[Tom Vaught]

Tim, here you go.

Picture is worth 1000 words

Tom V.

[Shiny]

OK, help me here.

Is the benefit of this bypass config just to avoid a flow restriction when running only the mechanical pump?

If, OTOH, the electric pump is used to add more flow at the carb, it can only do this by adding more pressure, right? So does this mean you MUST also have a regulator to stay carb-friendly?

I measured the inlet pressure that overwhelmed my needle and seat, and it was 12 psi sitting still. I guess if a mechanical pump typically delivers 5-7 psi, there is some margin to allow maybe 4 or 5 psi "boost" from the electrical pump. If flow through the mechanical pump is directly proportional to pressure, then the electric pump could almost double the flow ?? So does that electric pump have a low-pressure output or do you need a regulator somewhere to stay carb-friendly?

Mike

[glhs#116]

Originally Posted by Shiny

OK, help me here.

Is the benefit of this bypass config just to avoid a flow restriction when running only the mechanical pump?

If, OTOH, the electric pump is used to add more flow at the carb, it can only do this by adding more pressure, right? So does this mean you MUST also have a regulator to stay carb-friendly?

I measured the inlet pressure that overwhelmed my needle and seat, and it was 12 psi sitting still. I guess if a mechanical pump typically delivers 5-7 psi, there is some margin to allow maybe 4 or 5 psi "boost" from the electrical pump. If flow through the mechanical pump is directly proportional to pressure, then the electric pump could almost double the flow ?? So does that electric pump have a low-pressure output or do you need a regulator somewhere to stay carb-friendly?

Mike

I think if you had some weird sort of parallel setup between the two pumps you might have the situation you are talking about of the pressures adding. Both would be pushing at once at the carb.

But this isn't that. All of these setups are series. So the mechanical pump has nothing between it and the carb whether or not there is an electric pump behind it. The mechanical pump in normal operation sucks from the tank. In other words it causes a vacuum behind itself (well, a pressure drop, technically) strong enough to pull fuel forward to feed itself. On the other side it produces pressure to the carb that it regulates to whatever pressure. If you have a pusher (electric) pump somewhere between the tank and the mechanical what it hopefully does is take away some of the effort of pulling the fuel from the tank and make it easier for the mechanical to draw. This is why Jay was talking about the "free flow" rates of the pumps. In most cases the mechanical pump could pump more fuel to the carb at the same pressure if it didn't have to work so hard to draw it from the tank. And also, in most cases the electric pump could flow more fuel if instead of pushing against a carb it was having the suction side of the mechanical sucking on its outlet. In other words, in series both pumps "help" each other and both should be able to flow more together than each could alone. But unless the electric really overwhelms the mechanical it shouldn't change the pressure the mechanical puts out on its outlet side against the carb.

I think the way to think about the bypass and check valve is that it does two important things.

First, if the electric isn't running then the fuel has two paths to get to the mechanical from the tank. It can pass through the check valve and it can pass through the pump. Each of these is probably a little more restrictive than plain hose but having both open probably means there's no more restriction than the mechanical had before.

Second, when the electric is turned on and pressurises its outlet side, the check valve prevents that pressure just sending fuel in a loop back to the tank and makes sure it gets upstream where it will help.

Sam

[Tim Corcoran]

Originally Posted by Tom Vaught

Tim, here you go.

Picture is worth 1000 words

Tom V.

Thanks Tom