SU fuel line return restrictor diameter

[JavelinZ]

Hey everyone, I have what seems to be a bit of a dilemma. I'm currently working on fabricating some extruded aluminum fuel rails to replace the factory setup for the SU carburetors for some of the guys in my local Datsun club. They want to keep the fuel return system which is fine. What I just found out last night is that there is a fuel line restrictor in the return section of the factory rail to help keep the fuel from percolating or vapor locking the fuel system while idling in traffic when it is hot(which is an issue here in Arizona). I have a read number of threads that mention this restrictor but not a single one mentions the actual diameter of this restrictor. I would think this hole diameter is critical to maintaining the correct fuel pressure as Pressure = Force/area.

 

So my main question is; what diameter is this restrictor? so I can incorporate one into my system.

 

Or does the smaller diameter of the return line (3/16"??) act as the restriction?

 

I was going to post an image of my design but photobucket is acting up right now and won't show the image.

Thanks for your help.

Edited July 3, 2012 by JavelinZ

[beermanpete]

The return line has a 0.043" diameter metering orifice at the end of the fuel rail.

[Tony D]

The function of the orifice, btw, is due to the variable delivery of the stock fuel pump. Watch what your fuel pressure does at 4500 compared to 6500 and 2500 and idle.

 

The orifice doesn't change, but that pump delivery sure does!

 

Without it, you wouldn't have any fuel pressure at all at idle. And at 6500, you would be running lean lean lean from low pressure as well

Edited July 6, 2012 by Tony D

[JavelinZ]

Hey thanks for the reply beerman, much appreciated.

 

Tony, thanks for the clarification on the function. I'll definitely check the fuel pressures at those speeds after I get these done as the guys all asked for a fuel pressure gauge.

 

Over the past few days I had basically resolved to drill a .050" diameter hole in the restrictor I am making. I figured that would work fine and be close. So I'll have to get a 1mm drill bit then. I doubt that .007 difference in hole size will really matter but I might as well make it accurate.

Edited July 7, 2012 by JavelinZ

[Tony D]

It's more for your general education. It's an eye opener. The FSM gives a volume test free-flow at 2500 if I remember correctly.When you see what the stock system does, when you start substituting parts you know your minimum goals (like on your fuel rail! Should function same as stock with the right orifice in there.)

Not to cross-pollinate too much, but long ago we made a billet rail counterbored to accept Rochester Carb Jets for the return orifice, in a hole tapped 1/4NPTfor the return fuel barb.

Then you can tailor your fuel pressure by reaming or replacing a removable piece and not your rail (which makes goin back where you were on step previously kinda hard!)

Just a thought. Cheers!

Edit:

"It's all been done before!"

Edited July 7, 2012 by Tony D