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If you can have two gauges, it's useful to have a "normal" (for car
folks)
vacuum/pressure gauge. This shows you how much work the turbo is doing
relative to
the pressure outside.
I'm not sure I follow this. What information do you get from a boost gauge
that you dont get from a MAP gauge? Arent they both telling you manifold
pressure in different units?
The primary difference is that a boost gauge considers atmospheric pressure as its zero point. This way all you're indicating is boost. It doesn't really matter what the unit of measure is, but for some reason the auto folks considered psi to be the standard, so that's what gets talked about when "boost" is being discussed. In aviation, the atmospheric pressure gold standard [for this country] is in-hg, and for my way of thinking, looking at boost alone doesn't offer any really useful information. What we are really interested in is manifold deck pressure, and because we operate in a 3-dimensional environment, that pressure is directly related to the atmosphere at our particular altitude. A properly sized turbo will remain in its normal operational range throughout our environment, so knowing boost separately from atmospheric pressure is a useless piece of info, IMHO. Getting back to my earlier thesis, I intend to redline the MAP on my engine at 34"hg. This is extremely conservative compared to what the auto racing crowd does to their rotaries, and that's fine by me. I need to be more conservative as I don't have the luxury of being able to pull off to the side of the road when I lunch the engine due to internal pressures getting too high. In a nutshell, having both a boost gauge and a MAP gauge is like putting gravy on your mashed potatoes and gravy... sooner or later you'll figure out that the extra gravy doesn't do anything for you other than add weight <g>.
<Marv>
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