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SkyKing, Something doesn't add up in all of these charts.
In the standard Cessna charts, notice that any given %HP at any given altitude is a constant PPH usage. For instance, 65% HP is 140 PPH - regardless of altitude, RPM, and MP. And whether it is above or below standard conditions makes no difference. The constant is that the leaning must be done the same in every example.
Conversely, for any given PPH burn, you can use the chart to identify the %HP regardless of altitude, RPM, and MP. Again, the leaning procedure must be a constant for this to work.
An intercooler has the same effect as changing the conditions from 20C above standard to 20C below standard.
Cessna’s POH says 158 PPH will give you 75% HP. Yet your chart says 176 PPH will be 75%. Doing the simple math of 176/158 = 1.113, and 1.113 * 75% = 83.4% HP. If you believe the Cessna charts, then you're actually running at 83.4% HP when you think you're at 75%.
You also report an increase in airspeed for the same constant %HP. How is this possible? To increase airspeed, you have to either reduce drag or increase HP. Since intercoolers do not reduce drag, they must be running at an increased HP.
The fact that you are burning enough fuel to equal 83% HP - and you are getting the expected airspeed increase of extra HP, this should cast doubt on the claim that you’re still running at 75% HP.
Try finding a power setting that gives you a fuel burn of 158 PPH (50 ROP). I'll bet you get the Cessna airspeeds for the 75% HP setting at that particular altitude.
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