[goathead455]

I don't see why you can't use the factory vapour return line as a vent back to the tank with the fuel command center,there should be very little fuel going through it.
I agree that it would be much too small for use as a return line.

[dataway]

Maybe my terminology is wrong .. but a 68 GTO has a "return line" ... is that a "vent" line? I thought later models had a return line, AND a vent line.

I'm assuming the "AC" you guys are talking about is not Air Conditioning ...

Having to run a separate new line back to the tank might be a deal breaker for me ... as I'd like to keep the car as correct as possible with the exception of the FI.

These system recirc so much fuel that they need that large of a fuel return to the tank? Sounds like a typical installation on a vehicle using the original mechanical pump and the FI pump would have two fuel returns?

[dataway]

But I'm also seeing the quote below in the pdf

"The Fuel Command Center is the ultimate
fuel delivery system. Use your stock fuel
tank, fuel line to the pump and the stock
pump. Plumb a fuel line from the stock
pump to the Command Center using low
pressure hose and connections. Then
plumb a high pressure line (with inline
filter) to the EFI Throttle Body. No return
line is required."

So they mean no return from the EFI ... not no return from the Command Center.

Someone needs to make up a nice full schematic of the system showing stock fuel tank connections, all vent lines, all return lines etc.

[goathead455]

There is a vent line back to the tank from the command center,not a return line.If your car has the small line from the fuel pump back to the tank,you should be able to use that line for the vent from the fuel command center back to the fuel tank.

[STEELCITYFIREBIRD]

I believe I see where the confusion exists.

You will always need a return line to the tank.

>If using the command center it is only a vapor line back to the tank.Factory line,like an A/C car, in good condition will suffice.

>If using an inline/intank electric pump it will be a liquid return line , and as JLMounce found, the factory vapor line is too restrictive to be used as a liquid return line.

Did I clarify correctly?

[goathead455]

That is exactly how I understand it.
Thank you

[7T2]

I used the vapor return line off the charcoal emissions canister on my '72 formula-
its 3/8", and flowed enough bypass to prevent pressure creep at idle w/a 155lph pump. it entered the tank at the top where the gas gauge sender was located. not sure if first gen birds have this line or not.
you need a fuel reservoir in the pressure side to prevent starvation going around corners if using the stock tank pickup. i fabbed a sump in the bottom of my factory tank positioned right below the return line, so returned fuel filled the sump, curing the prob of fuel starve going around corners with less than half tank.

[dataway]

I think the confusion for me is based on the earlier models .... we have a fuel return from the stock mechanical pump, but no "vent" or vapor line back to the tank.

Stock setup on the early pontiacs is 3/8 supply to the mechanical pump, 1/4" fuel return from the pump.

Sounds like this system would require a 3/8 vapor/vent return back to the tank from the Command Center ... which would be a pain on an older car that would require a fitting to be added to the stock fuel tank for the Command Center return.

Wish they could set it up like my late model truck ... has a high pressure pump in the tank, and a return line ... that's it, no command center, no surge tank etc.

[7T2]

look at the install instructions for the 4003 fuel command center on link below.
doesn't look like any return is needed back to the primary tank if using that part?

http://www.fitechefi.com/default.asp.pg-FormulasCharts

[The Jury]

Originally Posted by STEELCITYFIREBIRD

I believe I see where the confusion exists.

You will always need a return line to the tank.

>If using the command center it is only a vapor line back to the tank.Factory line,like an A/C car, in good condition will suffice.

>If using an inline/intank electric pump it will be a liquid return line , and as JLMounce found, the factory vapor line is too restrictive to be used as a liquid return line.

Did I clarify correctly?

Yes this is the clarification that I'm looking for. I also feel that since this is primarily a vapor vent line that this would work fine. If I remember correctly the 68 fuel tank has a vent line at the top to the atmosphere that would vent any pressure created in the tank and I don't believe the command center would add too much pressure to the stock tank.

We'll give that a shot, safety is my primary concern.

[krisr]

Originally Posted by 7T2

look at the install instructions for the 4003 fuel command center on link below.
doesn't look like any return is needed back to the primary tank if using that part?

http://www.fitechefi.com/default.asp.pg-FormulasCharts

Looks like it's got an inbuilt regulator into the tank so it's already returning.

The VE Holden Commodore (Pontiac G8 over there) has a similar intank pot with a pump/surge tank/regulator all inside, the only line coming out is a feed to the rail. I'm running one in my car and have had zero issues with it. As a matter of fact, the ohms on the fuel level sender works with an Autometer gauge too, but if you run your sender to the ECU, calibrate empty/full and use an output to drive a gauge using PWM, you dont need to worry about the ohms at all then.

OEM LS powered cars run a returnless system as it's all in the tank or some cars had a filter which acted as a regulator and returned up near the rear axles.

[STEELCITYFIREBIRD]

Originally Posted by 7T2

look at the install instructions for the 4003 fuel command center on link below.
doesn't look like any return is needed back to the primary tank if using that part?

http://www.fitechefi.com/default.asp.pg-FormulasCharts

http://www.fitechefi.com/virtualoffi...ofJune2015.pdf

Check out the text in red on page 2,Titled"CommandCenterVentPort-IMPORTANT"

[7T2]

Originally Posted by krisr

OEM LS powered cars run a returnless system as it's all in the tank or some cars had a filter which acted as a regulator and returned up near the rear axles.

didnt know gm had dead head systems in LS apps- good info.
i considered a returnless (to primary tank) system that routed line off mech fuel pump to holley float bowl mounted on small (pint or so) alum fuel cell mounted under hood. gravity feed to inline pump through the rails, then back to the cell. in case there was an issue, i could replace efi system with carb to get home. ended up going return system to keep fuel cooler.

Originally Posted by STEELCITYFIREBIRD

http://www.fitechefi.com/virtualoffi...ofJune2015.pdf

Check out the text in red on page 2,Titled"CommandCenterVentPort-IMPORTANT"

my post was about fuel returns- adequate size and need for

[dataway]

Ok, here's a basic diagram showing typical 68 GTO fuel system.

The questions .... vent line necessity and routing, C.C. "return" line necessity and routing.

Seems to me a person could probably sneak in another factory style 3/8" hard line back to the tank along the same route as taken by the stock OEM lines.

Feel free to download the pic and draw on it.

[John62]

Correct me if I'm wrong but I think the confusion is the return line goes near the bottom of the tank, so if it is below the fuel level it can not vent the expansion of air [fumes] in the CC back and forth as the CC lowers or rises with fuel. So it can not be used regardless of size. But a vent line, usually the third line in a system and often only 1/4" stops at the top of the tank and should be able to vent air [fumes] back and forth. I wonder if you could just disconnect the return line above the axle and turn it down and use it from the CC as a vent, and cap the return line going into the tank, as long as you have a vented gas cap. You can actually leave the vent open on the CC but being under the hood it is unsafe and not recommended.

[dataway]

Jeez, you'd think they would set this up so you just need a high pressure in-line pump spliced into the stock fuel line, using the stock fuel return, no vent, just a hardline that deadheads in the FI unit. Remove or bypass the stock mechanical pump altogether.

Might be easier just to install eight Crower or Hilborn stacks

[1966socalgoat]

On my unit I go straight from my mechanical pump into robbmc fuel canister with built in efi pump which goes to an ls filter/ regulator, reg returns back into fuel canister and the fuel canister has a rtn line that runs down the passenger side to my fuel cell. Works great and I can go back to carb in 30 minutes.

[JLMounce]

Originally Posted by dataway

Jeez, you'd think they would set this up so you just need a high pressure in-line pump spliced into the stock fuel line, using the stock fuel return, no vent, just a hardline that deadheads in the FI unit. Remove or bypass the stock mechanical pump altogether.

Might be easier just to install eight Crower or Hilborn stacks

A couple problems with this. pretty much all inline style pumps are designed to be used with a return line and as such they run at 100% all the time. They don't like to be started, stopped, slowed and sped up constantly like what happens in a high pressure dead head application. You'd burn up the pump incredibly quickly and then they'd have you, an angry customer that just got stranded on the side of the road, calling wanting to know why the pump failed after only 700 miles.

You mention using the factory return here, but trust me, that's not going to happen. First, not every car of this vintage had a return line. In a lot of cases you had to have a car with AC for that option to exist. Second, for these cars that return is a vapor return line and although fuel condenses and you do get fluid in that line, it was not designed to carry a large flow of fuel. Putting it to use as a true fuel return line will lead to pressure creep which injectors nor needle seats can cope with. In both cases you get a rich enough mixture to stall the engine.

The reason they aren't advocating using the factory hard line is because nobody knows the kind of shape it's in. You're using a line that could possibly by 40 years old and was never designed to have pressure acting on it from the inside out. As a company that's a massive liability. So you get new line with the kits.

In regards to the command center, it does not necessarily have to be vented back to the fuel tank, but it does need to be safely vented, which is not really in the engine compartment. It is a tank itself after all and you must account for expansion and contraction of the fuel and air levels in the tank. That's physics and there's no way around that.

[dataway]

So the 1/4" return line on a 68 would not be enough flow return to keep a inline pump from failing? (So essentially not a dead head)

I wasn't aware that the fuel return line on a 68 was just a vapor vent, I thought it was the fuel return from the positive displacement mechanical pump.

Hate to seem dim ... but what is the "AC" you guys are talking about? Surely it's not Air Conditioning.

I thought ALL mid to late 60's had a fuel return from the mechanical pump.

[JLMounce]

Originally Posted by dataway

So the 1/4" return line on a 68 would not be enough flow return to keep a inline pump from failing? (So essentially not a dead head)

I wasn't aware that the fuel return line on a 68 was just a vapor vent, I thought it was the fuel return from the positive displacement mechanical pump.

Hate to seem dim ... but what is the "AC" you guys are talking about? Surely it's not Air Conditioning.

I thought ALL mid to late 60's had a fuel return from the mechanical pump.

The line is 1/4" outside diameter that is crimp bent, so it's not designed to flow a whole heck of a lot. Some fuel is always in that line but it's designed to return vapor from hot fuel to the tank. The vast majority of fuel pumped by the mechanical pump goes to the bowls, not back through the vapor return.

The FiTech inline fuel kit instructions requires a 5/8" return line minimum.

AC does mean air conditioning. Many cars in the mid 60's that didn't have AC would likely have a single feed line without a return. I know this is true of Firebird's that did not have the AC option checked.

In regards to that tiny fuel vapor line from the factory, think about how the pump is actually working. We'll use the Walbro 255 as an example. This pump flows 255lph of fuel at 42psi, slightly less at the 58psi that the FiTech unit commands. That's enough fuel to support over 600hp at wide open throttle. Many of us here don't drive our cars around at wide open throttle all the time and even fewer of us drive around with engines that make 600hp.

driving around town casually you may only be using 25-50hp and only need fuel to support that amount of power. That means at any given time you may only be using about 10% of the fuel flow being delivered. If that flow is hindered in the return it raises pressure in the feed and with EFI applications creates a fueling oscillation, which is what happened to me when I tried to use the factory vent line for all of about the 2 feet attached to the tank.

1. Engine fires cold and is using a lot of fuel
2. Engine starts warming and fuel requirement for idle reduces
3. More fuel is being sent back to the tank and the small line reaches the point of being pressurized
4. The pressure in the return line is added to the pressure at the injectors
5. Pressure continues to build as fuel requirement reduces
6. Extra pressure begins adding additional fuel to the engine, creating rich run
7. This is where the ECU steps in and tries to reduce pressure at the unit and fuel in the line by reducing pump output.
8. Engine coughs and sputters as air/fuel mixture becomes to rich
9. engine catches as the ECU has pulled fueling and it stumbles back to life. It repeats 6-9 until either the engine dies or you turn the car off. In the mean time the pump is being stressed and the engine is going from a state of being washed in fuel to being incredibly lean over and over.

This is a bit long winded, but this is what I went through and I wasted a bunch of time and money because I tried doing this cheap to begin with instead of correctly. I gave up the last month and a half of cruising season and paid several hundred more than I needed to because of this. Do it once, do it right.

If you don't want to do the entire fuel system, spend the money on the command center and just run the vent line that it needs. It's the easier of the two options and allows you to easily go back to factory if you need to.