My boats been running 160 and jumps to 180 at times. My manifolds get real hot and sometimes they make noise like they boil inside. It has a after market 90 riser that seems to move very little water and the bypass moves all the water out the riser. Is there a way to re pipe a line out of the top of the manifold and tee the bypass or to the riser to move the water from the manifold better.

swap the hoses at the thermostst housing so most of the water goes to the lower part of the manifold first..sounds like some one got it backwards.

so take the cold water line and put them on the bottom of the manifolds and the back hot water lines at the stat go to the riser.

The boats running 160 lowest and 180 highest by the gauge. With a laser temp gun i get 200 deg at the ex manifold with a cold water hose after its back from a drive. And 150 to 170 at the top of the riser

Boat runs around 55-57 mph but after a few passes it with have a hard time to get over 49mph