Fuel pouring out the front of the carbs

[Captain Obvious]

According to the 74 FSM (page EF-9), the electric fuel pump should deliver 1400 cc (85 cu in) in one minute or less. And also on the same page, it lists the pressure spec at 4.6 psi.

No idea if that's what they actually did, but that's the spec.

[Zed Head]

Seems like a good reference for people looking for aftermarket electric pumps.  I haven't had a carbed Z but if I did I'd probably get rid of the restrictor and use an FPR on the return line set to 4.6 psi if I went electric.  It's probably already been discussed a few hundred times.

Just filling my brain for future possibilities...

[Mark Maras]

Keep in mind that carbs don't rely on fuel pressure at all.  Volume is all they need and enough pressure to push or pull the fuel from the tank to the bowls. Extra pressure will try to only defeat the purpose of the needle and seat valve.

[Captain Obvious]

Carbs DO rely on pressure. You need to overcome the cracking pressure of the float valves. It might not be a lot of pressure required, but it's non-zero, and the system will not work without it.   

Too much is a problem, But no pressure is a problem as well.

[Zed Head]

At first brain wave it makes sense that you don't really need much pressure, from a static bowl perspective.  Fill it up, the float rises and seals, you're done.  But if you get in to the weeds you also have to consider how much fuel can pass through the float seat orifice at full throttle.  Seems like low pressure might cause a fuel starvation problem at high load.  You need pressure to push through that hole and keep that bowl full.

That's why I like to dig in to what the factory engineers determine.  They have engines to play with and tools to figure out the best numbers.

[Captain Obvious]

It's not the weeds at all. You need pressure.

Enough to overcome the cracking pressure when the valves are closed, and enough to overcome the orifice restriction that is the needle valve.

Tables and tables and charts and formulas for the pressure drop across an orifice. Reynolds number, temperature, flow rate, viscosity..... Depends on a whole bunch of factors, but the bottom line is you need enough pressure  to push the fuel in under all conditions, but not so much as to force the valves open when you don't want them to be open.

It's a little mini-control loop with pressure being the energy that makes it all work. If it worked without pressure, it would be a perpetual motion machine.

[Zed Head]

1 hour ago, Zed Head said:

much pressure

 

[Zed Head]

1 hour ago, Zed Head said:

you don't really need much pressure, from a static bowl perspective.

 

[Mark Maras]

 @Captain ObviousPlease define "cracking pressure when the valve is closed". I'm not familiar with the term. My point about the carbs not needing pressure is they can operate with a gravity fuel feed to the float bowls. If the float bowls will fill to the proper height at full throttle using 3.5 psi why up the pressure to 4.5?

 In the late 60s we ran a small dragster with a carbureted 327 c.i. Chevy. No fuel pump. Just a one gallon, non vented, fuel tank with a pressure gauge and a small bicycle air pump. We'd pump 3.5 psi into the tank, warm it up, do a burnout and just before staging the car we'd give a couple of more strokes on the pump to bring the tank pressure back up to 3.5 psi. We never experienced a fuel starvation problem.

[Jeff Berk]

As long as we are talking about fuel pressure and orifices, can I hijack this tread for a personal issue?

My 260z has both mechanical and electric fuel pumps, no fuel rail, and the fuel return line was disconnected. I'm at 4.5 psi using an old Holley fuel pressure regulator who's adjuster is not working (internal spring worn out?). I'm using two round-top SUs. I'd like to lower the pressure to see if it correct a rich mixture situation. If I don't run the electric pump, the pressure drops to 3.5 psi but I periodically have the engine start to miss until I switch the electric pump back on.

Any opinions on if I should just replace the fuel pressure regulator with a new non-return one or with a return type and reconnect the return line?

Edited July 2, 2021 by Jeff Berk

[Mark Maras]

I'd reconnect the return line. I'm surprised the engine starts to miss at 3.5 once in a while. Have you tested the pump for volume? A restriction in the supply line can up the pressure and lower the volume.

[Zed Head]

I don't have a good grasp on how a deadhead FPR works.  Holley says it works by an adjustable restriction.  Seems like the pressure would then vary with fuel pump volume, and fuel pump volume would vary with voltage, and voltage typically varies with RPM on a Z car.  Doesn't seem like the deadhead system would be very consistent.  I think it also puts more load on the pump, since the pump is pushing against a blockage that doesn't flow very much.

https://www.holley.com/blog/post/how_to_choose_a_regulator_for_carbureted_vehicles/

Essentially, the deadhead style regulator uses a restriction in order to lower the fuel pressure by simply decreasing the fuel flow. This is accomplished with the use of a diaphragm and seat that restricts the fuel flow and uses a spring to provide pressure adjustability.