Fuel pressure on Carb motors

[priobe]

I got a question. Why is fuel pressure important on carb motors if all that is happening is fuel entering a chamber to get sucked in via the jets.

As lon as the bowl is keep fuel with fuel why would pressure matter. Wouldn't volume be more important?

[blykins]

Too much pressure will blow the needle off the seat.

[priobe]

I understand that, but why have it set at 7 psi and not 5 psi or 4

If the value pressure is set below the blow point of the needle does it make a difference.

[strictlypersonl]

Originally Posted by priobe

I understand that, but why have it set at 7 psi and not 5 psi or 4

If the value pressure is set below the blow point of the needle does it make a difference.

The fuel is forced into the bowl through a relatively small orifice. In order for the delivery to keep up with maximum expected demand, the pressure must be adequate.

[PurpleVenom]

2nd that. I recently witnessed what happens with too low of fuel pressure. If the pressure drops below demand, the engine will lean out. Lean is bad. Fuel bowls should stay half full throughout the RPM band has been my experience.

[Hotfingrs]

Like the others have said, too much pressure you blow the needle off the seat. Fuel will come out the air vents, go into carb and wash cylinder walls down and/or get on the exhaust manifold and catch fire. Too little causes a lean condition as stated and you still lose an engine...your choice lean or fire.

[jhv48]

Fuel pressure has to be high enough to keep the bowls filled to the prescribed levels under all conditions, but low enough not to overpower the needle and seat. Usually between 5-7 lbs is sufficient.

[priobe]

ok, not to have a debate here. But pressure is 1 thing and volume is a another.

Generally when pressure increases volume drops and vice versa.

So, if you have a pump that can deliver more volume at say 3 psi, why would you run your regulator at 6- 7 psi (where the volume drops off)

Again please dont ask for model # of of where this information can be obtained. This is only a example

[CobraEd]

It's like volts vs amps. Same thing.

You could have a 10,000 cfm volume pump but if it only generates 1/4 psi, it will never open the float valve and the car will not run.


.

[Jerry Clayton]

The inlet valves are NOT OPENED by fuel pressure!!!Gravity causes the float to drop, opening the needle seat. With larger diameter needle/seat assys, too much pressure will force them to leak overrunning the carb at the vents.
Also--too much volume/pressure can cause a carb to syphon fuel thru the vent at higher engine rpm.

I like to set carbs up with around 3 1/2 to 4 1/2 pressure and reasonable needle valve assy for the type of driving the car sees.

[CobraEd]

Originally Posted by Jerry Clayton

The inlet valves are NOT OPENED by fuel pressure!!!Gravity causes the float to drop, opening the needle seat.

You are right, . . . . I forgot!!


.

[Bobcat]

Let`s go back to basic theory . A fixed displacement pump ( fuel , hydraulic or whatever ) delivers volume , or flow , in gpm , lpm or whatever . A restriction in the circuit creates the pressure , or more precisely , pressure drop across the restriction . In this case , the restriction/orifice is/are the openings in the float needle valve . The pump will deliver a flow of so many gpm across the orifice at a certain pressure drop directly related to the pump flow . Assuming the pump flow is constant and you could vary the orifice , you would see the pressure drop increase or decrease as the orifice size was decreased or increased . The float provides in effect , a back pressure check that varies somewhat the orifice size ... although it is fairly crude .
Why would volume drop as pressure increases ? Pumps are not 100 % efficient and always have some internal leakage ... either through the internal valves ( diaphragm pump ) or past the gears or pistons . The needle vale also isn`t a 100% bubble tight seal when closed .
Also , most fuel pumps have an internal relief valve that limits the max pressure ... as do some piston pumps .


Bob

[priobe]

Originally Posted by Bobcat

Why would volume drop as pressure increases ?

Bob

Ok, take a water hose, turn open the valve you have alot of volume, basically no pressure.

Apply your thumb over the hose outlet ( restriction).

Pressure increases / volume decreases.

[Bobcat]

Pretty much correct , except that a water hose doesn`t work exactly a pump . The hose has no internal bypass ( unless it ruptures ) . If you put a variable cut off valve on the output of the hose with a pressure gauge teed in and close it , you will see whatever the system pressure is . Watch the gauge as you open the valve and you will see the pressure fall off as the flow increases .
In a hydraulic system , if you have enough hp behind the pump , you can push as much flow through the orifice as you want up to the flow capacity of the pump . You just increase the pressure drop ( delta p ) across the orifice . The reason pump flow drops off as pressure increases is because all pumps have internal leakage from clearance between the gears to housing , pistons to sleeves and housing etc and on diaphragm units ( fuel pumps ) , across the check valves . Other components in the system can and do have internal bypass/leakage . All pump circuits have a relief valve somewhere in the outlet ... or you will have a disaster if the output is closed off totally . I`ve seen gear pumps literally explode when the output was blocked .

Bob

[Barnsnake]

In a mechanical (engine driven) fuel pump, there is a stretchy diaphragm. It is capable of maintaining the same pressure at zero flow or at a significant percentage of its flow capacity. In effect, it has its own accumulator. That's the key reason you seldom need a pressure regulator with a mechanical pump on a carbureted engine.

[priobe]

ok

So carb engines do not need a specified pressure to operate correctly like FI.

However, the only reason pressure is indicated is so you dont blow the needle out of the seat.

Carbed engines are more concerned with volume, filling up the fuel bowl as fast as possible

Correct?

[Jerry Clayton]

A fuel pressure guage on a carb eng will tell you if the pump is working.

A fuel pressure guage on a carbed engine will tell you why the float bowls are running over

Adequate fuel supply on a carbed engine is indicated by the timing slips

[mdross1]

At Holley for example their testing has shown their needles have enough float pressure to stand up to 7 psi.Most of the driving done will not need that kind of pressure.With a fuel pump and fuel lines with enough capacity the few times we get into the rpm's requiring large amounts of fuel then maybe max pressure is where one wants to regulate the fuel pressure.

[LoBelly]

Originally Posted by priobe

ok

So carb engines do not need a specified pressure to operate correctly like FI.

However, the only reason pressure is indicated is so you dont blow the needle out of the seat.

Carbed engines are more concerned with volume, filling up the fuel bowl as fast as possible

Correct?

yes (afaik)

when we fitted the Holley Blue electric fuel pump we simply wound down the pressure regulator until fuel stopped dripping through the carbie - don't have a pressure guage fitted at present.

LB

[Rick Parker]

For automotive applications with a mechanical pump, the MAXIMUM available pressure is limited by the spring that controls the lever which rides on the pump eccentric. Holley's seem to be able to accept 6PSI, sometimes a little more. Webers on the other hand don't like more than about 3.5-4 PSI. Part of this has to do with the fact that Webers use a metal on metal neddle & seat with a float that doeas not have a lot of leverage. Holley's on the other hand utilize a Viton tiped needle & seat with a float capable of exerting more pressure.