Will this fuel route setup work??

mike reeh

Senior Member
Okay here's the plan:

I got an older Holley pro-jection setup (2bbl 670cfm TBI)..

I plan to install it on my '77 K20 sb400.. the dual tanks setup, raises some questions and possible problems.. here goes.

Holley recommends all of this extra equipment (lines/more pumps/splicing/hacking/drilling/etc) to accomodate dual tanks... out of the question. Im thinking that this will work fine: keep entire fuel system and dual tank setup intact and functional, and have the mechanical pump feed the electric pump which will sit upstream obviously.. I will have the return line Tee into the factory return line from the mechanical pump.

In theory, this should work, eh? It elimates all hassles like dealing with plumbing the fuel pump way back at the tanks, having the fuel return to the correct tank, and best of all it can be easily converted back to stock/carbureted.

Id love to hear your input/suggestions. thanks guys

mike reeh


Endicott, NY
I don't think it'll work. You are best to byte the bullet and do like Holley says. The company has spent the money on R & D and knows what they are talking about. I have never heard of anyone using a Mechanical Pump and an Electrical Pump. Mechanical Pumps can not keep up with the pressure demand of the fuel injection systems. If they could, everyone would use them as they are cheap. Think about it. Do what you want, but I don't think your proposal is viable.

Chuck Smith

2000 Club Member
Well, a friend put a TPI in his 77 Vette. All he did was put a block off plate on the engine block after removing the mechanical pump. He mounted the holley electric pump on the frame back near the tank.

I think you could easily do the same thing. Just mount the pump on the frame rail, after the tank selector valve. Plumbing wouldn't be very difficult. Both the feed and return lines are right there on the frame rail.

IMO atempting to run 2 pumps, would be more complicated, and you don't know for sure how it will work.

Plumbing it this way the return line under the hood, and near the pump would lead to the correct tank, as would the feed come from the correct tank, depending on the position of the dual tank valve. I can't see any problems with this set up.


mike reeh

Senior Member
Okay now we're getting somewhere.

Chuck I posted the same question on two other UBB systems (this being the smallest) and really got no-where. You bring up some good points.

Let me start by saying that my intention for running the mechanical pump is to simplify the installation. I want the electric fuel pump up front, if possible, and it is common knowledge that electric pumps are pushers, not pullers. In essence, the mechanical pump will simply prime the lines between the tanks and the electric pump..

There is ONE grey area, that might be the deciding factor. Let me explain. OBVIOUSLY (and a lot of people dont understand this) both types of pumps will supply ENOUGH fuel for the engine. Its the amount of pressure required by the carb/efi that sets them apart. Neither pump will supply more or less fuel to the engine. BUT with the high pressure EFI system, there will be a constant flow of fuel bypassing the TBI because of the pressure regulator. In other words, a big LOOP of fuel going from tank -> efi -> tank. Basically unused fuel. The grey area is that I dont know how many GPH that loop is, on average. (at idle/cruise)

IF that loop of fuel is a considerable amount, than I could see the electric pump being starved, especially at idle. If its more like a trickle, than my plan is feasible.

Heres another something to think about. If its so important for the pump to be below the lowest point of the tank, than that means the pump is REALLY bad at sucking fuel. The mechanical pump will flow MUCH more than gravity alone. In this case, the plan will work.

Chuck: Great idea, just mounting the pump after the selector valve. For some reason I didnt give that idea much consideration. Probably because the Holley manual says to do it a completely different way.

Yeah I just reviewed the instructions and it insists on two pumps, one for each tank, BEFORE the selector valve!! I cant imagine the valve being able to withstand 15-20psi. I think that if I cant get the point where Im at least reasonably suret that my idea will work, I'll mount the pump like you said. Re-routing hoses should be easy. I'll have to run just one hot feed for the fuel pump.. thats easy.

Well, keep the suggestions coming, Ill keep you guys updated. Thanks again chuck

mike reeh
-not putting email address anymore, too much spam!!


PlowSite.com Veteran
By running a mechanical pump you are complicating the system, not simplifying it. The electric pump HAS to be in back near the tanks and they HAVE to be mounted BELOW them. The electric pump must be at the bottom of a loop from the tank to the pump and from the pump back up to the fuel line for the EXACT reason you state, because they are pushers. There must be something at the head of the pump so it can push the fuel to the TBI. They also must have fuel at the tail of the pump that can be pushed.
OBVIOUSLY you dont understand that the mechanical pump CANNOT supply enough VOLUME to the TBI for the same reason I stated in the other forum. IT IS NOT A CONSTANT FLOW PUMP! It cannot supply the necessary flow at the given pressure. Holleys electric pump you have is rated at 300PPH @ 15psi, Holleys High output mechanical pumps in comparison is 35PPH @ 4.5psi @ 1000rpm to a max of 70PPH @ 4.5psi @ 7000rpm. Your stock pump is LOWER. There is a problem here I would say. Therefore using 2 pumps when the one will do the job is complicating the matter. Not only this, but the mechanical pump will bleed pressure out of the system. A big NO in EFI.
If you omit the electric pump, the motor will not run, and therefore it will not supply enough fuel for the given pressure.
Not sure why this even matters, but the big LOOP of fuel going from tank -> efi -> tank is going to be constant GPH because the electric pump is at a continuous flow. The return line back to the tank is supposed to have a max pressure (based on memory of the Holley system) of 8psi. It will hardly be a trickle because the regulator is bleeding off pressure/unused gas at above 15psi.

Pump being below the tank for one reason is so it want go out prematurely. So it will be in a trap and will not have to be primed everytime. And yes it is REALLY bad at sucking fuel, we/you already stated electric pumps are pushers, not pullers.

The current selector vavle will not handle 15psi, unless it is a '87 truck, which if that was true we wouldn't be talking about Holleys TBI. If I remeber correctly Holley recommends getting their dual tank kit which includes a new selector valve. Putting the electric pump will after the current selector probably will cause problems due to it not being leak/seal proof. I won't say it won't work, but I have my doubts.

John DiMartino

PlowSite.com Veteran
You are asking the electric pump to pul fule all the way from the tank,thru the mechanical pump and push it to the TBI every time you turn the key on.The electric pump runs the instant the key is turned on,while the mechanical wont until you crank it.Then the mechanical pump has surges as it feeds the electric pump.I dont see thsi working,just my opinion.I know you dont want to hear this,but either do it right and buy 2 pumps or just use one tank and install a transfer pump from the other to fill the one your sucking from.You will have to change the selector and valves too.There is a way to make it work using a fuel resevour,with the mechanical pump,but you will have to buy some parts,it still might be cheaper than changing everything over.E-mail me or call me 845-778-0501 for more details,I know this works,done Ive before.This would also eliminate the fuel starvation at low fule levels when cornering or stopping fast.