view. Also they get worn over time.
Pick your battles. I would go with the alternator cooling at minimum first, than fuel pump and mags second. Consider when you get it flying, do some temp surveys & see how hot the mags, pump and alternator are getting. The tighter your cowl (plenum) the more discrete cooling tubes are needed.
I suspect soft seals allow air to spill over the baffle anyway and cool the mags. However if your workmanship is excellent and have minimum leaks at the back baffle seal, you might fry toast your magnetos with out cooling tubes? Who knows. If factory planes have them I wounder why. I would love to ask a magneto engineer. I suspect they are all retired.