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Originally Posted by Bicyclops
Using online density altitude calculators, it appears that I am seeing something like an inch or two more MAP than the expected absolute pressure.
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Remember, the calculated
available MP would be equivalent to measuring at a point before the filter, fuel control venturi and butterfly, intake manifold plenum, (if it has one), and the intake tubes, all of which offer some restriction. Since we actually measure at one of intake ports in a cylinder head, there will be a significant difference between what is available and what is seen on the EFIS. There is no good way to guesstimate the total intake tract loss since everybody's system is different, but there are examples; my notes say an RSA-5 fuel control alone is responsible for a pressure loss of about 12.75" H20 at 1560 pph, or 0.94" Hg. An FM-200 is about 4" H2O, or 0.3" hg, same conditions.
So, if you're seeing more on the EFIS than any reasonable calculation, well, it would have to be magic.
Quote:
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So, for my flight at 13500', 74degree, 30.07 alt setting , I see a calculated absolute pressure of just over 18" and my MAP gauge was reading 20.5 at wide open throttle.
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Yep, that would magical. If I use your inputs with an assumed a 3.5"D intake and IO-360, pressure before the filter would be about 18.1" Hg. Mr. Short's report of 17.4 displayed on the panel is probably pretty accurate.
BTW, 74F at 13,500 feet? Is it really that hot over California?