Could be, but - I don't think so, Jim.

Most of the time if you are talking "Hg boost you are talking absolute pressure.  If you are talking "psi" boost then you are generally talking differential pressure "gauge" pressure(absolute - ambient = gauge).  66" HG of manifold pressure at sea level = (66-30) = 36 "Hg boost = 36/2 = approx 13 psi boost - that's a lot of boost for our application! Dave would  be producing considerably more HP than an NA 13B.  Something on the order of more than 260 HP! Hot Dog!!

On the other hand, if his pressure was absolute of 36" then 36-30 = 6"hg of boost or approx 3 psi or so.  This would give Dave approx 200HP instead of the 260HP or more that a 66" Hg figure would give.

But, perhaps Dave is producing 260HP - however, that was not the impression I got.

Ed A









----- Original Message ----- From: "Jim Sower" <canarder@frontiernet.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Monday, September 05, 2005 1:51 PM
Subject: [FlyRotary] Re: Another great flying day = another day of troubleshooting

If you're defining boost as over pressure, his SL pressure was 30" ambient + 36" boost 0 66" MAP and at 10k' he's got 26" ambient + 26" boost = 54" MAP
I think he's talking 36" MAP at SL and 26" MAP at 10k.
Either way, MAP at altitude is a LOT less ... Jim S.


Ed Anderson wrote:

Well, actually it is absolute pressure that the spark plug is reacting to.  In this example, the absolute pressure in the intake is 1.234" HG higher at 10,000 with your 26 " boost than at sea level with your 36" of boost..  This would contribute to an increased pressure in the combustion chamber when it is compressed over the sea level compression pressure. Therefore, the spark plug is facing a tougher task at 10,000 than at Sea level (in this example).
 Ed

    ----- Original Message -----
    *From:* David Leonard <mailto:wdleonard@gmail.com>
    *To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
    *Sent:* Monday, September 05, 2005 12:57 PM
    *Subject:* [FlyRotary] Re: Another great flying day = another day
    of troubleshooting

             Actually, Dave you had a higher pressure ratio (manifold
            to ambient) at 10,000 feet with 26" boost than at sea
            level with 36".  At sea level the pressure ratio would be
            36/29.92 = 1.20.  At 10,000 ft ambient pressure = 20.57 "
            hg , so the pressure ratio would  be 26/20.57 = 1.26.  Not
            much greater, but it was higher by approx.1.23" Hg. (1.20
            -1.26 = .06 *20.57 = 1.234" Hg).

            Ed A

    Right.  But what does the plug know about the ambient pressure?  I
    thought that it was just the absolute density of the charge that
    was contributing to SAG.

    --     Dave Leonard
    Turbo Rotary RV-6 N4VY
    http://members.aol.com/_ht_a/rotaryroster/index.html
    http://members.aol.com/_ht_a/vp4skydoc/index.html

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