Mechanical fuel pump limit

[h20hamelan]

I have fitted electric, for only one reason. With an inertia switch, in the event of an accident.
I had convinced myself of this years ago, but, I wonder what the chances would be of the engine still running and a line rupture at the same time.
Further contemplating, the wee peanut on-top the shoulders has the thought of the opposite. Or a line rupturing, and the engine and mechanical pump still running.

Both unlikely, both possible?

Maybe another good point for the electric. Is you can have the pump on or off as you wish. And run it to empty fuel if your car has been laid up for more than 6 months with this modern hydroscopic crystal forming fuel.

[promotor]

Many thanks to all for your thoughts. Looks like there is some life left in the idea of using the mechanical pump.

The car is intended just for road use, no racing.

My fuel pump has been overhauled with apparently ethanol safe diaphragm and valves. Whilst overhauling it I increased its pumping capacity by using a gasket between the pump body and diaphragm: when I tested it it does move a noticeable amount more fuel.

The pump is using the original fuel line diameter so maybe I will look to increase this if I struggle with fuel delivery at higher revs. I'm planning on using mapped ignition - a Lambda sensor would be a useful tool to know if there's any leaning out at high revs so that may be on the shopping list.

I will likely fit a pressure gauge to the fuel line somewhere just to verify that I've got the right pressure and not loosing pressure at the higher end of fuel demand.

Good thoughts on the cut-out switch for an electric fuel pump - mechanical pumps do this in a simpler fashion though! However, I will look to include something like this if I do feel that a mechanical pump isn't up to the job.

Glad I posted this here as there's plenty of good info!

[2cams70]

I’m not really sure how you are trying to increase the flow rate by installing a gasket but I could be missing something. You may conceivably be able to increase the flow rate by increasing the pump stroke but by doing so you’d be over stressing the diaphragm. The standard pump would seem to have plenty in reserve so why try and change things unless testing suggests otherwise.

When people start playing around with fuel pressures you need to be careful. The static pressure (i.e when the carburettor needle and seat is closed and flow is not happening) will be be a lot different to the dynamic pressure (i.e when the carburettor needle and seat is open and flow is happening). What is the carburettor actually doing when you take the measurement and what is the reference point you are working to? I’d suggest in the data manual for the carburettor when they quote a recommended pressure it’s the static one.

[rgh0]

Rohan, are you still running the stock fuel tank? I'd imagine the choke point would be the metal pickup
tube within the tank.

i have a fuel cell with a larger outlet yes the small orginal outlet would have been the new choke point if i still had it.

cheers
Rohan

[alanr]

Whilst overhauling it I increased its pumping capacity by using a gasket between the pump body and diaphragm: when I tested it it does move a noticeable amount more fuel.

Not sure about this at all.It seems like a bad idea to me and will just create extra issues.
Quite apart from the increased effect on the diaphragm travel and therefore reliability, what about the fuel pressure increase and it's effect on the needle valves in the carbs and if they will cope? You will almost certainly need to install a fuel pressure regulator of some sort if you do this.
I just see consequential problems....

Alan

[promotor]

Indeed, I share concerns about proceeding with caution with anything fuel related, and it is in the back of my mind to keep checking on the function of the pump in use.

While overhauling the fuel pump I wondered what could be done to increase volume as it wasn't quite as good as tests undertaken on an old tired diaphragm fitted to the same pump body : fitting the gasket did increase delivery volume. The diaphragm rod wasn't contacting the linkage correctly so wasn't moving the diaphragm until taking up "slack" in the linkage. The gasket just moved this position. It did yield an increase in volume over the original diaphragm but not to the point where the pump lever was hard to operate so I doubt I've re-invented the wheel! The gasket is only about 0.020" thick.

I haven't increased the pump stroke over stock as the jackshaft is still the same and the linkage is all the same.

Fuel pressure will be checked to make sure all is good.

That's the issue with remanufactured parts - they don't always match up to old standards.

[Dellortofan]

If you have an OE quality mech pump I'd stick with it if working up to 150bhp . Even bigger Dells like low pressure.
If going to electric you are best with a rotary style pump (pierburg or OE manufacturer) that does not sound like an ak47 with a fine filtration steel body filter before it.
it needs to be expensive then...
A lot of cars been in long term storage don't run as the electric pump has seized.