I agree with most of what you say here but want to make a couple comments.
Because the lines act as separate fuel rails coming from a single
source both require regulators to re combine to a common return line.
With a single regulator one line would have to be dead ended and the
only way to purge any air or vaporized fuel is then through the
injectors on that bank.
I’m away from home so can’t go look at the OE preHE rails, now hanging on
the garage wall, but as I recall the two rails were connected by a crossover
so could, in principal, work with a single regulator on the left side.
However, that would make the right bank run at a little higher pressure due
to resistance of the crossover hose.
This means in the event of vapor or air lock the engine would have to
run on one bank in order to purge the line.
This is a bit of a simplification. I look at it this way:
The fuel has the opportunity flow in alternative directions at several
junctures. First, it can flow to one rail or the other. Then within each
rail it can flow forward or backward, since each rail looks like a
racetrack, i.e., forms a loop. Now, if a vapor pocket forms at some point
in one of the rails during post shutdown soak back the flow path that it is
in will offer more resistance to flow. Consequently, the fuel will
preferentially flow the other way, leaving the vapor containing path starved
of liquid fuel. Any injectors in that path will be spitting and sputtering
and nearby cylinders will be misfiring. That’s why there is often hard
starting and rough running with this setup when restarting after a brief
shutdown period. If the engine starts it will eventually clear because of
higher pressure and the cooling effect of fresh fuel coming to the rail.
Having leak free injectors, regulators, and non-return valve is very
important… keeps the rail pressure up thus reducing possibility of vapor
Very early cars had adjustable regulators,
Which had to be adjusted individually with flow to the other one pinched
This change to the later single regulator is well worth while
installing on earlier cars.
Indeed, but the HE regulators can’t be used unless the entire HE EFI system
including the ECU is used. The reason is the HE regulators are
non-adjustable and have a higher set pressure than the preHE injectors need.
And you can’t solve the problem by using HR injectors too, because they flow
considerably less than the preHE ones.
I must give credit to Roger Bywater who advised me when I retrofitted an HE
rail to my preHE. This project is described in a write-up at my website. In
addition to greatly improving hot starts, it eliminates many clamped hose
joints thus reducing chances of fuel leakage and fire.
BTW, I initially used 2 preHE regulators in parallel so as to allow each to
operate at the design nominal opening. Later, one failed and rather than
buying a new one I re-plumbed it with the remaining good one. It runs fine,
even though it obviously has to be open further since it’s handling the
entire return flow.
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