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boost gauge...what exactly is it measuring? 850 1997

posted by  davidm on  Tue Jan 8 13:10 CST 2002
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The gauges we use in the Volvo world are not strictly boost gauges (which would only measure the output of the turbo, always a positive number when the engine is on). The Volvo measures the intake manifold pressure and displays the balance between the vacuum produced by normal aspiration, and the positive pressure produced by the turbo. That's why you can get a less-than-an-atmosphere reading when the turbo is in a low-boost state. The turbo is producing positive boost, but not enough to completely offset the vacuum of piston/valve aspiration. Turn the key off, and you have perfect balance (no boost, no vacuum): the gauge is straight up and down on stock Volvo gauges, at "0" on VDO, Auto Meter, and other instruments.

In my case, the controller generally limits sustained manifold pressure of my car to 9 lbs psi....right about where you see your gauge. A full atmosphere would typically be about 14-15 lbs depending on how close to sea level you are. I've seen an occasional spike to 14 or so, but personally, the 9 lbs I get are plenty.
--
(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)






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