In a message dated 97-06-05 15:21:16 EDT, sean_johnson@milacron.com (Sean
Johnson) writes:
> I'm thinking that if I space the fuel pump out a little more, I can reduce
> the stroke, dropping the fuel pressure into the 3-5 psi range. It's not a
> linear relationship though, so getting the proper spacer distance from
> someone would be great!
Sean:
Changing the stroke won't fix the problem. A brief description of how a fuel
pump works will help explain.
A fuel pump consists of a fuel chamber, spring loaded diaphram, inlet check
valve, outlet check valve, and an actuating arm, which is driven by a cam
lobe, usually on the camshaft.
To start the description, assume the fuel chamber is empty, and the actuating
arm is on the low point of the cam. As the cam rotates, the arm pulls the
diaphram backwards, against the pressure of the spring. This creates a
suction which closes the outlet valve, opens the inlet valve, and draws fuel
into the chamber. When the diaphram is fully retracted, the cam rotates,
allowing the arm to fall to its lowest position, allowing the spring to push
the diaphram forward. This creates a pressure which closes the inlet valve,
opens the outlet valve, and pushes fuel to the carbs. The needle valves in
the carbs control the rate of discharge. At idle, the diaphram will hardly
move. If the diaphram is not in the full forward, or discharge, position, ie,
it still has fuel, the arm will be held off from making contact with the cam
lobe till later in the cam's rotation. When the arm again makes contact with
the cam lobe, it will return the diaphram to the full suction position. In
this instance, the diaphram will only take a partial stroke. On the other
hand, if the car is consuming large amounts of fuel, the chamber will be
empty when the arm starts it's power stroke, and the diaphram will make a
full stroke, drawing in a full charge of gas.
As you can see, the arm movement only controls the maximum amount of gas that
can be drawn into the fuel pump chamber - whether it gives the diaphram a
full or a partial stroke. Spacing the pump will only reduce the maximum flow
rate, not the pressure. If the stroke is too short, the pump will discharge
fully before the return stroke, and there will be a brief moment of time
during each stroke that no fuel will be delivered. At low speed, this may not
be a problem, but might severely limit high speed performance.
To control the pressure, the spring will have to be replaced (or the pump
itself).
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
|