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Fuel pressure
Ham - 20/2/08 at 08:14 AM

Hi all,
I am currently changing my fuel pump from mechanical to electric, I have purchased a facet solid state pump(mounted at the tank outlet),
Do I need to install a pressure regulator?
Engine is a X Flow running twin 40's

Cheers

Mr Whippy - 20/2/08 at 08:35 AM

not normally as they cut out once up to pressure. Some also allow you to adjust the cut off.

Jenko - 20/2/08 at 08:37 AM

A pressure reg is always recomended - you are lookin for around 3.5 psi at the carbs......

britishtrident - 20/2/08 at 08:49 AM

You do not need a pressure regulator with this type of pump and DCOE carbs.

Regulators are a bad idea if not required as they place a restriction on the flow.

Xflo mechanical fuel pump is also fine to use with DCOE carbs.

Mr Whippy - 20/2/08 at 09:20 AM

Bet your glad you asked

I used that type on the Falcon after using the point’s type (unreliable). Never bothered with a regulator with either type and had no issues. If you’re using a fuel return I'd use the standard mechanical pump as due to the pressure never building up the electric pumps just run and run. The solid-state pump you interested in is designed for standard float carbs so won't flood them.

Ham - 20/2/08 at 09:23 AM

Thanks guys,
This pump is non adjustable, the reason I am replacing the mechanical pump is that on long straights on the track at around 6500rpm I experience fuel starvation .
The new pump does not give output pressure, if more than 3.5 psi what will happen?

Cheers

Mr Whippy - 20/2/08 at 10:14 AM

if the float valve is not seating right it might force fuel past the valve and flood the float chamber. You'll soon know about it as the car will feel like the choke is on and get a bit smelly. Unlikely to be a problem though and you can get exactly the same problem with the original mechanical pump.

alexdj125 - 20/2/08 at 11:06 AM

I've got a xflow with dcoe 40's and a facet fuel pump and I was told that I HAD to use a pressure reg, seems to run ok, or should I pull it off?

Thanks, Alex.

Mr Whippy - 20/2/08 at 11:58 AM

Who said you had to and why, did they even give a reason? Feel free to fit one if you like as there is no downside so long as it can keep up with the flow rate.

I would not bother myself as if it runs then no problem. The only thing I'd suggest is you fit the inertia kill switch used in fuel injection cars to stop the pump in the event of a crash and the fuel line being cut. You wouldn't like fuel spraying all over the place would you.

alexdj125 - 20/2/08 at 01:31 PM

Hmm, good point, I can't remember being given a reason! think I'll go pull it off and see what happens!

britishtrident - 20/2/08 at 02:33 PM

quote:
Originally posted by alexdj125
I've got a xflow with dcoe 40's and a facet fuel pump and I was told that I HAD to use a pressure reg, seems to run ok, or should I pull it off?

Thanks, Alex.


Depends which pump you fit different Facet pumps produce different output pressures the normal Road Facet Pump solid state produces 2.5 to 3 psi.
http://www.completeautomobilist.com/parts/fuel+system/

Fitting a higher pressure pump and a regulator is pretty pointless.

I am very much against regulators because they are not 100% reliable. Internally they have moving parts which wear, if the regulator is worn it will leak and especially in a no flow situation the pressure will build up to full line pressure.

In contrast in a fuel pump the output pressure is dictated by spring in compresion --- the pressure cannot rise above the rated pressure of the pump.

The higher pressure Solid state Facet pumps are i theory rated at a higher flow rate but this isn't true in practice because if you reduce the line pressure with a regulator you are in effect putting in a restriction that chokes the fuel flow back down to the same as standard road solid state pump.

What really determines the max fuel flow rate of a pump undr perfect conditions is much the same as what determines how much power an engine develops --- basically the size x number of strokes per minute.

To maximize fuel flow it is essential to pay attention to the suction side of the system.
Any restriction on the suction side will lead to vapour locks forming due the fuel instantly turning to vapour when the pressure is reduced much below atmospheric.

The fuel line from the tank to the pump should be adequately sized. Only relatively coarse "screen filters" should be used on the suction side. And the tank should be properly vented to avoid a semi-vacuum forming inside the tank.

[Edited on 20/2/08 by britishtrident]

PaulBuz - 20/2/08 at 02:34 PM

I'm running twin 40 DHLA's on my engine.
I also have a fast road Facet pump (4.5 psi?)
Without a regulator , the pressure overcomes the float needle, with fuel pouring out of the carb.!

britishtrident - 20/2/08 at 02:40 PM

quote:
Originally posted by PaulBuz
I'm running twin 40 DHLA's on my engine.
I also have a fast road Facet pump (4.5 psi?)
Without a regulator , the pressure overcomes the float needle, with fuel pouring out of the carb.!


When going through tuning parts catalogues you have set aside mind set that words such "high pressure" and "competition" have any meaning.

A high pressure fuel pump is pointless if the carbs can't cope with the extra pressure when the engine is stopped but pump switched on.

Much the same goes for the high pressure oil pumps sold for Xflos -- they only give higher pressure because the relief valve spring has a spacer under it, a bit of a con really.

[Edited on 20/2/08 by britishtrident]

Jenko - 20/2/08 at 05:52 PM

Sorry, I'm going to disagree with those who said you don't need one......

Weber's like having a consistant flow of fuel, Something like a filter king is designed for the job (plus adds some more filteration to the fuel).....Depending on which pump you have, those facet pumps can deliver upto 8 PSI, which is way too much....

A reg helps with any surge you may see, and ensures that the needle valves in the carbs never get overpowered....

If you have a <4psi pump then it's not stricktly required, but generally good practise.....

britishtrident - 20/2/08 at 07:21 PM

quote:
Originally posted by Jenko
Sorry, I'm going to disagree with those who said you don't need one......

Weber's like having a consistant flow of fuel, Something like a filter king is designed for the job (plus adds some more filteration to the fuel).....Depending on which pump you have, those facet pumps can deliver upto 8 PSI, which is way too much....

A reg helps with any surge you may see, and ensures that the needle valves in the carbs never get overpowered....

If you have a <4psi pump then it's not stricktly required, but generally good practise.....


A filter king won't make a difference to pulsation --- fuel is more or less incompressible. The only way to reduce pulsation would be to fit something like the huge double pumper low pressure SU pump as used for many decades by Rolls-Royce.

Provided the maximum pressure on the float valve is not exceeded Webers only care about the level of fuel in the float chamber.

Regulators without a return bleed cannot be depended on in non flow situation.
The non-flow situation cannot be regulated reliably because all regulators contain a valve with wearing parts which will at some point leak.

Jenko - 20/2/08 at 07:55 PM

Thats a fair point, but it's the higher than 4psi pulses that can have an effect on the needle valve...Too much pressure can effectively stop the floats returning...I'll admit you would need a very specific set of circumstances to get this if you had a low psi pump.........

The purpose of the reg is to ensure this never happens, so you have to be 100% sure that your fuel pump is incapable of producing no more than 4psi.......Even the 'fast road' Facet pumps are rated between 4 and 5.5 psi (which means they are especially stable), and strictly speaking thats too much for a weber.................

I would always go for a higher rated pump and fit a reg...that way, you can 100% guarantee you are getting the correct pressure at all times.........

Paul.

britishtrident - 21/2/08 at 07:36 AM

Normal road Facet pump output of 2.5 to 3.0 psi.


Fitting a Fast Road or Competition pump won't make the car go faster. If more flow rate is needed a bigger pump and or fuel lines is the answer not a higher pressure pump.

Most fuel starvation issues arise because of problems on suction side not the high pressure side of the pump.

Jenko - 21/2/08 at 10:20 AM

Of course more pressure does not make you go faster!......

Read any weber spec, and it shows that around 3.5 - 4psi is the optimum Fuel pressure......Anything over this and it 'CAN' effect the needle valve and float operation.#

So, the specs are below...Unless said person has the smallest rated pump, then there is a possibility of over pressure........By fitting a pump giving slightly higher pressure than required (and regulator), you fully cover yourself for any fuel requirement, plus, if the engine is ever upgraded, you don't need to worry about fuel.....

Spec Road Road Fast Road Competition
Pressure2.0-4.0 3.0-4.5 4.5-7.0 7.0-10.0

Clearly anything above fast road must have a reg...

Stu16v - 21/2/08 at 11:12 PM

Plus, whilst the pressure of the smallest pump will probably be more than adequate speeds and on a rolling road, it is quite possible that fuel starvation might manifest itself if the car is capable of accelerating hard (which is exactly what seveneques are all about). Mainly because the fuel pump is now trying to push a 'column' of fuel against the acceleration.
IMHO that is why it is best to have a pump of slightly higher pressure than required, and mount the regulator as close too, or better still in front of the carbs