boost pump question

[grubac]

Hey Everybody

I've got a very basic question.

In the Lycoming IO-390 specification it states that the boost pump outlet pressure limit, when connected in series with the motor's fuel pump, is 35 psi maximum (when the motor's fuel pump is not working) and is 14 psi minimum (when the motor's fuel pump is working at maximum conditions).

I completely understand the maximum boost pump outlet pressure limit. I don't however, understand the minimum. What would happen if the boost pump provided less that 14 psi during maximum operation conditions?????

Any answers would be much appreciated.

Grubac

[Wayne Gillispie]

it developes pressure up to the pressure relief valve setting and then starts recirculating. The pressure on the inlet side of the mechanical pump will be the same at this point. Once the engine is started the mech fuel pump inlet pressure will decrease as fuel flow increases. At full power on climbout(max operating conditions)if your boost pump outlet pressure was indicating less than 14 psi that may indicate you have faulty pump or a restriction in your fuel lines/filter/screen before the pump. If your mechanical fuel pump quit pumping you want to have adequate flow/pressure from your electric boost pump to keep the engine running until you can land. Lycoming does not know what boost pump set up we use. I am trusting that Van's has already tested my setup since I built per the plans. If one deviates from the plans one may want to measure these pressures before flight. The mechanical pump on the IO-540-D4A5 can have a minimum inlet pressure of -2 psi. By setting the minimum outlet pres at 14 psi this gives us some margin to guard against vapor formation and lets us know we have a properly sized boost pump for our engine, especially if the mech pump goes out.

[grubac]

Hey

Thanks a lot for the reply. Now I completely understand why the booster pump is needed. I'm not sure I understand how the mechanical pump can have a negative pressure though.

I'm also looking to have a return fuel line to the tanks. I can't, however, decide where to install the "T". I've heard of some installing a T between the mechanical pump and the fuel servo and of others that have installed the T before the mechanical pump. Does anyone have any experience with this? Does installing the T at the mechanical pump actually involve drilling a hole in the mechanical pump?

I have a IO-390 which has a precision fuel servo. I know that, technically speaking, I don't need a fuel return line, however, I would still like install one to be sure.

Any help is much appreciated.

Grubac