Fuel pump theory...

[302efi]

I have a Holley blue pump w/reg on on my carbed 306. The pump is mounted right by the tank and the reg is mounted right at the carb. Something just came to mind. Tell me if this is right :

Say I'
m running 7psi of fuel pressure at the carb. My question is, the pump is still pushing 12 psi of fuel until it gets to the reg, right ???

So, no matter what I do, the pump will still pull the same amount of gas from the tank, no matter if the reg is set at 2 psi or 12 psi, correct ???

No wonder I get bad mpg

 

[85_SS_302_Coupe]

well, the regulator is there to controll how much fuel the carb gets, so either it's only going to burn 2psi and the rest goes back through a return line to the tank, or something's not set up right.

 

[Michael Yount]

I think the carb set up may different 85-SS; I think that type regulator simply takes a pressure cut - I don't think it has a return line.

302efi - the question is whether or not the regulator is set for a pressure level that's expected by the carb. Seems most I've piddled with want 4-7 psi - so you sound about right, although on the high end. The carb is only seeing whatever you set the regulator at. That pressure doesn't "force" gasoline into the engine. The fuel gets "pulled" into the engine because of a pressure difference between the air pressure 'pushing' on the fuel in the fuel bowl, and the air pressure of the air passing through the throat of the carb. Jet size can impact things, but pressure (within a reasonable range) shouldn't impact fuel economy. There's a needle valve on the fuel bowl inlet, and the float inside the bowl closes off that needle valve keeping too much fuel from entering. The fuel in the bowl is at atmospheric pressure - not line pressure. On carbed cars pressure problems usually show up under full throttle - the engine's need for gas outruns what the pump is capable of delivering. Or they show up because needle valves/floats can't cut off the over-pressure that's being delivered to the carb. You'd know that because gasoline would be overflowing the carb.

Tell us more about how your car is modded, how it's driven, what traffic is like and what your gas mileage is.

 

[tmoss]

I had a slick mod setup on my Holley regulator - it was vacuum modulated (run vacuum to the top of the regulator diaphram) so that pressure varied with vacuum, just like our EFI pressure does - at WOT pressure was 7#, at idle it was about 3#.

 

[Michael Yount]

That is slick Tom - makes life a bit easier on those needle/seat valves.