L84 fuel percolation

[Jim L.]

Re: L84 fuel percolation

Hi Dan,

Does the 'spider' valve shut off fuel when the engine is shut down?

For certain it is supposed to close completely on the early spiders, which have gravity operated check valves. On Sting Ray spiders with the spring operated valves, the valve should still be forced closed; however that all depends on the strength of the spring.

Only 'loose' fuel would be the tiny amount in the spider itself to 'cook off'? When the 380 unit starts, it gets a shot of fuel from the main fuel pump which then refills the spider with slightly pressurized fuel to help get rid of the boiling residual fuel in the spiders.

During a hot restart of a 7380 unit, you'd be holding the throttle open and the cam on the bellcrank would trip the microswitch. That, in turn, would prevent the starting fuel bypass valve from opening. So, the scenario of the engine mounted pump momentarily pressurizing the spider to stop the percolation woudn't happen.

On a hot restart, the engine is actually slightly flooded anyway from residual fuel in the nozzle lines. That's the reason for going to WOT to restart.

Seems like an insulating wrap to shield the spider lines would be a step in the right direction, just look ugly

When a solution to the percolation problem is finally developed, no matter what it is, I have a hunch the result will be, um, a less attractive FI unit.

Jim

[James J.]

Re: L84 fuel percolation

Jim,
Thanks for your patience in attemping to explain things.
Since there is no accelerator pump I fail to understand how fully opening the accelerator before starting would have any affect at all, except on initial cold start which would set the choke.
The hot start problem that I described preciuosly has been happening since the introduction of ethenol into the fuels.
Also Jerry Bramlett checked the anti-siphon valve and found that it was functional.
When it has been sitting for a few days and is stone cold it turns over several times, maybe 8 or 10 revolutions as you suggested before firing.
With the race fuel or on cool days with pump gas the engine will hot start, with partial pedal applied, with no problems at all, even with the hood closed.
It is important to me, to keep the engine conpartment looking as original, no insulation etc.

[Dan H.]

Re: L84 fuel percolation

Hi Dan,



For certain it is supposed to close completely on the early spiders, which have gravity operated check valves. On Sting Ray spiders with the spring operated valves, the valve should still be forced closed; however that all depends on the strength of the spring.


During a hot restart of a 7380 unit, you'd be holding the throttle open and the cam on the bellcrank would trip the microswitch. That, in turn, would prevent the starting fuel bypass valve from opening. So, the scenario of the engine mounted pump momentarily pressurizing the spider to stop the percolation woudn't happen.

On a hot restart, the engine is actually slightly flooded anyway from residual fuel in the nozzle lines. That's the reason for going to WOT to restart.


When a solution to the percolation problem is finally developed, no matter what it is, I have a hunch the result will be, um, a less attractive FI unit.

Jim

Jim, I forsee a 'check valve' mounted ontop of each nozzle!
Dan

[Jim L.]

Re: L84 fuel percolation

Jim, I forsee a 'check valve' mounted ontop of each nozzle!
Dan

You know....... Dan, that's an interesting idea. Seriously.

That would establish a certain minimum pressure in the spider and the nozzle lines that's higher than what it is today. You might not be able to make the pressure great enough to stop percolation under all conditions without totally screwing up the fuel delivery. Still, you could mitigate the problem somewhat and there'd be value in that.

Very interesting.....

Jim

[Jim L.]

Re: L84 fuel percolation

Since there is no accelerator pump I fail to understand how fully opening the accelerator before starting would have any affect at all, except on initial cold start which would set the choke.

See an earlier posting. The residual fuel in the nozzle lines creates a slight flooded condition when the engine is hot. Opening the throttle admits enough air to clear the flooded condition and it shuts off the starting fuel circuitry so excess, additional fuel isn't sprayed into the engine.

The hot start problem that I described preciuosly has been happening since the introduction of ethenol into the fuels.
Also Jerry Bramlett checked the anti-siphon valve and found that it was functional. When it has been sitting for a few days and is stone cold it turns over several times, maybe 8 or 10 revolutions as you suggested before firing.

If Jerry has validated the anti-siphon valve and if you need to spin the engine several revolutions for a cold start, then that's good enough for me.

With the race fuel or on cool days with pump gas the engine will hot start, with partial pedal applied, with no problems at all, even with the hood closed.

Hmmmm..... "partial pedal". Try wide open throttle next time (and every time) you need to do a hot restart.

It is important to me, to keep the engine conpartment looking as original, no insulation etc.

I'm very sensitive to and sympathetic with this line of thinking. I feel the same way. Ultimately, though, I'm going to do whatever it takes (short of buying expensive fuels) to keep driving my fuelies. If that means some gawdawful wart grows on the side of the fuel meter or the spider gets wrapped in wet burlap bags, then so be it.

The fuel percolation problem is a design issue with the Rochester FI units, plain and simple. It's time for some re-engineering of the design.

Jim

[Dan H.]

Re: L84 fuel percolation

You know....... Dan, that's an interesting idea. Seriously.

That would establish a certain minimum pressure in the spider and the nozzle lines that's higher than what it is today. You might not be able to make the pressure great enough to stop percolation under all conditions without totally screwing up the fuel delivery. Still, you could mitigate the problem somewhat and there'd be value in that.

Very interesting.....

Jim

Jim, 8 tiny electric shut off valves, like the reto fit safety anti siphon valves on the early units. See you in San Jose!
Dan

[Clem Z.]

Re: L84 fuel percolation

i know nothing about FI but from reading these post i get the idea that there is not enough fuel pressure in the system on restarts to prevent fuel boiling. since the fuel pressure is determined by how fast the FI fuel pump turns would using a mini hi torque starter like race cars use to spin the engine over faster making more fuel pressure solve the problem?? BWTFDIKAFI

[Jim L.]

Re: L84 fuel percolation

i know nothing about FI but from reading these post i get the idea that there is nor enough fuel pressure in the system on restarts to prevent fuel boiling. since the fuel pressure is determined by how fast the FI fuel pump turns would using a mini hi torque starter like race cars use to spin the engine over faster making more fuel pressure solve the problem?? BWTFDIKAFI

The hot restart issue isn't due to a lack of fuel or of a lack of fuel pressure. The residual fuel in the nozzle lines creates a slight flooded condition. To compensate, the hot start procedure is to open the throttle wide when cranking.

However, the nature of the gear pump in the FI bowl is such that, if you ignore pumping losses due to gear clearances and bleed holes in the pump design, the pump is capable of infinite pressure at stall. No, practically speaking, it won't actually develop infinite pressure because there are losses. But it can generate PLENTY of pressure regardless.

Interesting idea, though.

Jim

[Michael G.]

Re: L84 fuel percolation

If I picture the whole thing correctly, when the fuel pump stops pumping, the lack of pressure in the fuel lines allows the spring in the anti-siphon valve to close. This cuts off the supply of fuel to the nozzle lines. The fuel in these lines cannot flow into the cylinders, though, because it wold create a vacuum in the nozzle lines. It therefore remains in those lines until one of two things happen, either 1) the car is re-started, re-pressurizing the fuel lines, thereby opening the anti-siphon valve and forcing the fuel in the nozzle lines into the nozzle or 2) the heat from the manifold radiates onto the nozzle lines, vaporizing the more volatile components of the gasoline in those lines. This vapor has a greater volume than the liQuid it replaced, so it forces the non vaporized gas components into the cylinders, leaving the nozzle lines partially full of vapor, and slightly flooded cylinders.

To test this theory, yesterday, I measured the temperature of the surface of those lines at shut-off. They ranged between 120 F and 135 F and got slightly hotter (~10 F) as the engine sat for a few minutes. The boiling point of some of the hydrocarbons in the soup that is modern gasoline is below 130 F, so it is very possible that the above is actually happening.

I now have all the components of my test apparatus, to seel if I can both cool the spider and insulate it. I intended to install it this weekend, but unfortunately, my #5 exhaust valve rocker stud just pulled out, so I've gotta fix that first..

I'll keep you abreast of my progress.

Mike

[Duke W.]

Re: L84 fuel percolation

The FI pump, like the oil pump, is a "constant volume" design. Delivery volume is approximately linearly proportional to speed.

The pump is designed to deliver more fuel than than the engine needs at any speed and load condition with actual delivery volume regulated by the spill valve, which drops delivery volume/pressure based on the venturi vacuum signal and manifold vacuum (ratio lever control) and other control features during cranking.

Excess fuel out of the pump is "shorted" back to the fuel bowl. It's very similar to how the oil pump/pressure relief valve works, except the FI fuel delivery pressure is variable to meet the engine's fuel need based on air flow and load.

At idle, delivery pressure is only about 0.5 psi, and similarly very low during cranking, so it's easy for fuel in the spider to percolate.

With typical summer fuel vapor pressure, at 130F about 10 percent of the fuel's components are at or above their boiling temperature.

Modern EFI systems maintain 30-90 psi in the fuel rail, and delivery is contolled by injector open time. Fuel delivery rate to the rail is constant and greater than engine demand at any speed and load condition - say about ten times greater at idle. Excess fuel is shorted back to the fuel tank by the pressure regulator, so the fuel rail is constantly being cooled by fuel from the tank. The combination of cool fuel and high rail pressure make vapor lock on a modern engine very rare.

Duke

[Paul Y.]

Re: L84 fuel percolation

Duke, I agree with you 100%. You hit it right on the head. That's why I am having only about 10% minor problems with my injection. All of the ignition is dialed in right and the injection is freshly restored and I'm putting around 1-2K miles on a month. Life is good. I'm going to a cruise this afternoon with about 3000 cars if we don't get rained out. Of course my fuely doesn't melt if it gets wet. Cheers, Paul