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My post had nothing to do with how the gauge reads, but the gauge reads manifold pressure relative to ambient pressure (that would be straight up on the later gauges). My airplane has a manifold pressure gauge that reads absolute pressure, as all manifold pressure gauges in aircraft do. Yes you can buy absolute pressure gauges for connection to the intake manifold on turbocharged cars. I use one when checking the boost on our three 700 series turbo cars. I have a tee in the line to the boost gauge (under the instrument panel) that connects to a rubber tube that is coiled up by the fuses and has a golf tee plugging it.
What my post stated is that turbocharged engines lose power at altitude. The Volvo engines all have a fixed wastegate that is not adjustable from the passenger compartment. As you gain altitude, the power output of the engine diminishes, but not as much as with Normally Aspirated engines. Some aircraft can be overboosted at low altitude, and you have to use the throttle carefully while watching the manifold pressure gauge. I have not operated anything with an adjustable wastegate, but they do exist. An adjustable wastegate lets to operator keep the engine at sea level power output to well over 20,000 feet. Cars generally operate at much lower altitudes than aircraft do, and the wastegate is factory set so the engine will not be damaged at maximum sea level boost. This fixed setting sacrifices the maximum power available at higher altitudes.
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