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Running over square.

(edit - everything below makes little sense as my boost guague is not accurate enough for the calculations I was trying to make - ignore).

I am getting mixed messages about running over square on a Rotax 914, Airmaster in flight variable pitch.

To complicate things, I only have a boost and fuel pressure gauge and not an MP gauge.POH (Performance data for variable pitch propeller) says 31 MP for 5,000 rpm, 35 MP for 5,500, 39 MP 5,800 (max 5 minutes).

So firstly, to adhere to the above, do I have the conversion correct?Ambient or resting MP = 29.92 Hg. I have read that boost at rest is 14.7 psi (which show 0 boost on the gauge)1 psi = 2.036Hg

When I go fuel throttle on take off I get about 6 psi which equals 14.7 + 6 = 20.7 * 2.036 -= 42.1452 (seems a bit high)

If I want to achieve 31 MP, 31 / 2.036 = 15.22 - 14.7 = 0.5 psi on boost gauge.

If I want to achieve 34 MP, 35 /2.036 = 17.19-14.7 - 14.7 = 2.49 psi on boost gauge

The above seems awfully complicated and also a bit low on the boost readings.

Notwithstanding that it would be a lot easier to get an MP gauge (but that will take time), do I understand the above correctly?

Is there any easier way?

Tell me what I am getting wrong here.

  • Re: Over Square and boost to MP conversions

    by » 3 months ago


    You understand it correctly but you have simplified the problem;  it is even more complicated.
    You forgot to consider the altitude and the Barometer in your conversions.
    Boost is Gauge pressure relative to local ambient.
    MP is Absolute pressure relative to a Perfect Vacuum.

    14.7 psi is 29.92 only if the altimeter setting is 29.92 and then, only if you are at Zero MSL (Sea Level)
    The pressure drops ~1psi for every 2000 feet of altitude.
    The pressure drops ~1psi for every 1"Hg of Barometric pressure.
    So if you are on the ground at 1800 MSL and the Altimeter Setting is 29.72, you are already 1 psi low.
    At 29.92 and 6000 MSL will need to add 3psi of "Boost" just to get back to 30"Hg which is really No boost at all.
    Boost might be useful on the track but is nearly worthless at altitude.

    You Seriously need a Manifold Pressure Gauge!!!
    That is the easier way!  Way Easier!!!


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


    Thank you said by: RotaxOwner Admin

  • Re: Over Square and boost to MP conversions

    by » 3 months ago


    Thanks Bill, I too have since found out that the boost is really only an indication and not nearly accurate enough for what I was trying to achieve.  Will be getting an MP guauge.


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