The poser loss issue is directly the reduction in atmospheric pressure. The
EFI computer keeps the proper air/fuel ratio and the developed power drops
proportionately.

Only one device eliminates this problem - a turbocharger. With the turbo you
can control the manifold pressure and there is adequate compression capacity
to overcome the atmospheric pressure reduction.

The reason that you so strongly notice the reduction in power as compared to
other vehicles is explained in my posts on gearing and engine power issues.
The key problem is the amount of additional thrust available from the
engine/powertrain combination above and beyond the thrust needed to move the
vanagon under the conditions required. Hint: Professional hill-climber racers
run very high numerical ratio final drives compared to flattrackers. Your
vanagon probably has 5% reserve thrust capacity after paying the altitude
penalty.

Frank Grunthaner