I checked my (Dynon) MAP sensor today. It reads high by 0.45" HG. That is bigger error than I would have expected.
So I reduced the MAP values in Post #17 by the 0.45" HG delta, but the numbers still seem a bit high, based on your comments in Post #19.
Here are the revised data points. RV-8, YIO-360-M1B, 1 Mag & 1 EI, Snorkel, at WOT:
21.5" MP, 2450 RPM, 9500' MSL, 10771' DA, 208 MTAS/181 KTAS.
20.0" MP, 2440 RPM, 11500' MSL, 11985' DA, 205 MTAS/178 KTAS.
Yes. Still too high. Why? Hmm. Maybe I'm missing something here.
(Review intake tract data)
Duh...I was sloppy. Sorry.
The 1.06" Hg (14.4" H2O) tract loss quoted previously (deltaP, airbox prior to filter vs MP at #3 primer port) was a value for 2500, WOT, derived (as a cross check) from two other deltas. However, I also measured the airbox/MP deltaP directly, at three different altitudes....
and tract loss decreases with altitude. That detail would close some of the gap.
Here's a link to the original spreadsheet record:
https://www.danhorton.net/VAF/Scoop vs Snorkel/
Note Line 31. It's what I grabbed in a hurry while slopping down some coffee before work.
Now look at Line 18, the measured tract pressure loss for three altitudes.
Extrapolate from the 6500 and 10500 loss, and tract loss is roughly 9.2" H2O at 9500 and 8.4" H2O at 11500. That's 0.68" Hg and 0.62" Hg.
9500 ft static is 20.98..call it 21.0. Factoring density, Q is 1.13. 80% of Q is 0.9. 21.0 plus 0.9 = 21.9. 21.9 less a tract loss (mine, not yours) of 0.68 is an estimated MP of 21.2. You're reporting 21.5. Difference is 0.3" Hg
11500 ft static is 19.41..call it 19.4. Factoring density, Q is 1.05. 80% of Q is 0.8. 19.4 plus 0.8 = 20.2. 20.2 less a tract loss of 0.62 is an estimated MP 19.58. You're reporting 20.0. Difference is 0.42".
Right now I don't have an explanation for why you're still higher than the estimates. There are quite a few variables. That said, your new corrected MP's are a lot closer to reality, as are Steve's.
Steve, had a chance to check calibration?
BTW, as a reality check on the estimates, here are panel shots from a trip to PA last week. Note these MP figures are for a horizontal ram airbox, not a snorkel.
9500, peak EGT, WOT. Static would be 21.0 Including adjustment for 23F above standard, recovered Q would be 1.174 x 75% = 0.88 (the 75% is computed from the spreadsheet). The total before the filter is thus 21.88". The tract loss was 0.68", so the estimated MP would be 21.2". I'm actually showing 21.3, pretty close.
11500, 60 LOP, WOT. I show 19.5" at 180 KTAS. Static would be 19.41. Including adjustment for 30F above standard, recovered Q would be 1.03 x 75% = 0.77". The total before the filter is thus 20.18". The tract loss was 0.62, so the estimated MP would be 20.18 total less 0.62" tract loss = 19.56" I'm showing 19.5"