This study looks into interaction between the quasi-phonemic vowel length contrast in Scottish English and its word-prosodic system. We show that under the same phrasal accent the phonetically short vowels of the morphologically conditioned quasi-phonological contrast are produced with significantly more laryngeal effort (spectral balance) than the long ones, while the vowels do not differ in quality, overall intensity or fundamental frequency. This difference is explained by employing the concept of "functional load". Duration must be kept short to mark the short vowel length, while both word-stress and phrasal accent require lengthening. Therefore, the additional laryngeal effort in the short vowels serves a prominence-enhancing function. This finding supports the hypothesis proposed by Beckman that phonological categories of word-prosodic systems featuring "stress-accent" are not necessarily phonetically uniform language-internally.