mech pump as regulator?
 

My camaro's block is not bored for a fuel pump rod so I can't run a mechanical pump. I plan to install a pusher pump at the tank and I want to maintain my return line and keep the stock fuel line routing to the carb. Anyone see a reason I couldn't use a stock fuel pump as a 'regulated' bypass at the block?

I assume the return valve in the stock mechanical pump is simply a pressure release spring valve? I would probably need to know at what pressure the mechanical pump will bypass fuel, but I could determine that mechanically with a pressure guage then set my pusher pump to this pressure or slightly above.

I considered the trick three port regulator mounted on the fenderwell with the braided fuel line, etc. But that stuff always looks so shiny at first then begins to grunge out after a while. If the stock pump would work it would look stock, save some cash and maybe be a little innovative.

I dont think that would be good for the pump. You could just get a return style regulator.

Why?

And yes I could get a return style regulator but why be like everyone else?

Why? because it works.

How about, mount a 3-port regulator on the fuel pump block-off plate? Make a little bracket or something.

'How about, mount a 3-port regulator on the fuel pump block-off plate? Make a little bracket or something.'

I could live with that.

And I was asking 'WHY' it would be bad for the pump?

I'm still not certain the mech pump couldn't be used, but further pondering has got me thinking there's a fine line between innovative and ghetto, and this line's getting blurry.

I don't know for sure but I think the return on the mech pump is just fed via a "calibrated" hole, not through a fancy spring-and-ball regulator.
The mech pump feeds a fairly constant pressure through the way it's designed. A spring pushes on the membrane - delivering fuel, and the rod intermittently "resets" the spring and membrane - sucking fuel.
I think your idea should work, and I don't think it'll be bad for anything. The only thing I can think of is that with a pusher pump, the pressure variations at the carb will be larger when driving up or down steep hills. But if you adjust the pusher pump pressure to the middle of the range that the carb wants, I believe it will be good enough.

I may wind up cutting an old mech pump open to verify how it works then go from there. Thanks for the input.

Naf- I think that's an interesting question. Of course, it all depends on how the factory mechanical pump regulates pressure. On an aftermarket pump with no bypass you can push the one-way valves open with only about ~1PSI of input pressure so that would never work as a regulator. I think ti would depend on the factory bypass system, which would have to open at the proper pressure AND have enough flow potential through the bypass to keep up with an electric pump.

Aw, heck, bolt it on, hook up a fuel pressure gague at the carb and take a drive. See how it does. Wouldn't be the first time somebody has used simple back-woods methods to see if something will work or not.

aint broke dont fix it :)

Is broke, got to fix it.

It's just a matter of blowing into the return port of the pump to find out how it works. I bet you'll feel there's only a restriction there, things are usually made as simple, cheap, and dependable as possible. The pressure regulation is inherent in the way it's made, with a spring pushing on the membrane, just like a pressure regulator would be made. Spring tension and membrane area determines the pressure it gives.
Like Damon says, just do it and try it out, it'll most likely work, no reason it shouldn't, as long as the pusher pump has the capacity for both the return bleed and the engine's demands.

It'll probably be a few weeks before I get to it but I'll try it and give a follow up.

Thanks for the advice, guys.

I thought they control pressure by shutting off the mechanical pumping action?

It doesn't "shut off", it just can't push any harder.

It doesn't shut off as per se, but yet in a way it can be said to do so.
When the demand is small the pump just rides on the top of the cam pump profile. With a large demand it rides on the whole profile giving full stroke.
The top of the pump is a chamber with valves in and out. In the bottom of that chamber is a membrane. Under the membrane is a spring pushing the membrane up, thus feeding fuel at constant pressure. The membrane is forcibly pulled down by a lever which in turn is pushed on by the rod and the cam. So the sucking action is of unlimited strength, being forced by the cam, but the feeding action is smooth and regulated by the spring's force. If the fuel is not consumed from the chamber then the membrane simply stays down and cam gets no work to be done, just being touched on top of its profile.
Try to block the outputs of a spare pump and see what that does to the feel of the lever as you push it. Releasing the outputs makes the lever pop out again.

Everything so far works fine. I reused the little fenderwell mounted pump and regulator that the PO supplied with the engine. Running it through the mech pump shows a continuous return flow (I used a cheap, clear filter so I could see the flow-will replace it with something more durable later). I'm getting a good 5 psi at the carb. The in-tank pump was working but at very low pressure, maybe 1-2 psi.

I'm not really concerned with mounting the little electric pump so far from the tank, it was dead-headed off the fender forever before I got it and it's receiving flow from the in-tank. It does put a lot of 'stuff' in a small amount of real estate and I will probably move it to the rear for a cleaner install. Maybe splice it into the circuit for the in-tank, depending on current draw.

I did run into an issue with the metal fuel line. It was supposed to have come off an LG4 and 'looked' the same as the one on my pontiac, but I could not get it to run behind the water pump. Yes, I had intended to cut it back and re-flare for the inlet extension on the carb. It turned out to be a nearly perfect fit behind the alternator. Did all thirdgen lines run behind the water pump?

I didn't think any of them ran behind the water pump.

Weird, my 85 runs behind the water pump, tucked in next to the pass side mount.

The line I bought from classic tube runs behind the water pump, nice and tucked out of the way. Before that I made one that just ran behind the alternator. Can't tell what mine was stock though.