Which do you use, nerve-wracking or nerve-racking? I hadn't seen the latter before. However, it seems to be the preferred use, and the OED has wracking cited from 1909, while racked is from 1893.

They are alternative spellings. There are a a couple of other similar idiomatic uses, such as (w)rack and ruin. Dictionary.com says that it comes from OE wræc "misery, punishment," and wrecan "to punish, drive out"

There's also a usage note,

quote:

The use of the spelling wrack rather than rack in sentences such as she was wracked by grief or the country was wracked by civil war is very common but is thought by many people to be incorrect