Fuel Return With DCOE Carbs

[zcarnut]

bj, read what I wrote….I said 260Z not 240Z….I don’t know how the 1973 240Z electric fuel pump retrofit circuit functioned but I am aware of how the 1974 260Z electric fuel pump circuit functions. If you don’t believe what I said then maybe you might believe what Nissan wrote.

 

Here’s what the 1974 260Z service manual, section EF Fuel System, page EF-7 says:

 

Electric Pump Operation

 

This system controls the operation of the electric fuel pump according to the engine speed. It receives the engine speed information from a voltage generated by the voltage regulator. When the engine is running below 400 rpm, the electric fuel pump cut relay #1 remains OFF and the pump will not be operated. While cranking the engine, the electric fuel pump relay #2 remains OFF and the pump will not be operated.

 

Under normal engine running conditions, both the electric pump and the mechanical pump are operated.

 

As an owner of a 260Z I can verify that the electric fuel pump is on while the engine is running. Its “humming†sound is very evident.

[bjhines]

EDIT:::: Zcarnut is exactly right!!!

 

You can be sure if you confirm that the Alternator cut relay is a NORMALLY ON or N-OFF relay.

 

There are plenty of other people who describe it as you say.. But I am not sure that the translation is correct.

 

EDIT:::: Zcarnut is exactly right!!!

[Tony D]

Back a page ago, solution #3 which was stated 'wouldn't work' is EXACTLY THE BEST WAY TO RUN IT!

 

I know there is no 'best' but in this case, there IS---the regulation of the fuel pressure regulator is a BACPRESSURE REGULATION SETUP. This will (if you install a fuel filter before the carbs like you should) keep fuel pressure as constant as possible even with a plugging fuel filter and if you are using an EFI pump, will run the pump at the lowest possible amperage, and keep fuel flowing like a madman as it's only pumping to 3.5psi. If the filter plugs, the pump then can compensate and up it's pressure through the operation of the backpressure regulator (which will see a lower pressure at the carburettors and close off to raise it!)

 

On High-HP cars in japan, this is how they are all run. Up to a given HP. And that level is very low. I'm not even getting into running a surge tank and then feeding fuel from that, but in high-g corners with a stock tank it's a requirement. I have starved cars running stock SU's in hard lefthanders with less than 1/2 tank and two fat guys in the car... Running lean at speed and load is not something you want to do.

 

If you are seriously considering a fuel system, then ditch the stock rail altogether and plumb it up with adequate sized lines, and if you are worried about heat on track events or even highway blasts up to 2 hours (one fuel tank duration) consider a cooling coil and 1 gallon cool-can. I made one for a vapor-locky 73 (with round tops, BTW) which took a 7# bag of ice. Made it out of a 1 gallon igloo cooler and some coils of copper tubing. This more than cooled the fuel going into the carbs for longer runs down I15 to Vegas, and cured the cars vapor lock issues coming off the highway into stop-n-go. A 7# bag of ice would turn to a gallon of 100-110F water on a 120F day in about two hours. This was with a return line, so yeah you were cooling the entire tank. Anybody who has done dyno tesing on less than 1/4 a tank and had problems keeping AFRs constant will appreciate this hint as well... Easy to make, cheap, and will keep the car cool on the track. It doesn't have to be in the engine bay, but for safety I'd not be putting it in the back where the spare tire went...

 

 

IN SHORT: Any system that puts the regulator in between the fuel source and fuel demand point will suffer more fluctiations than one which regulates pressure at point of demand by backpressure requirements.

 

An example from the petrochemical industry: API672 requires constant oil pressure to the bearings of high speed rotating equipment (duh, huh?). The specified method for this is to place the PRV at point of demand with relief back to sump. Any filter media between the pump and the point of use then, becomes irrelevant with a positive displacement pump of sufficient pressure capability to overcome filtration restriction. I have seen 8 bar oil pump pressure, a drop across filters of 5.3 bar, and SILL have the required 1.7 bar at point of oil injection to the bearings.

 

Later API specs have placed a secondary relief on the outlet of the pump now, to limit it's output to the pressure rating of any piping carrying the oil to the filter, but the effect is the same---the fluid is deliverd to the point of use with NO fluctuation whatsoever, and the lowest electrical demand possible on the pumping motor. Even if you are using a 15psi pump like a Holley Red, this will allow for PLENTY of filtration blockage and still deliver your 3.5 psi. If you regulate off the pump...and the filter clogs... Eif you regulate after the filter and the return line opens...

But, if you regulate AFTER the carbs in a backpressure retention scenario, the only eventuality which will cause low pressure is a broken fuel line to the front of the car, a plugged filter will have to have 11psi delta to affect fuel pressure to the carbs, and a broken return line will cause no issues except for the fire out back...

 

Same thing for Fuel Systems.

[Tony D]

The 260 has float bowls of considerably less volume than the 73's. Under hard acceleration the single pump will not keep them at proper fuel level.

 

A 73's larger bowls will hold level better, but when shut off will cause more of a hot start issue due to the massive fuel they can percolate into the intake tract, and how long it takes to reprime them. A 74 will perc only so much, and will refill to operating level (for idle) much quicker, but will also suck down under WOT much faster due to the smaller bowl volume.

 

Operation of the 73 and 74 systems is slightly different, they do not operate the same way.

[cygnusx1]

I run solution #5 on my FI car. It seems to work fine on the FI. I am about to use the same setup with a low P pump for my Weber car in progress.

 

I got to thinking that there may be a way to modify the Webers to get some fuel circulation in the bowls; other than the fuel that is coming in to replace the fuel consumed.

 

You could tap the bottoms caps of each bowl and tie them to a return line regulated by a valve, tied to a vacuum source. At high vacuum, you could "flow" a portion of the bowl fuel back to the tank. At low vaccums you keep the drain-backs shut. It would need to be calibrated with an orifice so as not to outflow the feed + the consumption. Hmmm. A bit complex but a decent solution...IMHO.

[bjhines]

ZCARNUT!!!,

 

I MUST make amends.... I rechecked my harness and found that the Alternator relay was always-ON, no matter what was energized. I also had low resistance across 3 of the terminals. I opened it up and found that the alternator relay was welded closed and the coil is burned as well as shorted to ground. I obviously did not confirm coil operation on this one.

 

YOU ARE EXACTLY RIGHT.. All this time I was completely wrong about the contacts on the alternator relay operation. The system functions exactly as the manual states even for the 240Zs. The Alternator relay is N-OFF(unless the contacts are welded closed:)).

 

♥♥♥♥ .. That is embarassing.

 

Here is a picture of the burned alternator relay that screwed me up.

[bjhines]

Tony D's system would use a regulator like The Aeromotive 2 port A2000. http://aeromotiveinc.com/products-page/regulators/carbureted-regulators/a2000-2-port-bypass-regulator/ It is designed to go on the return-end of a matching fuel log for a Holley carby. It may be problematic for Webers though. The Aeromotive fuel log is significantly larger than the fuel lines and slows flow down to keep from causing Bernoulli/venturi effects at the carby inlet tubes. You might not get the pressure you intend on the skinny inlets for Webers with all that fuel flying past.

 

The typical Bypass regulator works the same way except the pump side tees out to the carbys. This would work like the Aeromotive A1000 regulator. http://aeromotiveinc.com/products-page/regulators/carbureted-regulators/a1000-carbureted-bypass-regulator/ The return is still after the internal valve and everything else. The downside is that it does not constantly refresh the fuel rail with cool fuel. You could still use a restricted bypass off the Tee as long as it flowed very little fuel.

 

The older style of hookup with NO return is generally prone to inconsistent operation under extremes. The VERY NICE, well made Aeromotive A4 regulator will give you good performance with no return as long as you use a carby specific pump. http://aeromotiveinc.com/products-page/regulators/carbureted-regulators/a4-carbureted-regulator/

 

 

 

...

[Tony D]

I run solution #5 on my FI car. It seems to work fine on the FI. I am about to use the same setup with a low P pump for my Weber car in progress.

 

I got to thinking that there may be a way to modify the Webers to get some fuel circulation in the bowls; other than the fuel that is coming in to replace the fuel consumed.

 

You could tap the bottoms caps of each bowl and tie them to a return line regulated by a valve, tied to a vacuum source. At high vacuum, you could "flow" a portion of the bowl fuel back to the tank. At low vaccums you keep the drain-backs shut. It would need to be calibrated with an orifice so as not to outflow the feed + the consumption. Hmmm. A bit complex but a decent solution...IMHO.

 

Coments as follows:

With an EFI system, and a deadhead like that to the injectors...well "it's your engine do as you please".

 

On the 'some other method' for Webers, it's been done offroad for years where you remove the floats, epoxy in a wier with fuel cel foam and 'dry sump' the fuel delivery. Gives EXTREMELY consistent fuel levels in VERY ROUGH off-roading environments. But we're not agreeing on fuel pump and regulation placement, so let's not even get into that one!

[Cobra_Tim]

We currently run a "type 5" on the IT car with a -8 line as the feed and a -6 line as a return. Now this is on SUs but the theory should be the same. We seem to be running into a supply issue where the return is "robbing" the supply of fuel under high demand scenarios. Under a full hard run in 3rd gear we seem to lose combustion in 3 if not all 6 cylinders after a shift to 4th. We have an ATL cell using one feed supplying a mallory filter. It then goes to a Mallory pump sitting at cell level. From there it goes to Mallory return style FPR on the firewall on the passanger side (this is the onlu -8an line.) The FPR is where it splits the feed and return. The feed supply a small fuel block where I have the test port and 2 -6an lines feeding the SUs and the return uses a -6an line that runs to the other "feed" on the cell.

 

I bought a retunless style regulator to try and remedy the situation but am really unsure if this is the way to go. Perhaps someone here could give some input.

[cygnusx1]

Coments as follows:

With an EFI system, and a deadhead like that to the injectors...well "it's your engine do as you please".

 

On the 'some other method' for Webers, it's been done offroad for years where you remove the floats, epoxy in a wier with fuel cel foam and 'dry sump' the fuel delivery. Gives EXTREMELY consistent fuel levels in VERY ROUGH off-roading environments. But we're not agreeing on fuel pump and regulation placement, so let's not even get into that one!

 

Quite a few people have run a dead headed fuel line in EFI cars here. It works. The only drawbacks are that it take a bit of extra time to purge hot fuel after a hot start, as long as your rail volume is sufficient. No biggie, but depends on where you live relative to the equator and sea level.

 

My suggested Weber circulation method has little to do with consistent fuel delivery. The concept is designed to allow more frequent fuel changes in the carbs to keep the average temp of the bowl fuel down.

[Daeron]

I bought a retunless style regulator to try and remedy the situation but am really unsure if this is the way to go. Perhaps someone here could give some input.

 

Tim, thinking about it more, what you need to do is run from the fuel pump to the junction block, and run two lines (plus the pressure gauge) out of the junction block. Each line goes to a Tee at the carb; one branch goes into the carb, the other goes out to a FPR with two "inlets." Two backpressure regulated circuits, and then one fuel return line from the regulator.

 

Seeing the graphic; and recognizing that we were running #5 ( ) helped me visualize the system much more.

[Tony D]

Using a backpressure regulation scenario, it is the only one where full pump capacity can be delivered to the carbs.

 

Every other scenario either restricts the flow to the carbs, or dumps capacity before reaching the carbs. Once restricted or dumped, it's never available for the carbs to use.

 

People who have changed to a backpressure regulation system have been amazed their stockish pumps do well with far more horsepower. When it all is available to the engine (carbs) you don't get much better than that.

 

For SU's you can "T" the pump to supply round the back and front of the engine to the respective carbs, and run the regulator between them with return back to the tank---similar to the looped stock EFI system (note Nisssan uses Backpressure Regulation for the stock FPR...) You can run them into "T"s or whatever...

 

But you will notice the fuel pressure remains constant and your fuel pump amperage draw can be used as a direct correlation to fuel flow if you have good datalogging.

[Cobra_Tim]

Tony, Like this?

 

 

Right now I have mine setup like Mallory has in figure 3B... 3A would be more appropriate?

http://static.summitracing.com/global/images/instructions/maa-4309.pdf

[Tony D]

Tony, Like this?

 

 

Right now I have mine setup like Mallory has in figure 3B... 3A would be more appropriate?

http://static.summitracing.com/global/images/instructions/maa-4309.pdf

 

The diagram you have is pretty much functionally idedntical to 3A. I'd go with 3A. A plugging filter will not affect fuel flow to the carbs, in 3B, the regulator will sense pressure at the inlet to the filter---you never know what you will be getting downstream of it.

 

With 3A, as the filter plugs, the return on the regulator closes to keep pressure (and flow) constant to the carbs.