"Steve Brooks" <steve@tsisp.com> wrote:

"I've been reading these emails between you and Rusty and John with great
interest.  I am really confused now.
I have a boost gauge made by VDO.  It reads both suction 0-30hg and boost
0-+25hg.  The center scale (0) is at the 1 o'clock position or there about.

I thought that this is what I need to monitor the turbo boost level, but
now, I don't have a clue.
I'm not sure what this gauge is going to show me."


Hi Steve, That gauge shows the entire range of deck pressures available, with suction being throttle settings less than WOT, and the boost side showing, well... boost.  The difference here is that you've got a gauge that actually reads inches of mercury... when people are talking specifically about boost they usually discuss it in terms of psi.  Your gauge should show 0 when the engine is shut down... that zero point is equal to ambient atmospheric pressure on a more typical manifold pressure gauge.  At sea level on a standard day with the engine shut down, a MAP gauge would show 29.92"hg, your boost gauge would show 0 because it's referenced to ambient pressure.  Once you start the engine and you run it at idle your boost gauge is going to show some level of vacuum, probably something like -20"hg.  Your manifold pressure gauge is going to show 10"hg... that's ambient less that 20" of vacuum.  This is because the engine is acting as an air pump and it is sucking on the air in the intake manifold... the pressure is reduced because the throttle plate is mostly closed and only a small amount of air is being allowed to enter it.  As you open the throttle the vacuum will start to diminish (less vacuum will be a lower suction number) so at 1/2 throttle you'll probably be reading something like -10"hg... the MAP gauge will read 20" now.  At WOT you'll be back to 0 (actually there are some friction losses, so you may well be showing -1 or -2"hg) and the MAP gauge will be back up to ambient (actually 28 or 29"hg). As the turbocharger starts to pressurize the intake air (make boost) your boost gauge will move into the positive area, and the MAP gauge will start to read pressures greater than ambient.  When your boost gauge is showing +10"hg of boost the MAP gauge will be showing 40"hg.    To convert the (VDO) boost level to a psi reading, just divide it by 2... 10"hg = 5psi of boost.  So, your VDO gauge will most certainly tell you exactly what's going on inside your intake manifold, and it is in units that are compatible with what you're used to seeing on a typical MAP gauge in an airplane.  You can instrument your plane however you like, and your VDO will do the job. I'm used to seeing MAP (that's "Manifold Absolute Pressure") so I will use a more typical MAP gauge and only concern myself with absolute deck pressure. Up at altitude (say 10kft) if I push the turbo to the max I'll probably be able to maintain my 34"hg redline.... on the VDO instrument I'd have to know what ambient pressure is (about 20" at 10kft) and push the turbo control until I'm seeing 14"hg.  The problem with your VDO is that I don't always know what ambient is so I don't know what the absolute pressure is that the engine is seeing.   14" of boost at 10kft altitude is just fine (as it's only 34" of MAP)... 14" of boost at sea level is a different story, as that's 44" of MAP, well beyond my self imposed MAP redline.  I hope this clears up your confusion.

   <marv>