S30 fuel return diameter - musings

richiep

richiep

Club Member
Presently, my Z doesn't run a fuel return - I deleted it when I did the rebuild 6 years ago and fitted the triple carb setup. However, I've just become aware of some AN fittings and neat little bits of associated clamp hardware that would allow me to run a return in a tidy, minimal way (plenty of you know how OCD I am about the car...). Considering I have a NOS fuel tank that will be replacing the smaller custom one currently on the car, I could take the opportunity to add a return too while doing the tank. The general view seems to be that running a return is a good thing.

My question - is there any particular benefit/advantage of running a smaller diameter return as per originally fitted to the 240Zs over the larger diameter used on the 260Z? I currently have 5/16 / 8mm line and hose feeding the triples. Early returns were 3/16 / 5mm correct? I'm guessing that the restriction might create a small pressure differential that prevents too much fuel bypassing the carbs? (how much that actually is of real world consequence I don't know)

The obvious difference here is that I'm not using the original SUs, but Dellorto triples, so the fueling requirements (and thus maybe potential return) are going to be different.

Whether I go for smaller or the same diameter return obviously affects what hoses, fittings, pipe etc. I need, so informed technical opinions are welcome! :eek:
 
I think it probably depends what kind of fuel regulator you use - if you have the filter king type thing which restricts flow to control pressure, you need a relatively restrictive return line to keep pressure high enough between regulator and carbs.
Whereas if you have a bypass regulator, you want a fat return line in order to allow it to bleed of as much as pressure as it wants to the return line.

Personally, in my experience bypass regulators work much better. (i.e. I only managed to get stable target pressure to my triple Dellortos with a bypass regulator!)

I've got an Aeromotive bypass regulator, and it's docs talk in terms of minimum return line sizes. From memory, I've got 8mm return line which was slightly below the min that Aeromotive recommended, but it works fine.
 
I have my regulator on the return line after the carbs, I'm using the original fuel rail but drilled out the restrictor to let the regulator do the work.
 
if you have an open return and put the regulator before the carbs then you cant regulate the pressure effectively
 
No, the behaviour of a bypass regulator is to keep the pressure in the controlled line constant (the line from tank to carbs, or from tank to carbs to regulator depending whether you have it before or after carbs) .
Excess pressure is bled of into the bypass (which is the return line). As the carbs demands vary, the amount being returned varies.

The regulator is a simple chamber with open in and out ports - no valves etc. The chamber is just part of the line. It has a pressure sensitive valve to the bypass (return) line which opens and closes progressively as the pressure rises and falls in the chamber.

There is no difference in pressure between in and out on the regulator - just a difference between the in/out line and the bypass line.
 
Jons explanation is spot on,

Look at it this way, the regulator doesn't "know" what is fitted after it, it can only control what is in the control line (prior to it) and bleed off excess pressure to the return
 
Richiep,
Looking at the OEM setup....it doesn't matter too much what size the pipe is. The OEM set-up had a restrictor on the OEM fuel rail, where the return pipe fitted on. Following that, maybe to consider, is add a restrictor into the fuel pipe, such as a tight fitting brass slug with a small dia hole in.
 
I should be able to sort something out in terms of adding a restriction by way of the outlet fitting of the regulator. I'll just look at fitting one with a reduced diameter.

Of course, that's if I'm allowed out to play in the garage any time soon. Kerry just gave birth to our second daughter last night - Skye Elizabeth Parker. I'm so outnumbered now!...
 
Ian Patmore said:
Richiep,
Looking at the OEM setup....it doesn't matter too much what size the pipe is. The OEM set-up had a restrictor on the OEM fuel rail, where the return pipe fitted on. Following that, maybe to consider, is add a restrictor into the fuel pipe, such as a tight fitting brass slug with a small dia hole in.
Click to expand...

But that leaves no scope for adjustment and how do you know if its right or not, carbs are very sensitive to pressure
 
But that leaves no scope for adjustment and how do you know if its right or not, carbs are very sensitive to pressure
Click to expand...

I haven't mentioned anything about adjustment and pressure, as its already been mentioned in the preceeding posts.....Grasshopper
 
But Richie is asking specifically about a return line and position of regulator, if you fit a regulator correctly you don't need to mess around with a restrictor which on its own is not sufficient and suitable for triples to work at their optimum......a bit like a sump vent connected to an exhaust system........Grasshopper
 
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