flyrotary@lancaironline.net Mailing List Archive

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

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

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

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

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

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

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

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.

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.

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

John Slade (all leather, no gas)

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

Rusty (all gas, no leather)