Is it ok to pressure test my alum exhaust manifolds with my (residential) hose? It probably has 40-50lbs of pressure.

I didn't think home pressure was that high, mine is @ 15 psi

That is A LOT of pressure, I'd vote NO. On my Stainless Marine manifolds, there was probably 100sq.in on either side of the casting, you're looking at 4-5000 pounds of force. I'd pressurize to a couple psi, with air, maybe a bike pump, and dunk the manifold in water, look for bubbles.

Huh????? That is like saying you have 1000 square inches of plumbing in your in your house and there is 50000lbs of force.The force is spread out.
It is still 40-50psi and a good set of manifolds can handle that pressure with no problem.

I had the first gen stainless marine bbc manifolds and that is exactly how I tested mine. My water pressure built up to about 30 psi.

BTW--on mine, the aluminum bottoms were fine and I found a crack in one of my ss riser pipes (the inner pipe).

Good Luck, FT