  We present new Spitzer IRS observations of HD164270 (WC9, WR103). A
quantitative analysis of the UV, optical, near- and mid-IR spectrum of HD164270
is presented, allowing for line blanketing and wind clumping, revealing T* ~
48kK, log L/L_sun ~ 4.9, dM/dt ~ 1e-5 M_sun/yr for a volume filling factor of f
\~ 0.1. Our models predict that He is partially recombined in the outer stellar
wind, such that recent radio-derived mass-loss rates of WC9 stars have been
underestimated. We obtain C/He ~ 0.2 and O/He ~ 0.01 by number from optical
diagnostics. Mid-IR fine structure lines of [NeII] 12.8 and [SIII] 18.7micron
are observed, with [NeIII] and [SIV] absent. From these we obtain Ne/He ~
Ne^+/He = 2.2e-3 by number, 7 times higher than the Solar value (as recently
derived by Asplund et al.), and S/He ~ S^2+/He = 5.1e-5 by number. From a
comparison with similar results for other WC subtypes we conclude that WC9
stars are as chemically advanced as earlier subtypes. We consider why late WC
stars are exclusively observed in high metallicity environments. In addition,
we compare the UV/optical/mid-IR spectroscopic morphology of HD164270 with the
Planetary Nebula central star BD+30 3639 ([WC9]). Their UV and optical
signatures are remarkably similar, such that our quantitative comparisons
confirm similarities in stellar temperature, wind densities and chemistry first
proposed by Smith & Aller, in spite of completely different evolutionary
histories, with HD164270 presently a factor of ten more massive than BD+30
3639. At mid-IR wavelengths, the dust from the dense young, nebula of BD+30
3639 completely dominates its appearance, in contrast with HD164270.
