OK, so the more accurate conversion is 

20psi=40.7 in Hg

so in your example

20psi boost = 70.6 in MAP (Manifold Absolute Pressure)

It help in your article, turbos work as a boost ratio.  So this turbo it then making a boost ratio of:

70.6 / 29.9  = 2.36

So as you go up in altitude you multiply the ambient pressure by 2.36

So at 17,500 ft the pressure is half, and so is the MAP, or, 35.3 " MAP

Losses are not porportunately reduced, and the turbo may be less efficient in that range so actual MAP may be less.

On Sat, Jul 6, 2013 at 11:57 AM, Patrick Panzera <editor@contactmagazine.com> wrote:

I'm editing an article about a bone stock, turbocharged direct injection car engine that boasts 20 lbs of boost.

I want to convert that to MP so us airplane junkies can wrap our heads around it.

Thanks!

Pat 

On Sat, Jul 6, 2013 at 11:44 AM, David Leonard <wdleonard@gmail.com> wrote:

Simply convert 20 psi to Inches of Hg (its about 2 to 1 but not exactly) and add it to 29.9.  So the MP should be about 69 inches, give or take depending on losses and where in the system you measure.

I keep my plane less that 45 inches MP.  You have to be more rich and skilled than me to go higher without detonation.

On Sat, Jul 6, 2013 at 11:14 AM, Patrick Panzera <editor@contactmagazine.com> wrote:

Friends,

Hypothetical question. 

Let's say it's a standard day and I'm at sea level.

I've installed a turbocharger and have also connected the stock automobile turbo boost gauge, along with my traditional manifold pressure gauge.

I ground-run the engine to full throttle (where the MP would normally be reading 29.92- minus some efficiency loss) and the boost gauge is reading 20 psi.

What should the MP indicator be reading?

Thanks!

Pat

--
David Leonard

Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net

--
David Leonard

Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net