how long does the exaust pipes have to be with a 235 at 50 duration camshaft so water doesn't get in the motor chevy
big block

Reversion is more dependent on lob sep and cam overlap. Post all the cam specs.
What matters is how far back the water mixes with the exhaust.

If you dump some water out of the exhaust before it gets to the ss pipes this could help stop reversion. I put a custom made 1/2 "tee on the hose connecting the exhaust manifold with the ss pipe EMI exhaust. You only need a very small amount of water to cool any rubber exhaust hose. My cam is a 236 duration @ .050 No problems here and I have the short ss risers