Fuel pumps and surge tank

[Kbert]

I am converting a carbed engine to fuel injection using a GM TBI. The vehicle has two fuel tanks with a stock electric fuel pump in each. There is a switch on the dash to switch between the front and rear tank.

For my conversion I have decided (based on advice from here) to run a surge tank with an EFI pump to supply the engine downstream from the stock fuel tanks. I am doing this to prevent any problems with fuel pressure/flow when switching from one tank to the other.

My question is: will the stock carb fuel pumps be sufficient to keep the surge tank full?

Kbert

[billr]

Yes, but be careful where you return fuel from the FPR to. It should go back to a tank at atmospheric pressure, which probably means one of the main tanks; but that means you will have to switch the return line from tank-to-tank as well as the pressure line and pump power. I'm thinking the surge tank will be at "carb pressure", about 5 psi

[Kbert]

My original thought was to have one port filling the surge tank, one returning fuel from the tank to the stock tank (both of these through a six port tank selector valve), the fuel pump output from the surge tank and then a return from the fpr to the surge tank. I believe this is the standard setup.

Are you saying that using stock carb pumps as lift pumps might have trouble feeding a surge tank that has a higher pressure efi pump drawing from it?

Just for rederence I rebuilt the TBI with an 18 psi fpr spring.

Kbert

[billr]

I think what you describe will work OK. The FPR will be returning to a pressure slightly above atmospheric, since the surge tank will have some pressure higher than the main tank to get fuel to flow; but I don't think that will be an important factor. Maybe not a factor at all, the FPR balances FP and reference pressure (manifold pressure). I'm thinking FPR regulation doesn't depend on back-pressure in its return line much at all. (Within reason, of course; if you blocked off the FPR return line it would surely affect regulated pressure!)

[Kbert]

I agree. I think it will work fine as long as the stock pump without any restrictions save for fuel lines themselve can out flow the efi pump working against the regulator.

Thank you.

Kbert

[nathaninwa]

My 86 Chevy that I fuel injected from being standard carb was setup with no pumps in the tanks and a return style engine mounted carb pump

It also has a stock 6 port switching valve.

I mounted an external 255 along the frame rail just after the switching valve and have had zero fuel problems when switching tanks by simply sucking out of each tank

[DaveEFI]

The late Jaguar XJ6 had twin tanks and early Lucas EFI. You could perhaps copy what they did. It would depend on whether your surge tank is below the main tanks or not - as to whether you need low pressure pumps?

[billr]

I, too, have had no problem with an external pump mounted near the stock tank and simply sucking from the bottom of the tank. I kind of assumed the OP wanted to retain the stock in-tank "carb" pumps and senders for ease and to not have to add a port to the bottom of the tanks.

Why add a bottom port and not just suck through the OEM port? I wouldn't rely on that if it is up on top, even if the stock pump was removed to eliminate restriction though it.

[nathaninwa]

That was a concern of mine, sucking through the top of the tank with an efi pump. Talked with a local automotive shop about it and they mentioned lots of 80's vehicles did just this with no lift pump. I was leary, since this is my tow rig, but so far no issues and it's been about 1500 miles and about a dozen tank flips while cruising at 65mph with no issues

[DaveEFI]

I thought all simple high pressure pumps had to be gravity fed - ie below the bottom of the tank?

[billr]

It depends on the pump design. A positive-displacement pump (roller, gear, vane, piston) should be able to suck fuel uphill until the suction pressure equals the vapor pressure of the fuel and the pump effectively "vapor locks" with cavitation on the suction side. Depending on the temperature of the fuel, I would guess a pump can lift it several feet, at least. And, a PD pump will try pretty hard to self-prime, even if the "over-the-hill" suction line initially has some air in it.

However, centrifugal or turbine fuel pumps seem to be common these days. These are simpler pumps that are cheaper and more rugged, but they are far less tolerant of disturbances on the suction side. In fact, they generally have great difficulty in getting a prime if they initially are not flooded. You better be sure there are no leaks on the suction side, and leaks can go un-noticed, since a leak at the highest point may never result in a fuel spill.

[nathaninwa]

My setup is just a 40 dollar 255lph pump off eBay. I was concerned about a prime, but it picked it right up and made pressure the first without issues.
I do know that if fuel becomes a problem that prepump is a good place to check

[Kbert]

It is my hope that by using the stock in tank pumps and six port switching valve to fill and return from a surge tank that i won't have to do any tank modifications to the stock fuel tanks.

Kbert

[Six_Shooter]

You'll be much farther ahead to forget the surge tank, and just replace the intank pumps with high pressure pumps. however, if this is a GM with intank electric pumps they might be teh same as TBI pumps, since the TBI also runs on low pressure (9 to 12 PSIG). Many late carbed GM vehicles used pumps that would later be used for TBI duties.

What your propsoing with multiway valves just sounds like problems waiting to happen when simplifying the setup can reduce any possible problems.

The only thing that using a surge tank will help alleviate is the little hiccup that will likely occur when switching between tanks. You could modify the system a little and set it up more like some of the dual tank Diesel GMs in the mid/late '90's where the rear tank feeds the front tank and the pump is switched on by a circuit that monitors fuel level in the front tank.

[Kbert]

Its an early eighties ford, so no high pressure pumps.

The switching valve is already on the vehicle stock, and it is a six port with returns.

We had a work truck that was a gm that had the rear tank filling the front tank system. It was nothing but trouble.

And I thought you approved of this setup (minus the return to the stock pumps as I thought I only had a three port valve):

You'll likely fee fest to use a surge tank here, if you don't wasn't to modify the fuel tanks. The existing pumps and valve can be left alone then and use the surge tank as a sort of buffer that will allow the pressure to be raised to where it needs to be for the TBI, and a page to return the fuel to from the TB. A float can be added to turn the intake pumps on and off so that there's no need for the overflow return back to the main tank that is typical in a surge tank application.

Kbert

[Six_Shooter]

I don't recall the thread that was in, but there's likely more context to it than that.

I'm not a fan of surge tanks, they make things more complicated than they need to be in most cases.