Injection Return Line

[BillS]

I was talking to a guy today, who for some reason was determined that he could take his 3.1 engine and block the outlet port on the rail, or alternatively lead it back into the filter where the low pressure pump delivered fuel and the high pressure pump picked it up (this is an engine swap into an MG, so not using an in tank pump).

I told him that might create all sorts of havoc, but was unsure exactly why running an early style rail as a returnless system would be a disaster.

Opinions?

[RedZMonte]

most guys go the other way lol... They usually make the returnless into a return system.

[WrathOfSocrus]

Well these are made up numbers of course - but if the high pressure pump is 100 PSI and the return goes to an FPR holding 50 PSI (the other 50 going to the return) and you impede the return by a low pressure pump at say 25 PSI, you will have effectively backed up the return and introduce extra pressure at the rail (in this hypothtical 25 PSI). That is of course assuming the low pressure pump doesn't fail pumping against a pressure double its own output, and of course that there is no such thing as friction or vacuum modulation of the FPR in effect.

An FMU could do the same thing and probably last longer doing it.