dual vs single entry fuel rail

[Murray_Calavera]

Hi all,

I've pretty much got my shopping list sorted for the new fuel system, there is only one thing left to work out... dual entry or single entry setup for the fuel rail?

I've been strongly recommended by a few people to run dual entry with the center return setup. Their main argument tends to be that due to the length of the rail, I might see a pressure drop across the rail and I could have a cylinder slightly lean out at times. 

This doesn't make a lot of sense to me, the way I see it if the fuel pump can keep up and the reg is doing it's job then the fuel pressure should always be equal across the rail. 

The more I look around the more I find people running both setups, and I can't seem to see any downsides performance wise either way. 

I'm keen to hear some more thoughts on the matter. 

[NEO25T]

I had planned twin feeds for the rail when doing my fuel system cause i think it looks cool but just did the 1 feed front and return rear for simplicity

[bcozican]

I run a single entry on 400rwkw rb25 setup for 8 years successfully with no fuel related leaning issues. Tuner said the same if I do it again or change it to go twin entry with centre return for same reason you stated / safer. He also said he doesnt see any drop leaning issues across cylinders across mine on dyno so no need to change so.ething that isnt broken

[GTSBoy]

4 hours ago, Murray_Calavera said:

This doesn't make a lot of sense to me, the way I see it if the fuel pump can keep up and the reg is doing it's job then the fuel pressure should always be equal across the rail. 

It's a big and complicated explanation that I can't be bothered running all the way through.  Suffice it to say, with enough fuel flowing through a skinny fuel rail, there can be times when the pressure at the outlet end is lower than at the inlet end and this could cause problems.  Centre return simply halves the length of the rail and halves the qty of fuel flowing in each half of the rail.  This leads to ~ 12% of the possible pressure drop from a single entry.

[burn4005]

Eh, overrated. I have added a pulsation damper to the pre rail feed at the 1650cc inj are pretty choppy though. And ecu uses live rail pressure measured at the regulator in the fuel model

I'm at 500awkw on e85 in an r34 gtr.  stock lines, stock filter and stock rail. 

I have an egt sensor on each cylinder and they're all flat on a long pull. 

Not sure where this pressure drop issue comes from I mean it is a physical effect but your fluid velocity would need to be massive for it to be important over a 1m long straight rail. Yes, the rail pressure will be higher than what the regulator sensor sees due to fluid flow mechanics, and the effect gets stronger the high the bypass rate but low viscosity warm fuel travelling in a straight rail this gradient will be very small. 

The twin entry rails with very high fuel demands introduce strong turbulence at the centre return affecting the two centre cylinders. It's better to have the fuel travelling in one direction. 

A large enough rail with a bore size to suit the delivery (should be larger than the hose used to feed it so no supply issues exist) is more than adequate. 

I just did a quick calc and the drop accross a 1m long straight extruded tube of 15mm diameter with a flow of 10l/min of room temp ethanol is 0.145psi. The real effect will be even less as at high duty cycles you are dropping roughly 1/6 of the flow as you cross each injector so the differential decreases.

This is one of those mates brothers cousins dogs auntie said things but until someone puts a sensor pre and post rail to prove it I'm a non believer and am feeding my engine well over 800hp worth of e85 without an issue. 

Edited December 14, 2018 by burn4005

[K_arlstrom]

I agree with the calculations above, and stock fuelrail (rb26) is even larger. 17 or 18mm if I remember correctly. 

[Dose Pipe Sutututu]

@burn4005 how far is your FPD from your rail? I had mine just after the 2nd fuel filter and about 50cm away from the fuel rail and instead of dampening the fuel pulsation it made it worse! This only occurred at about 3500rpm at low loads.

Ended up pulling off the vacuum reference to the FPD and surprise surprise lean spot gone at that RPM hole gone! Mind you all the lines in my engine bay are braided Teflon lines which doesn't do a good job of absorbing vibrations instead aids it. 

[burn4005]

my Damper is only a small OEM style bosch one with an adjustable pre-load installed just before the fuel rail that I'm sure is quite compressed at elevated pressures so would have little effect in high boost high power operating zones. i was only concerned with low pulse widths at relatively low fuel pressures, which is where you were seeing your issues. I'm also using a progressive cavity twin screw pump that provides very linear fuel delivery, whereas a turbine style pump will struggle a bit with very choppy fuel pressure. the walbros really do not like being run slowly, whereas the Pierburg stays very linear in its supply.

there is no manifold reference line on my FPD, and I still have a few meters of Gates rubber line in my fuel system that would also provide a dampening effect.  I use the VP M2 additive that seems to keep the rubber in good condition and I never leave E85 sitting in it for extended periods.

and yes, running all solid line would certainly provide a much noisier fuel pressure do to the lack of elasticity.

Edited December 18, 2018 by burn4005

[Dose Pipe Sutututu]

Thank for that!

I might play around with my pump duty cycle, I have both wallahbro 460s running on PWM.. and see if moving my FPD closer to the rail would help.

I'm using a Radium FPD-R, an inline unit.

[MagicMikeZ32]

I think the original pressure drop theory came about when there was a greater drive to push smaller injectors (before readily available cheap 1000cc+) further, with stock pumps (before readily available cheap high flow pumps).

Not so much pressure drop caused by rail length, pressure drop by lack of delivery.