Y-pipe sizing — is a 3" Y-pipe too much for a 4.8 or 5.3?

Speedway's rough rule of thumb: about 1 inch of total pipe diameter per 100 hp across the system. A 500 hp engine wants about 5 in total — dual 2-1/2 in pipes, for example.

By that rule, a single 3 in pipe is sized for roughly 300 hp, which suits a stock to mildly modified 4.8 or 5.3. It's a rough estimate, so use it as a starting point.