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Talk to me about fuel pumps

68stang289

68stang289

Member
My Carter electric fuel pump took a crap on me and has forced me to look at replacement options. When I picked up the 70, the owner said the motor needed 8 psi of fuel pressure. That said, the single carter was pumping out 5.5-6psi according to my autometer gauge and the motor seemed to be running just fine. As all of us know, those electric pumps are noisy as can be, even when installed with rubber insulators. I see that some of the mechanical pumps on the market put out 8 psi max now. If I can avoid the noise and hassle of the electric I would like to, but I also don't want to starve the motor. What are the benefits of an electric vs a mechanical fuel pump?

I haven't found any good explanation for how the mechanical works in terms of psi it puts out. I would imagine they run 5-6 psi at idle with 8 psi under full load? Has anyone had good luck running those higher pressure mechanical fuel pumps with a more built motor? Anyone know of any mechanical pumps that put out more than 8 psi?

Thanks for the edumication guys!
 
If its a NA engine, I stay with the mechanical. Simple and reliable, if the engine turns over your pumping fuel no matter what the voltage is. IMO they will last over 100k. Not sure about the pressure comment, I would think at that pressure you would run the risk of fuel bypassing the needle valves. Its been a while for me on a carb though.
 
In terms of safety ;
A mechanical fuelpump will stop pumping when the engine shuts off due an accident for instance.
When you have an electrical fuelpump, you need the right "electrical wiring setup" so it disconnect the fuelpump from power in case of an accident.
They call it an " inertia switch " if I am right.
If you work with an electrical fuelpump , you have to have it worked through an inertia switch and triggered by the ignition system.So the fuelpump get activated for a few seconds if only the ignition"key" is turned on , and get continiusly activated when the engine is running.
( yes , poor explanation but I bet you get the picture of what i mean )
 
Actually, good explaination Bruno.

As for the mechanical pumps, they have by design an internal pressure relief that limits the pressure. It's not as good as an external regulator, but it works fine for carbs. he pressure will vary somewhat, from low at idle (5-6 psi, to reaching max pressure around 2000 rpm (cruise, not WOT).

Some do not have pressure limiting designs and are labeled along the lines of 'must use external regulator' These are high dollar mega volume pumps.
 
When you put the inertia switch in make sure it's working. A friend swapped factory efi in his Chevelle,on it's test run he hit a small bump and the switch kicked in. He couldn't figure out what went wrong until he checked the switch. Ran over the same bump and shut off again. Replaced the switch and no problems.
 
Thanks for all of the replies guys. I have read up some on the inertia switches and to be honest, the problems I've read that have been associated with them like KTJMcG mentioned was a big factor for me hoping to switch to a mechanical. I like the idea of the simplicity and reliability of the mechanical pump as well. I have read the needle valves can take up to around 10 psi in the Holley 4160's (which is what I'm running) before there starts to be a bypass issue due to the pressure.

67 evil eleanor - The motor is fairly built, but still NA - TRW .060 with big valve cuts, Crower solid cam, ported Offenhauser intake, Holley 4160, polished Lemans rods with ARP bolts, windage tray, Ford power parts rockers, stands and spacers, shell lifters, and steel roller timing chain.

apollard (or anyone else) - do you happen to know if the mechanical pump will put out the max 8 psi at WOT as well?

Thanks again for the wealth of knowledge guys!
 
a carb wanting 8 psi doesn't seem consistent with what i've read. was the owner trying bandaid with pressure as opposed to properly upgrading to support volume?
 
"buckeyedemon" said:
a carb wanting 8 psi doesn't seem consistent with what i've read. was the owner trying bandaid with pressure as opposed to properly upgrading to support volume?
Click to expand...

It honestly would not surprise me. He talked it up a lot about how he did "everything right" which included using speaker wire to wire up the electric fuel pump, using bailing wire to hold up the exhaust system, only one nut per seat total, you get the idea.
 
PO's.....so fun. 8 psi sounds high to me too. Sounds like some carb work needs to be re-accomplished.
 
8 is high for a street carb. 6 is more like it. As for pressure, the pump will maintain it's rated pressure at WOT unless you are using more fuel than it can pump. A 100 gph pump will support around 800 hp, so with a sbf, you would not have to worry about it with a good pump.
 
"apollard" said:
8 is high for a street carb. 6 is more like it. As for pressure, the pump will maintain it's rated pressure at WOT unless you are using more fuel than it can pump. A 100 gph pump will support around 800 hp, so with a sbf, you would not have to worry about it with a good pump.
Click to expand...

A 100 gph pump on a Mustang with a full 20 gallon tank, will last all of 12 minutes at WOT.
 
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