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5KT: Of turbos and altitude, also O2 sensor ...

	Took the old '85 5KT up to Tahoe on Saturday: turned 106K on the
	trip, but the old beast had no trouble keeping up with anything
	on the road uphill or down. I was pretty happy ...

	I was watching the boost gauge closely this time, and I noticed
	that altitude made a *very* significant difference.  At sea-level,
	it winds up to 1.4 bar at WOT most of the time (except when it's
	very hot).  At about 5,000 feet it wouldn't budge past 1.2, and
	at nearly 7,000 feet it would barely make it to 1.2.  At that point,
	it almost felt like an underpowered normally-aspirated car(!).

	OK, so this is no revelation to a lot of you folks.  But I'm
	curious about a couple of things.  Clearly, it appears that
	the boost regulation at the wastegate is based on relative
	(or "gauge") pressure, and that the display shows absolute pressure.
	It makes sense (I suppose) that a mechanical wastegate would
	sense relative pressure, but shouldn't there be some altitude
	compensation applied by the ECU?  Or is this just a turbocharger
	capacity (rather than wastegate-imposed) limitation?  Or is there
	something wrong with my car?

	Also, my OXS warning light on the dash turned on halfway through
	the trip: Bentley says "replace O2 sensor every 30,000 miles". Can
	I test the thing to see if it's actually dead (since the warning
	light's triggered by the odometer), or do I have to just replace
	it on faith?  Will it make a noticeable difference (I'm looking
	for motivation besides emission control to do it!)?


Arun Rao
1001 W. Cutting Blvd.
Pt. Richmond, CA 94804