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). This is the true indication of the engine's power setting. =20 =20 As for conversions, standard atmospheric pressure at sea level is 29.92 inches of mercury, roughly 30. Remember the vacuum gauge that went from = 30 to 0? It's the same range, and units, but turned around to show vacuum relative to the 0 starting point. The only conversion is to turn the numbers around. If someone says they have 20 inches of vacuum at sea = level, that's 10 inches of MAP. If they aren't at sea level, you have to = adjust by about 1 inch per 1000 feet. =20 =20 One psi is very close to 2 inches of mercury. So when I say I have 3 = psi of boost, that's the same as saying 6 inches of mercury. Since I'm at sea level, that's 36 inches MAP. Since I don't want to run my engine over = 36 inches MAP, I can run 3 psi at sea level. When I go to 10,000 ft, the outside pressure will be about 20 inches since you lose about 1 inch per thousand feet. Now, to normalize to sea level pressure of 30 inches, I = need to add 10 inches of boost, which will be 5 psi on the boost gauge. =20 =20 Make sense? Simple, but confusing. OK, now everyone check to make sure = I got it right :-) =20 =20 John Slade (all leather, no gas) =20 =20 I'm not sure I want to know what this means :-)=20 =20 Rusty (all gas, no leather) =20 =20 ------=_NextPart_000_0083_01C37842.8B757050 Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: quoted-printable Message

You seem to = know a lot=20 about turbos. Could you give some of us (eg me) a bit of basic = education?

If you want to = learn how=20 turbos work, buy a stock 3rd gen RX-7 from an original owner who has no = clue how=20 it's supposed to work. The real leaning begins when you add a = boost gauge,=20 and find that the sequential turbo system isn't working. By = the time=20 you diagnose the many problems it will have, change about 76 hoses and a = few=20 solenoids, you will have graduated the Mazda school of turbo=20 education.

Seriously, I'm = no turbo=20 expert. I have a good basic understanding of how they work, but = have a lot=20 to learn when it comes to sizing calculations and such. That sort = of info=20 is in the books that Todd mentioned, but I've never had occasion to be = forced to=20 learn it. In my case, I wasn't considering any turbo other than = the stock=20 Mazda unit, which has it's limitations for aircraft use, but it's cheap=20 (used). There are certainly better choices, but they cost a = bunch of=20 money.

I'm confused by the mixed units. = There's=20 talk about x inches of Manifold Air Pressure, then about so many PSI of = boost.=20 Are we talking about the same thing using different units?

I have to=20 confess that I was incorrectly using these units myself until Tracy = straightened=20 me out. As I understand it, there are only two units, which are = inches of=20 mercury, and psi. =20

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In the car=20 world, a traditional vacuum/boost gauge will show a 0 point in the = middle, with=20 vacuum to the left, measured in inches of mercury, and boost to the = right,=20 measured in psi. The vacuum range goes from 0 in the middle, to 30 = at the=20 far left, and the boost range goes from 0 to whatever they choose = as the=20 max reading to the right. This is a differential gauge, = meaning that=20 the reading is relative to the outside pressure. In other words, = when the=20 engine is off, it will always read 0, regardless of what the outside = pressure=20 is. I have one of these in the plane, because it will show me = how=20 much boost the turbo is making relative to the outside = air. This will=20 help me keep the turbo within it's normal operating=20 range.

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In the=20 airplane world, I believe the tradition is to only look at manifold = absolute pressure (MAP) in inches of mercury. This is = actually=20 simpler. The EM-2 will give me this info (eventually=20 <g>). This is the true indication of the engine's = power=20 setting. =20

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As for=20 conversions, standard atmospheric pressure at sea level is 29.92 inches = of=20 mercury, roughly 30. Remember the vacuum gauge that went from 30 = to=20 0? It's the same range, and units, but turned around to show = vacuum=20 relative to the 0 starting point. The only conversion is to = turn the=20 numbers around. If someone says they have 20 inches of vacuum = at sea=20 level, that's 10 inches of MAP. If they aren't at sea level, you = have to=20 adjust by about 1 inch per 1000=20 feet. =

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One psi is=20 very close to 2 inches of mercury. So when I say I have 3 psi of = boost,=20 that's the same as saying 6 inches of mercury. Since I'm at = sea=20 level, that's 36 inches MAP. Since I don't want to run my engine = over 36=20 inches MAP, I can run 3 psi at sea level. When I go to 10,000 ft, = the=20 outside pressure will be about 20 inches since you lose about 1 inch per = thousand feet. Now, to normalize to sea level pressure of 30 = inches,=20 I need to add 10 inches of boost, which will be 5 psi on the boost=20 gauge. =20

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Make=20 sense? Simple, but confusing. OK, now everyone check to make = sure I=20 got it right=20 :-)

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John Slade (all leather, no gas)

I'm not sure I=20 want to know what this means = :-)

Rusty (all = gas, no=20 leather)

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