When we are not stressing the words can will normally sound more like kn with very little vowel sound and can't will have the full vowel sound but with just a stop at the end. If we are stressing the word, usually to draw the listener's attention to that specific word for whatever reason, can will be pronounced with the full vowel sound and can't will be pronounced with either a slight popping sound (made with the tongue pulling sharply off the hard palate just behind the teeth) or else with the same stop as in the unstressed version. In either case in the stressed version the vowel sound in can't will be shorter in time duration than the vowel sound in can.

Does this make any sense?