Originally Posted by theduff
Im not familiar with using the inlet fuel pressure tap on an RSA-5 injector as a return line fitting but it stands to reason if you are feeding the servo with a -6 inlet fuel fitting and you use a -4 to return the fuel to the tank your just going to pump the fuel in a continuous by-pass and never develop enough inlet pressure to run the engine. I suspect the restricter fitting was to choke off some of the by-pass fuel so it would develop the proper inlet pressure.
I would like to hear from someone who is familiar with plumbing an RSA system in this manor and how it helps with Hot-Starts.
I use the -4 ORB tap on the RSA to feed a block that has both a transducer and a -4 return line to the tank. The return line has a valve to open and close the circuit. I use this to purge hot fuel from the lines up front after a heat soak and it works very well for this purpose. On really hot days, I will leave the return open until just before takeoff to insure no stumbles as I go WOT. This is a big issue for mogas, but not as much for 100LL with it's higher/lower vapor pressure.
While a wide open line would allow faster purging, it also creates the risk of starving the engine if you forget to close it before going WOT. I used a #60 drill bit for the restrictor in the return line fitting. This limits flow via the return to about 10 GPH @ 25 PSI. The mechanical pump delivers close to 45 GPH (boost is higher I believe) and the servo consumes less than 20, so there is no negative consequence if you accidentally leave it open.
I suspect that a wide open -4 return line to the tank would prevent the servo internals from seeing anything close to 25 PSI, though I have never done the calculations, as it seems like a bad idea. With my restrictor, I DO NOT see any pressure drop with the return open. Only downside is that it takes longer to fully purge the hot fuel.
I consider it a must have if running mogas in hot temps.