Two in series may not give much more
flow than one; depending on the back pressure vs the pressure at which those
flows are based. If those pumps are rated flow at 0 pressure, it is likely that
even the AL one is marginal.
I’ve done the math on the 20B. The
flow requirements depends on the cooling system design (obviously); but if you
were to design for a sort of optimum system for an aircraft, you’d like
to have 20 – 30 F temp drop around the loop when you are running about
85% power, say, 220 HP. So for a 50/50 EG/water mix, and 25F delta T; that
says 39.5 gpm. For pure water the number is 28.5 gpm
The only real data I have on my pump is
from the dyno runs. That showed 43 gpm at 5000; 48 at 6000. That is
without a thermostat, and on a large capacity system with presumably relatively
low back pressure. 25-30% less with a thermostat. Unfortunately, I
don’t know what it is on the airplane.