Optimization over 7 constants, running a function 10,000,000 times
You have several loops that are completely unnecessary. Remove them and you will see dramatic speedups. Take voltoalph and voltoenen for example:

voltoalph2 &lt;- function(v,c) {
  # v: a data vector
  # c: a scalar constant
  c(0, c[1L] * (v / max(v, na.rm=TRUE)))
}
voltoenen2 &lt;- function(v,c) {
  # v: a data vector
  # c: a scalar constant
  enhelp &lt;- 1 / c(1, abs(v / max(v, na.rm=TRUE)))
  c[1L] * (enhelp / max(enhelp))
}
require(microbenchmark)
v &lt;- 1:10000
microbenchmark(voltoalph(v, 2), voltoalph2(v, 2))
# Unit: microseconds
#              expr        min          lq        mean     median          uq        max neval cld
#   voltoalph(v, 2) 105004.652 106038.7245 108767.0230 106725.827 108326.7975 151482.035   100   b
#  voltoalph2(v, 2)     81.925     83.5825    141.3382     89.794     91.6065   5350.052   100  a 
identical(voltoalph(v, 2), voltoalph2(v, 2))
# [1] TRUE
microbenchmark(voltoenen(v, 2), voltoenen2(v, 2))
# Unit: microseconds
#              expr        min         lq        mean     median        uq        max neval cld
#   voltoenen(v, 2) 103595.526 105433.803 108797.6308 106882.274 110160.65 148550.397   100   b
#  voltoenen2(v, 2)    200.275    203.182    227.6582    207.932    220.57   1007.375   100  a 
identical(voltoenen(v, 2), voltoenen2(v, 2))
# [1] TRUE


The vectorized (non-loop) versions are over 1000, and 500 times faster (for voltoalph and voltoenen, respectively), and provide identical output. percentcorrect calls each of those functions two times, which makes the speed improvement have twice the impact.