# this is bad (also obsolete)
[ "$var" != foo -a "$var" != bar ]
# this is good (and fully POSIX compliant)
[ "$var" != foo ] && [ "$var" != bar ]
# you probably don't need to care for systems where this was necessary
[ "X$var" != Xfoo ] && [ "X$var" != Xbar ]
Yes, they were certainly less efficient than modern engines. However, ICE engines have always had trade-offs between power output per size, mechanical efficiency and reliability, and for this vehicle the last one was certainly of most importance.