Mechanical v Electric fuel pump?
 

My car was fitted with an electric pump many years ago by a previous owner when the engine was replaced with a 1978 Dodge 360. I need to replace it. It had been VERY noisy and now it's failed. I had planned just buying a new electric pump and have just redone the wiring and fuel lines with that in mind. There are a bewildering array of electric pumps available and I have been trying to find the quietest that will do the job.

However, it seems that the quietest option could well be to ditch the electric and revert to mechanical pump which is, of course, how the car would have left the factory anyway.

So I am curious, why do owners fit electric pumps to replace the mechanical ones unless they have greatly uprated the power of the engine and need higher fuel flows or PSI? Is there some other benefit that I am not aware of?

David

The main advantage of an electric fuel pump is that you don't need to churn the engine over to fill the carb when the car hasn't been used for a while.

Ah, yes. I hadn't thought of that.

David

I am away from the 3-4 months of the year. So this may be an issue but I suppose I can just disconnect the coil HT lead and turn the engine over on the starter to fill the carb float chambers then reattach the coil.

David

I don't think that would really help anything. The main problems with turning the engine over cold until fuel gets to the carb is wear on the starter and possible minimal cylinder lubrication.

On further reflection it isn't just after long periods of standing. It gets fairly hot under the hood and fuel evaporates when the engine is shut down. An electric pump will also help with hot starts. I think the Filton engineers must have had good reason to replace the mechanical pump with the SU.

There are many opinions on the electric pumps. Several have become infuriated with the SU and gone to Holley, Mallory etc. The latter have good output and are reliable but have been criticised for being noisy and may generate more pressure than you need, lifting the needle valve and causing a rich mixture.

The main problem with the SU was its mechanical points. The AUF404 was double ended and had twice as much to go wrong. The later, solid state, switching seemed much better and was a fairly easy retro-fit. I modified my pumps in this way and was very happy with the result. New pumps with this type of switching are available in the UK for about £300.

If you do go for a racing style pump you need to make sure it is mounted on a suitably compliant base with flexible hoses so vibration isn't transferred to the car.

Mechanical vs electric fuel pump
 

I fitted a red top Facet to my 407 20 years ago, it failed this year after 56,000 miles. I fitted a one immediately.
The Facet came with a pressure regulator which I soon found the most suitable pressure for my 313 engine. I mounted the pump under the off side wing near the servo. It is mounted rubber insulation bolts so the tap, tap tap noise is hardly heard on tick over. My car is still positive earth, Facet used to do both positive and negative pumps. They now only offer negative but it is a simple job to wire it up for positive earth.
It is an almost instant start as mentioned not turning over the starter motor.
I can see no reason not to fit, just makes life simple.

Good point, John. Thanks.

Thanks, David. FWIW attached is a pic of what was in the boot until I took it apart to set it the right way up! It's a Mallory 140 and supposedly very quiet. Since being turned the right way up, it's only managing 2.5PSI !!!!! I'd just put in another except it's rather expensive and overkill for this engine anyway.

I have been looking to see if I could fit a mechanical and it seems unlikely anyway. My car has its alternator sat on that side of the engine and there's less than 3" clearance between it and the blanking plate over the mechanical pump location.

Thank you, Nick. Facet are a strong candidate but my car is all wired and plumbed for the pump to go in the boot sitting marginally below the level of the fuel tank bottom. I think I'll have to stick to that setup for now (mostly because of the wiring changes I'd need to make) and will let you know what pump I install and how it sounds!

David

I have ordered a Mallory Comp 70 for the car.. I'll update after I fit it.

David

Electric Fuel pump.
 

David,
I do not see a pressure regulator on your Mallory which appears to be upside down. The Facet regulator is not their own I think it is a Purolator and could be fitted to increase / decrease your pressure.
Good luck and would like to hear how you get on.

Nick

LOL - yes, Nick, it WAS upside down. That's the way it was when I got the car and it was noisy, so I re-installed right-side up it after taking that pic. There is an external pressure regulator - it's the smaller canister you see in that picture - but with or without it, that pump is now only giving 2.5PSI and some occasional pulses of more.

David

I don’t understand why you would need any more - that’s around the same pressure as is produced by the SU pumps on the later models. What you need is volume, which is not the same as pressure. Bear in mind that the Carter Carb literally just dumps the contents of the float chambers straight into the manifold on kickdown and you’ll see what I mean.

I found the SU double ended pump that I used to replace an extremely noisy Facet to excellent. Quiet in that it was inaudible with the engine running, even at tickover, but just loud enough to give you that reassuring tick tick as you turn the ignition on. Mine did me 20 years and never gave me any trouble at all.

Thanks for your input and you may well be right, Roger, but I ordered a replacement pump anyway. The one on there should have been putting out 12PSI and 140 USgph. For what it's worth, my carb isn't the Carter - it's a Summit Racing M08600VS. Also, I tested the pump later to empty the tank. The flow from it seemed rather pathetic but I did not measure it except that it took over 5 minutes to fill a 5 US Gallon container so approx. 48 Impgph? That almost certainly would have been adequate but I decide to replace it anyway. The SU pump was tempting but a bit pricey here plus the one I have ordered physically fits the mounts of the one I have removed.

David

I agree with Roger on all points. As noted in one of my previous posts when my SU stopped working, as a temporary measure I fitted a Facet pump. It did the job but I disliked the noise it produced, despite the insulating and sound proofing I did during its installation.

David W: have you checked the petrol supply pipe to the carburettor which runs along the chassis? The location of the fuel pipe is conveniently situated for it to be trapped by an unsuspecting mechanic when jacking up the car.

Irrespective of what pressure you have in the pump, the flow will be reduced if the pipe has been squashed or deformed.

Brian

I have the car on stands at the moment, so I will take the opportunity to inspect the full length of the fuel supply pipe. Thanks.

I really hope to get this car back on its tyres by the end of the week even if I don't do all the little jobs that I had planned while it was on stands. This time of year is best for driving fun here in Florida!

David

An electric pump will keep running as long as it has power even if the engine has stopped because the fuel line has ruptured...

True but there are a couple of preventative systems: you can fit an oil pressure switch that cuts power to the pump if there's no oil pressure; and, you can install an inertia cutoff switch that cuts power if the car experiences an impact. I have the latter installed but plan adding the oil pressure switch at next oil change.

I don't think any car with an electric pump can be considered safe without at least one of those cutoffs installed.

David

That's an interesting idea but needs some refinement. If you just use a pressure switch then you will get no fuel until you crank long enough to build oil pressure. A less ideal solution than a mechanical pump.

I agree reverting to the original mechanical pump has advantages but to fit one back in my car now would be a big job and I want it on the road. I'll add it to my list of future 'improvements'.

David

I've had a Carter fitted in the boot of my 411. It makes enough noise so you can hear it fill the carb when you turn the ignition on, but once the engine is running you can't hear it. However, they seem to be unreliable and don't like being unused for long periods of time. My first one seized and just got hot - not ideal for a petrol pump, and it's replacement doesn't appear to working properly either. So I can't recommend them.

As for pressure, I don't know what the 313/318 engines need, but the 411 needs 5-5.5 PSI; I have a pressure regulator mounted inline next the carburettor unfortunately I can't tell you what flow rate is required.

As an aside, I once saw a 412 which was fitted with an electric pump which was used just to prime the carburettor before starting by pressing a button in the cabin, but still used the mechanical pump when running.

On mine when I swapped from SU to Facet I checked the specs of the carb to make sure I knew what it wanted in terms of pressure and flow. The silver top pump seemed most appropriate for my small block but a red top is probably what you need.
I bought the kit which includes new bobbins etc and a selection of springs from Facet so you can alter the pressure of the pump by unscrewing the bottom of the pump and swapping the spring over.
I had two goes at mounting it until I found a spot where it couldn't vibrate against anything.
On mine it's very quiet (inaudible when running) and I'm happy with it.
I didn't use a regulator and all seems fine.
On previous cars (not Bristols ) I've used them and mounted them in all sorts of places - so I wouldn't worry too much about whether it's below the bottom of the tank or not - they seem to cope.
Hope this is of some use!

Sorry to hear that , Kevin, especially as a Carter is what I just took delivery of and plan to install this weekend! My first two choices were back ordered so I ordered the Carter P4070 as it's pretty inexpensive and I could have it now. If it fails, I still have my first choice of a Mallory Comp 70 on order or I might even try a Carter-clone.

David

Thanks, Andrew. Supply chain issues have led me to obtain a Carter P4070 to install this weekend. If it is too noisy or fails, I have a backup on order. I do plan to mount it on rubber 'bobbins' to help reduce noise/vibration. I'll let the forum know how it goes.

David

I quite like the idea of an auxiliary low volume electric pump to prime a system with main mechanical pump. It would be easy to control with brief timer as used with diesel injectors. That said it seems to be a fix for an unlikely problem. I understand the concern of pump running and leaking fuel with engine off and ignition on but there are other safety issues I would deal with first. The most notable is lack of headrests in the early V8s but thanks to a suggestion from Giles Parsons I hope to be sorting this soon with some from an MGB.

David, I'd be interested to know how you plan to fit those MGB headrests. I have opted for an easier install that screws into the back of the seat which I believe is wood. There's a pic here http://www.bristolcars.info/forums/8...html#post10710 but they are not installed yet. I suspect the MGB ones would probably provide better resistance to a whiplashing head.

David

Still at the planning stage. I think a steel frame will need to be fabricated to transfer the load to the wooden sides. I believe Giles had the work done by a trimmer in Cornwall. I contacted him by email but had the reply that he couldn't remember the specifics of that conversion.
Knowing that the job has been done successfully I plan to get some second hand MGB seats and give it a go.

I don't see the need for a timer, unless you want it be fully automated with the ignition switch. You could just use a momentary on-off switch, which you press for a few seconds to fill the carburettor before depressing the accelerator pedal and engaging the starter motor, (ideally a reduction gear starter motor).


It may only be required when starting from cold when the car hadn't been used for a long time.

I rather fancy one of these devices: https://www.revolutionelectronics.co...Fuel_Pump.html
although I hesitate to add points of complication/potential failure.

David

When my Mallory order got cancelled, I quickly grabbed a Carter P4070 and installed that. I have a 100 micron filter between the tank and the pump. After the pump I have a Mallory deadhead regulator holding the fuel pressure to 5PSI followed by a NAPA Gold filter before the carb.. The new pump provides rock steady 5PSI through the regulator and everything seems to be working fine. The pump comes with a mounting bracket and rubber isolators to reduce pump vibration passing to the bracket. In addition, I mounted the bracket to the car using rubber 'bobbin' type mounts to further isolate it. The pump seems loud before firing up and can be heard above the idle but once above idle, the sound is lost.

I may try a mechanical pump as a project for next winter but for now the car is for driving although it needs some tuning.

David

Good news. With me there was also an element (for a while) post installation of consciously trying to see if I could hear it which doesn't help!