Fuel filter regulator connections

This is related to the fuel tank and lines section: 14 Fuel Tank and Lines


First, a little discussion on fluid dynamics.
Downstream of the fuel filter are fuel rail and fuel pressure regulator connections.
The outlet from the tank is a 10mm push on tube, and so it is that the appropriate size 10mm (Internal Diameter, ID) hose is used up to the inlet of the fuel filter. The outlet of the tank, by default, has a 10mm push on connector as well.

The inlet to the fuel rail on the engine has a tube with a ridge some way in from the start of the tube.
Any clip or clamp must be tightened on the far side of that ridge. If not there is a real risk the fuel hose will eventually work its way off. Then you will have a fire on the outside of the engine as well as on the inside.

The preferred way is to fit a black plastic connector which comes in two parts. One part fits round the fuel rail tube downstream of the ridge, the second part fits onto the end of the fuel rail tube. The two then screw together and form a solid connection to the fuel rail.
This connector is a 6-AN type, or 6JIC if you prefer, and that means a different hose to what we have laid so far.

The fuel pressure regulator can come with 10mm push on tails, or with 6-AN connectors. The variety supplied from AK is with 10mm tails.

The idea is that we have a hose from the filter to the fuel rail, interrupted by a 'T' piece which takes a branch off to the regulator. Given that the hose to the fuel rail needs to be a smaller hose for the 6-AN connector on the fuel rail, and that the hose to the regulator needs to be of the 10mm size for the 10mm tails, we have a difference of hose diameters in play.

Still with me? Well done. The next bit is interesting.

The regulator is set to open at 3.5bar pressure. So when the fuel pressure at the regulator hits 3.5 bar, a gate will open and some of the fuel will get fed back to the tank, which will relieve the pressure.
Excellent.
However, the regulator is at the end of a 10mm hose. The fuel rail is now at the end of a 8mm hose (actually it is 8.73mm for a 6-AN connection, but lets call it 8mm for now).
I don't have a Phd in fluid dynamics, but I do believe that there is a correlation between flow rate, pressure and the size of the hose. Assuming the flow rate to be the same, a difference in hose internal diameter must mean a change in pressure.
To put it simply, if the fuel hose diameter is different at the regulator to that at the fuel rail, they cannot be seeing the same pressure.

I discussed this with Omex (who supplied the regulator) and Merlin Motorsports (who supplied the hoses) and AK. Initially no one thought there would be a problem, but as the discussion developed, the tech folks at Merlin and Omex started to have doubts.

It is probably all totally fine. If there is a  difference in pressure in the hoses, it may be too small to be significant.

And it so happens that later, after IVA and 15 miles on the road, I am happy to report it is indeed all totally fine.

So this is how it went on the build.
The Omex supplied (via AK) regulator is actually made by an outfit called Glencoe Ltd. They own a brand called Sytec. I called them and they were very helpful. They supplied a 6-AN connector for the regulator. It is a weird fitting. It is standard 6-AN female on the outside, but the screw thread into the regulator body is 1/8NPT.
From Merlin, we have a converter out of the fuel filter that allows a direct 6-AN size 'T' piece. That means we don't have to break into the hose to fit the 'T' piece.

The converter and the 'T' piece made the hose connection higher in the chassis than I expected, but I do believe it is ok. For the next build I'll fit the filter slightly lower to accommodate.

So, what does it actually look like?

6-AN connector to the black two-part connector on the fuel rail













Brass converter and blue 6-AN 'T' piece downstream of the filter








8mm hose and 6-AN connector to pressure regulator. The lower hose is the 10mm standard hose back to the fuel tank connected via the 10mm push on connector.

The long loop on the hose to the fuel rail is to accommodate the engine covers when I get them.
The tie wrap on the filter to regulator hose looks awkward but functional.







After the body was fitted, the fuel pressure regulator was interfering with the hose connections to the heater. The regulator had to be moved to the far right hand side of the chassis cross rail.

Note the two redundant hex bolts were the regulator was originally.