Transient fuel pressure drop

[88V8]

Er, "New alloy tank" - fourth line , original post. But I didn't dwell on it, and yes, such a major change is material. As is that it replaced a much smaller (3 gallon) rectangular tank, that only had an outlet and a breather pipe, so that the returns had to be piggy-backed onto the breather. That produced similar much much more severe symptoms, as the breather was choked by the returns. Correcting that meant using a spare connection on the swirl pot for the PRV return - that worked!

The pre-facet filter is also new, as the original glass bodied filter cracked.

 

The Facet goes at full chat, all the time, circulating the fuel through the swirl pot and keeping it full.

 

Ivor, you describe the symptom most succinctly, "when you've been on a trailing or light throttle and floor it", though flooring it is not necessary. The lightest application of throttle precipitates the hesitation, nay, total lack of power for a second or more, as the fuel pressure collapses, then reappears, several times before normal service is resumed. It feels like the 'kangaroo juice' that novice drivers use when learning clurch control, and must strain the transmission a lot. The stalling on slowing is, I think the same thing, but without the clutch in there is no momentum of the car to keep the engine turning to catch and run as soon as fuel returns. I suspect I unconciously apply a little throttle as I prepare to set off again in a lower gear, the fuel presure drops and the engine stalls.

Carbs and I presume the M/u provide a richening effect on acceleration. I did wonder if the Bosch was weakening and wasbarely adequate, so this small demand outstripped it. But as you say, full chat, no problem, and it is well able to sustain 140psi as my faulty PRV experiment proved. But pressure is not the same as flow, and there is either a restriction somewhere, or bubbles are getting through.

 

Bosch noises: It usually starts with a high pitched rumbling, if I may put it that way, the noise ascends to shriek, as the pressure rises and settles at 105. Noise while running is difficult to say, as a stripped-out racer with minimal silencing is a noisy environment. There are similar shrieks as the pressure rises and falls, I think

 

ntc,

I would love to have a glass swirl pot. Fiddling with the connections the other night, and running the Facet with the swirl pot to Bosch outlet blocked, I had a plastic conector in the return line (it was convenient). It seemed to take a long time for the bubbles to clear in the return flow, as if the swirling was not as efficient as I hoped. It would be good to see what is happening in there!

The high-pitched rumbling sounds like cavitation/air bubbles.

 

As pressure builds, the pump note rises - and if I haven't used the car for a while, it does get quite high-pitched - then falls off once it's pressurised, but I'm describing the only noise I can hear in a trimmed road car, which is pre-start-up.

 

I come back, again, to the point that if the system is OK at sustained full chat, then it follows that it will not only do the pressure, it will do the flow-at-pressure, which as you say is quite another matter. So the problem isn't downstream of the Bosch, it's upstream. And it's not lack of capacity of the upstream system, it's not the filters or the Facet or the tank vent or the pipes, because it will supply on-the floor-max-revs-flow-at-pressure.

So something is changing in the upstream system, let's say taking heed of the pump noises, that it's aerating during periods of low demand.

 

So how could air be getting into the system?

If the Facet is drawing consistently from under the fuel, it can't be drawing free air, unless you have an air leak in the supply pipe to the Facet. And if you did, that problem would be there all the time, whether accelerating or sustained high revs, but it's not.

It seems that it's only when the system has had a period of not being called upon to deliver much fuel, that's when it manifests, that's when it aerates.

 

Despite your parsimonious reluctance to spend a piffling £100 on foaming the tank (which I can understand when you're probably thinking that you might still need to mod it) it doesn't sound as if there is surge in the main tank, because on a trailing throttle there's no G force - well, OK, in a corner, but this happens all the time, right, even when the tank is full ?

 

So the problem logically lies in elements of the upstream system which undergo a change of state during periods of low demand, independent of G. Another way to express it is that the aeration is autonomically generated when the supply to the Bosch is mismatched with its demand.

So might we postulate that when the swirl pot is filled and overfilled, somehow the fuel become aerated?

 

Looking again at your picture - a picture of something that looks more like aerospace engineering than part of an old banger - having a bit of trouble seeing where the black pipes go, but there appear to be two connections between the Facet and the swirl pot, why?

Also, the input to the pot seems to be towards the top ? would it not be better to put the feed into the pot at the bottom, then it would have no opportunity to aerate?

And I can't see where the takeoff is for the Bosch, but if it's at the top, that's where most of the air would be, but as I say I can't actually see where it is....

 

The Facet runs at constant speed.... well perhaps it shouldn't. Perhaps all that fuel sloshing around the swirl pot circuit is aerating - after all, even if there is no free air, dissolved air will come out of suspension on pressure drop, and there is a drop where it goes into the swirl pot, and another where it goes back to the tank, and if the tank's not full the falling fuel will pick up more air, so the more it goes around and around because it's not needed by the Bosch, the more potential for air. (you get the same problem with domestic central heating if it pumps over into the header tank, the resulting aeration causes corrosion)

 

And I repeat, your system doesn't aerate when supply and demand are well matched, only when supply is excessive.

 

So ..... when the Bosch suddenly wants more fuel, you have the excess fuel that is going back to the main tank, that returning column of fuel will creat a conflicting vacuum in the pot, conflicting with the Bosch which is also trying to pull fuel. Possible exacerbation.... not much....

 

However, all this supposition.... you've nearly pinned it down.... we can agree that it's the upstream side, the supply to the Bosch, so it's happening in the boot... do you know an intelligent, hardy, small person..... replace all the upstream pipe with transparent, including the air/fuel returns, and drive the car with said person in the boot, you won't need to drive fast or far from what you've said, small person's task being to note where and when the bubbles form, and recite same into a dictaphone for post-drive pondering over a dram or two.

Then your task - to ambush the bubbles and deflate them..

 

Ivor

Edited August 28, 2008 by 88V8

[john.r.davies]

Dearly beloved brothers and sisters,

 

Today's lesson is taken from the Book of Murphy, Chapter One, Verse One:

 

"And Murphy did say unto the multitude, "Kiss!"

But before the multitude could embrace,

Murphy said unto them,

"No, no, not like that cloth ears! Keep It Simple Stupid!"

And the multitude did wonder indeed at their own stupidity."

Thanks be to Murphy.

 

 

For indeed, it would have been better if I had KISSed!

And left out that manky little disposable filter between the tank and the Facet.

Remove that - and normal service is resumed!

 

I dissected that filter, and although the body was about two inches across, the filter element inside was no thicker than my finger!

No wonder it wouldn't flow.

 

Thank you all for you thoughts and advice.

 

I know where to come again when I have not KISSed!

 

John

[Guest ntc]

Dearly beloved brothers and sisters,

 

Today's lesson is taken from the Book of Murphy, Chapter One, Verse One:

 

"And Murphy did say unto the multitude, "Kiss!"

But before the multitude could embrace,

Murphy said unto them,

"No, no, not like that cloth ears! Keep It Simple Stupid!"

And the multitude did wonder indeed at their own stupidity."

Thanks be to Murphy.

 

 

For indeed, it would have been better if I had KISSed!

And left out that manky little disposable filter between the tank and the Facet.

Remove that - and normal service is resumed!

 

I dissected that filter, and although the body was about two inches across, the filter element inside was no thicker than my finger!

No wonder it wouldn't flow.

 

Thank you all for you thoughts and advice.

 

I know where to come again when I have not KISSed!

 

John

 

Good news John I would suggest that you raise the tank breather higher than it is ,come race day unless the tank is baffled or foam filled you could have a problem with surging.

[john.r.davies]

Why do you say that, ntc?

The tube to the breather is short.

Do you reckon that fuel will splash up into the breather and block it somehow?

I've reconnected the swirl pot, so surge affecting feed to the Bosch should (please, gods) not be a problem.

 

John

[Guest ntc]

Why do you say that, ntc?

The tube to the breather is short.

Do you reckon that fuel will splash up into the breather and block it somehow?

I've reconnected the swirl pot, so surge affecting feed to the Bosch should (please, gods) not be a problem.

 

John

 

Hi John

 

In past experience when rallying we had problems on a dry sump system on left handers climbing it turned out the valve similar to yours needed a head of air in the tube to function. Just thinking about your setup with a full tank you could have the same scenario

Regards

Neil

[john.r.davies]

Thanks, neil!

I'll be guided by your experience

Putting on a longer piece of tubing will be so easy.

 

John