Fuel Pressure : Return Line?

[zclubhouse]

I have a L28 with a mild cam, headers and dual flat top SU carbs. I am running a Holley Blue fuel pump and a holley adjustable fuel pressure regulator. I am starting to plumb the fuel system from the cell and I am wondering if I need to run a return line. Most of my research has confirmed that anything over 7psi of fuel pressure to the carbs has to run a return. The FPR has a range of 5-9 psi. Can I run 5psi with my setup and skip the return? Do I need to run 7 or higher? Any advice would be appreciated. Thanks

[zclubhouse]

nobody knows the answer to this question? I have looked around the internet and my service manuals for a while I still can't find what I need to know. I am ordering the plumbing on tuesday so I suppose I will just have to throw together a return. Any help before then would be fantastic. Thanks

[woldson]

Run a return line for future reference. If you need it ever it will be there.

 

I can not say diffenitively no. Their are some threads concerning fi and return lines.

[JSM]

I'm not running a return line on my SUs (round top). I've had no problems thus far but I think I'm right under 4 PSI.

[mark]

I never ran a return line on my webers.

[brokebolt]

Go here for a bit more on the topic:

 

http://forums.hybridz.org/showthread.php?t=103511

 

http://forums.hybridz.org/showthread.php?t=79631

 

Nothing is in concrete but information is key in making the decision.

[Evan Purple240zt]

I would run a return line to prevent any possible vapor lock problems from occurring.

 

Evan

[bjhines]

Regulators are dynamic devices. They are not able to compensate a static pressurized system.

 

The fact is that the fuel pump limits the peak pressure in a dead-headed fuel system. The regulator is only able to regulate pressure when there is a significant amount of fuel being fed into the carby. The problems arise when the carby no longer draws enough fuel to allow the regulator to do it's job. Fuel pressure will tend to rise above the regulator set pressure when the car is idling or the pump is running with the engine off. This is not a problem unless the fuel is forced past the inlet needles in your carby. Under high load conditions the regulator is able to limit pressure to the set level.

 

The whole idea of a bypass regulator is to eliminate the pressure rise when demand is low. This also allows the use of EFI pumps with much higher internal bypass pressures(50-60PSI). The pump is no longer the peak pressure limiting device. The regulator is able to bypass 100% of the fuel flow when demand is low to nil. Pressure creep is non-existant in a system with a large enough return line. The return line can(and probably should) be as large as the feed line in this case. The regulator will use this bypass only as needed to keep a constant feed pressure to the carbys under all circumstances.

 

 

 

 

 

Some of these bypass regulators have a port on the reference side of the pressure valve. This port is normally vented to atmosphere but it can also be hooked up to intake vacuum to allow even more sophisticated pressure control. Whenever engine vac is high the difference between the feed pressure and reference pressure increases and lowers the fuel pressure to the carby. This allows a dynamic control of fuel pressure over a wide range of conditions. It will add a great deal of complexity to finding the "right" setting for the preload spring.

 

The idea is to allow higher than normal feed pressure to the carb when demand is extremely high. The regulator will drop to a much lower pressure when the engine is idling. This allows compensation for smaller than ideal lines, or smaller inlet needle sizes. The peak pressure is set with the vacuum line removed and the reuglator sees atmospheric pressure. The peak pressure must still not be high enough to force fuel past the inlet needles. The low pressure setting is irrelevant due to the low fuel demands when the intake vac is high.

 

In operation these systems will show very different characteristics on you fuel pressure guage...

 

1. The dead head system: Fuel pressure will tend to rise when the engine is idling or cruising.

2. The bypass system: Fuel pressure is rock steady under all circumstances.

2a. The vac referenced bypass system: Fuel pressure rises under high demand and drops under low demand. Just like it should!!!!

 

 

 

 

 

The return line in the stock Datsun carby fuel rail is MUCH smaller than the feed line. There is also a tiny orifice in the return tube to limit the amount of flow to nearly nothing. This is NOT a regulated return line. It is a low-flowing, constant-bypass to eliminate vapor lock, or at least allow the vapors to clear the lines quickly without being forced through the carbys when starting the car HOT. This was a band-aid fix to a common problem among old cars with old technology and thinking.

[Mayolives]

BJHINES....Ding Ding..........Thanks John. Good information.