8. Let $\{a, b, c, d\}$ be a subset of $\{1,2, \cdots, 17\}$. If 17 divides $(a-b+c-d)$, then $\{a, b, c, d\}$ is called a "good subset". Then, the number of good subsets is $\qquad$
8.476.

Construct a regular 17-gon $P_{1} P_{2} \cdots P_{17}$. Then
$$
\begin{array}{l}
17 \text { divides }(a-b+c-d) \\
\Leftrightarrow a-b \equiv d-c(\bmod 17) \\
\Leftrightarrow P_{a} P_{c} \parallel P_{b} P_{d} .
\end{array}
$$

Remove one vertex, and the remaining 16 vertices can be paired into 8 pairs, forming 8 parallel chords. Any two of these can serve as $P_{a} P_{c}$ and $P_{b} P_{d}$, thus forming a good subset $\{a, b, c, d\}$.
Therefore, the answer is $\mathrm{C}_{8}^{2} \times 17=476$.