Fuel return pipe

[Gromit]

........well thats one way of checkin it.

[Baldeagle]

Simple solution fit carbs!

[Kevin Wood]

The fuel pressure regulator is referenced against either atmospheric pressure or manifold pressure (the third, smaller hose connection) so if the swirl pot is at a few PSI it makes no difference. I have a 2mm restriction in my return. I'd probably go for 1mm if I did it again because the LP pump still goes at a fair old rate but without the restrictor you could hear the piston smacking against its stop because it was pumping into no resistance.

Kevin

[Big R]

Thanks Kevin - that's the simple answer I was looking for.  

[deano.1]

Seems to me that it doesn't really matter where the restriction is as long as it gives the pump something to pump against

[bill shurvinton]

Just to put everyone straight. Its NOT a swirl pot. Swirl pots are to seperate oil and air as oil gets foamed up in an engine.

For a fuel system its a surge pot designed to deal with occasions where g forces uncover the fuel pickup in the tank. It contains enough reservoir to ensure that the high pressure pump does not run out of fuel to pump.

The fuel return serves 2 purposes. Firstly it vents vapour back to the tank. Aromatics in petrol boil at room temperature. When compressed to 3bar fuel gets hot and all sorts starts boiling off. The return ensures that you do not get vapour building up. I am not a fan of the returnless fuel rails that emissions regs have forced on OEMs, as there is nowhere for vapour to go other than into the engine.

Secondly it allows hot 'sour' fuel to be returned to the main tank to be cooled down. Ever wondered why the boot of your car gets so warm? This return is not vital, just useful, so flow needs to be less than the incoming pipe.

You do, as Kevin pointed out, need to also take into account the needs of the lift pump, which needs some back pressure to work against to be happy. In the case of some pumps you also need to stop it running flat out all the time or you will compromise life expectancy considerably. But you do not want to limit the fuel flow for when you floor it for a little bit.

Within those limitations there are a number of ways to do it. some are better than others and some of the bad ways to do it will still work, but its always worth trying to do it right.

Bill

[Westfieldman]

Within those limitations there are a number of ways to do it. some are better than others and some of the bad ways to do it will still work, but its always worth trying to do it right.

So in your opinion which is the better/best way to do this

[bill shurvinton]

There are 2 answers to this. The very best solution  is a properly designed tank with the surge pot built in. But if you have a carb pump and want to save money then a solid state lift pump into either a 1 litre surge pot feeding the HP pump or the K-jet pump and pot solution. Put a 1mm restriction in the return and you should have a fuel system that copes with everything.

Over 400HP fuel coolers start to become useful, but not yet convinced that a se7en needs it. No data to back this up.

[KerryS]

Ever wondered why the boot of your car gets so warm?

And there was I thinking it was the two damn big silencers under the boot floor that made it warm  

This must be why there is no rear seat heating on my tintop, 'coz it's over the fuel tank  

Seriously though, interesting stuff. The current WF tank has a surge pot  at the fuel take off point but when fuel is low even this isn't enough to stop fuel starvation on long RH bends (or roundabouts for our mainland European friends).

I presume the surge pot effectively replaces the LP pump in the SBD diagram and uses gravity to feed the HP pump?

Kerry

[Blatman]

The Westfield swirl pot couldn't be any worse if you tried. I can't believe they still make them like that. They could make all their tanks like mine and only ever have to make one design of tank 'cos it'll work with carbs or injection. It's a simple thing to do too...

This is how to do it. Cheaper than adding swirl pots and lift pumps etc, and less joints (or potential leak points) than a lift pump/swirl pot arrangement. Lighter too...

[Mike-SEiW]

I do not have a restriction on my return pipe from (what I call) the catchtank. I use a Facet solid state pump and it's been ticking away at full chatter for two years. We've run exactly the same system on the endurance racer (same type of pump) and that pump is at over 300 hours of rattle and abuse, and it's still ticking away nicely.

Also - a detail that seems to escape scrutiny, is that the fuel pressure regulator will regulate a DIFFERENCE of pressure between incoming and outgoing, so if you increase the return pressure through a regulator, the difference across the injectors is exactly the same. Since incoming and outgoing pressure difference on the HP pump is exactly the same, it too will never notice and pump exactly the same flow as before. The only difference is all lines are now at higher pressure, although in total low, but does in the n-th degree of theory, likely increase the risk of system failure.

Also, if you fit a restriction to the return line AND IF the consumption of fuel by the engine is less than what the pickup pump is pumping, the system will build to the maximum pressure the pickup pump can generate. Whether this is a problem or not, depends on the maximum pressure the lift pump can generate. If you use a K-jetronic lift pump, that pressure may be 150+ psi (which is seriously dangerous! hence the reason for a carb pump which will stop at whatever 7 psi or so that is normal for a carb'd engine).

[Kevin Wood]

If you use a K-jetronic lift pump, that pressure may be 150+ psi

Ah, good point. DON'T fit a restrictor if you're using a high pressure pump as the lift pump. These won't mind running into an open pipe anyway as they're a different beast altogether.

the fuel pressure regulator will regulate a DIFFERENCE of pressure between incoming and outgoing

The fuel pressure regulator should reference the pressure from the vacuum port on the regulator though, not the fuel return. 7 PSI from a red top in the return line will not change the regulated fuel pressure in the fuel rail significantly. It didn't in my installation anyway.

Sounds like the solid state Facet pumps are more tolerant than the mechanical interruptor jobs.

Kevin

[Blatman]

Sounds like the solid state Facet pumps are more tolerant than the mechanical interruptor jobs.

Most definitely, I would have said...

[Mike-SEiW]

the fuel pressure regulator will regulate a DIFFERENCE of pressure between incoming and outgoing

The fuel pressure regulator should reference the pressure from the vacuum port on the regulator though, not the fuel return. 7 PSI from a red top in the return line will not change the regulated fuel pressure in the fuel rail significantly. It didn't in my installation anyway.

Interesting data point. Let me think that through.

The FPR is basically a diaphragm, spring and a plate above a hole.

If we call the area of the outgoing A1, and the total area acting on the spring as A2, spring force is F, fuel rail pressure (incoming) is P2 and outgoing pressure is P1 the total formula will be:

P2*(A2-A1) + P1*A1 = F*A2

Thus, if P1=0, the fuel rail pressure is linear to the spring pressure.

However, if F is unchanged, and P1>0, actually, P2 must decrease. So in reality, the fuel rail pressure drops slightly.

However, flow through the injectors is affected by the square root of the pressure, so if pressure drops by 10%, actually, the flow is only decreased by 3.16%. The amount that P2 is affected by change in P1 is totally dominated by the area relationship of A2 and A1. Most likely, fuel pressure regulators are so designed that any change in P1 is likely to result in a very small change in P2.

But an interesting exercise which showed I had not properly thought it through, if my initial formula is correct, that is

[Dave Eastwood (Gadgetman)]

Also assumes that back flow can occur - no one way valves or anything. (Not that I have a clue whether there are or not     )