The 450 is also a turbine pump.

Have you confirmed you need more than one 450?

Yes, of course. One 450, or two 255HP's, can't hold fuel pressure past about 30-32 PSI. When the 255/450 both worked, it would hold pressure no problem, I actually had headroom in my fuel system then.

Interesting. The DW300 has a pressure relief valve that opens above 100psi. The 450 opens at 110psi. Food for thought.

That is indeed interesting. Given how much pressure I was running the pumps at, that may have been too much and made them bypass in boost. I backed them off to 60 base pressure now.

As a follow up, I powered up the pumps in tank, and the 255 was spraying fuel out of the plastic top of the pump! Looked fine by visual inspection, but leaked bad when pressured up. So weired, never seen that before. New DW300 is installed and working great!