Quote:
Originally Posted by Jay S
A mechanical pump fuel pressure is referenced to the atmosphere. Same as an electric. There is no reference back to the inlet pressure.
I have seen some old low boost turbo set ups where mechanical pumps ported a boost reference too the intake manifold. I think it is usually up on the side of body of the pump. Tom V probably has seen that also. I think I recall seeing that on Hugh Macinnes’s turbo charging book.
It is not usual to see the idle fuel pressure jump up a couple pounds with you turn the electric on though. But only a couple lbs, depends how the electric bypass is set, and how well the bypass on the mechanical pump works.
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Ahhhhh!!! Thanks for dealing with my confusion and answering.
I'll have to digest this, as I thought gas was pulled into the pump (or pushed in by an electric pump), the diaphragm started moving, pressure built up, the inlet valve closed, and gas exited through the check valve at the outlet.. so I've got to figure out why if the gas was above atmospheric going into the pump chamber, it wouldn't stay at that pressure.
I understand if the mechanical pump was "off" (pretend the lever wasn't being actuated), an electrical pump could push right through it. If it didn't work this way, an electrical pump couldn't prime a carb.
So you are probably correct about "relative to atmospheric, not inlet pressure", but how that works remains unclear to me.
I still somehow think if you plugged the outlet of the mechanical pump, you'd see the pressure generated by the upstream electrical pump inside the mechanical pump chamber, whether the mechanical pump was operating or not. And this pressure would be above atmospheric, any pressure generated by the mechanical pump would raise it even further. This is where I'm still stuck!!
I probably has to do with the way the mechanical pump pressure is limited but I'm not clear on all this yet....
hmmmmmm...
Mike