[Vwdiesel] Altitude compensation

Roger Brown r.c.brown at ieee.org
Fri Sep 2 22:50:50 EDT 2005

Libbybapa at wmconnect.com wrote:
> When I checked the compression on my TD vanagon all four were within 5 psi of 
> each other.  3 were 400 and the fourth was 395.  I was a little concerned, 
> but the engine ran fantastic, so I've continued to drive it.  My friend's 
> quantum checked out at 380 on all four.  I recently got thinking about the fact that 
> I live at about 7,000 ft.  When the altitude compensation is factored in to 
> my friend's engine his compression at sea level is 460 psi.  My TD vanagon 
> engine would read 475-480 psi!  Nothing wrong with that!
> I was also thinking about the recent discussion of Passats running on low, 
> mid or high grade fuel.  Someone mentioned that some people saw no performance 
> decrease or mileage decrease with the lower grades, others saw significant 
> difference.  I got thinking that probably the cars seeing no change were primarily 
> at high altitudes.  I'd love to have that member of the group post that idea 
> to the passat group.  It makes me think that I've been wasteing my money 
> putting high test in my ABA work van.

You can generally run 1 grade lower octane gas up high.  My Toyota 4Runner takes 87, but when I hit the high plains and mountain 
areas where 85 octane fuel is available, I use it and the engine works fine.  In fact one time I was still running on 85 in the 
tank by the time I got back to sea level and it ran fine.  I do find I get better MPG at the higher elevations, probably a 
combination of less engine power (ECU cuts back on fuel to match the lesser amount of air) and the thinner air has less density 
and drag.


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