fuel pump pressure

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    I was looking through various specs for fuel pump pressure and found some inconsistencies. It is important for setting up fuel pressures for the regulators if an electric pump is in the system. The 1935 Pierce spec sheet does not specify fuel pump pressure, but the 1934  specifies only 2 psi for both 8 and 12. I usually assume 1934 and 1935 specs to be the same, and the ’34-’35 parts book has the same fuel pump number. The 11th edition MoTors manual on the other hand specifies 4 1/2 psi for 1935-38. This is the same as specified for Packard V-12’s using EE-3 carb.

    If I remember correctly I checked to verify that my EE-3 would resist 4 1/2 psi without flooding the carb when I set up the fuel pressure regulator, although I set it a bit lower to 3 1/2 psi. I am cogitating on a mod to my system that would need to go to 4 1/2 psi with electric pump running.



    Fuel delivery systems must satisfy two conditions: Zero demand before start-up and maximum demand on a long mountain grade. Gravity systems; vacuum tank or Ford Model A, can deliver fuel at nearly fixed pressure during all conditions. Mechanical pumps deliver fuel proportional to engine speed; very low during cranking and increasing with engine speed.

    Electric pumps must meet both flow conditions with fixed output, often by using a regulator for pressure control. Too much pressure will flood if the float valve does not shut tight, but too little will starve the engine under heavy load.

    The electric pump my ’29 PA uses has a separate switch under the dash – not direct to the ignition. Separate switches allow the engine to start with the fuel already in the float chamber. Then this switch is used to provide flow when running, or run it just a few seconds on a hot day to prevent vapor lock before cranking.

    Setup of the carburetor will also have a large influence on the pressure needed or tolerated. If you want to dig deeper, send me an email. Herb



    Check PASB 1990-1 page 8 for some fuel pressure information. Hopefully it’s what you are looking for.



    Dave, thanks. Interesting, they manage to miss 1935 845 but presumably it is supposed to be the same as 1934 with the 2 psi pressure, although just to add to the confusion the original mechanical pump on mine will pump to over 3 psi, which should be determined by the spring force of the fuel pump diaphragm.

    My EE-3 with the original stainless steel needle (no rubber tip) resisted over 4 1/2 psi without apparent leakage to my surprise, but discretion being the better part of valor I will keep things at a lower pressure. 4 1/2 psi cold won’t be the same as when a hot carb bowl is boiling.

    Several years ago I witnessed the percolation phenomena when I was testing the new radiator core in summer heat – no fenders or hood. When the engine was nearing vapor lock I found the left side idle mixture adjust screw had no effect, and peering  trepidatiously down the carburetor I could see liquid fuel burping out of the main jet on that side from boiling. It would have been unpleasant if it had backfired at that moment. I am rethinking potential auxiliary electric pump system changes before the hood and fenders go on making changes inconvenient.



    I believe that the float bowl on an EE-3 is vented just like my UUR-2’s on my ’31 and 32.  This means that there is NO fuel line pressure in the chamber.  If you are vapor locking in the fuel line then increasing the pressure can help. If you are boiling the fuel in the float bowl then the bowl itself has gotten too hot.  I wrap my float bowls with a ceramic cloth “diaper” to help lower the temperature.  I also wrap the exhaust manifold for the same reason.  It’s also a good idea to wrap the fuel line itself.  Gas with ethanol makes all the cars more sensitive to vapor lock.




    Bill, I covered this in my articles on vapor lock a few years ago. Yes, the fuel line pressure has no influence on carburetor boiling. which is why a simple electric pump system doesn’t actually help with this. I am not a fan of insulation on the fuel line or the carburetor bowl to help with this, except for radiation shields with air gaps. The problem is that insulation slows down the transfer of heat but doesn’t stop it, so if a car heat soaks after a hot drive it will still heat up, just more slowly. Moreover, the insulation wrapped on a fuel line is going to absorb more heat radiating from a hot exhaust manifold than a shiny metal fuel line. The stagnant air under the hood rises to over 200 degrees for an extended time. When you get back in the car the insulation that slowed down the temperature rise of the carburetor bowl or fuel line now slows down how quickly it can cool down from the air that should start blowing around at around 130 degrees if you can get it going. The insulation’s effect of improving or exacerbating the problem becomes a question of how long the hot car sits before being restarted.



    My comments are really meant to apply to “hot driving” rather than “hot starting”.  We’ve driven our ’31 on some very hot days and have found that keeping the fuel as cool as possible before it gets to the carb helps delaying a vapor lock so bad that it stops the car.  While I’ve not tried it, Ed Minnie and John Cislak have created a system for recirculating the fuel from the inlet of the carb back to the tank where the fuel is cooler.  They also enlarge the fuel lines as part of the process.



    The only problems I have had on my cars myself are starting and running after a hot soak. My Pierce is not driving yet so I am trying to decide on an insurance system to install before the hood, fenders and runningboards go on making it much harder to to install. It could be I won’t need it but my tests so far indicate it is likely to be a problem. There are a couple basic ways I am aware for a recirculation bypass system, one is to just use an orifice at the carb inlet for the return line to bleed a fraction continuously of the basic mechanical fuel pump feed back to the tank to keep more circulation going through the pump and lines. This diverts some of the mechanical’s output  unless an electric is boosting it.

    I am thinking of a system,that uses the large excess of a rotary electric pump to recirculate only before and after briefly to cool the system and purge vapor after a hot soak. Most of the time it will operate normally on the mechanical without recirculation. It would be more effective as needed also for driving vapor lock. My original question was directed at trying to decide what a maximum pressure should be with the electric engaged. I think the answer is probably 1psi or so to make sure the electric doesn’t overwhelm the float valve in a hot carburetor and start overflowing.



    Fuel pump pressure should not be more than 2-2.5 psi. A pressure regulator would be helpful.  An electric pump as a pusher near the tank works better as a puller near the engine. I dove my 37 Airflow coast to coast across the desert at over 100 degrees without a problem. Problems worse with Ethanol fuel. You may find as I did in some circumstances in hot weather and higher altitude a mechanical pump just will not pull fuel without it vaporizing and causing problems. I prefer original when possible but their fuel not like ours today.

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