Mechanical or Electric fuel pump for RV8 in a Cobra

[martin_t]

Hi

I have a 3.5 Rover engine in a GD Cobra kit. I am using a Holley 390 and have not started the engine yet!!! Can anyone advise me if I should get a mechanical or electrical fuel pump? I have heard pro's and cons for Electric but not much about mechanical. Also if the concensus is Mechanical, where can I get one from and which would be best? I can find lots of electrical ones for sale but no mechanical.
I understand the Mechanical can be mounted in the engine bay and is no where near as noisy as the electrical.......

Advice appreciated guys

Martin

[ChrisJC]

Am I right in thinking that the mechanical pump is a camshaft driven pump? If so, only the very old engines had provision for that, P5 & P6 engines. Not even sure if it was on SD1? It needs the right timing cover and eccentric spacer on the end of the camshaft.

Electric is easier, and they're not that noisy, (well, EFi pumps aren't)

Chris.

[jefferybond]

Mechanical pumps actually bolt to the engine, on the timing cover. Your engine might not even have a suitable hole in the cover to fit one, so you might not have a choice.

I've fitted a Facet red top electric pump to mine, and it's been fine for several years. It's pretty quiet, provided you mount it on rubber bobbins, and make sure it's below the tank so that it gets a constant fuel supply and doesn't have to suck.

Jeff

[sidecar]

Mechanical pumps actually bolt to the engine, on the timing cover. Your engine might not even have a suitable hole in the cover to fit one, so you might not have a choice.

I've fitted a Facet red top electric pump to mine, and it's been fine for several years. It's pretty quiet, provided you mount it on rubber bobbins, and make sure it's below the tank so that it gets a constant fuel supply and doesn't have to suck.

Jeff

Hi Jeff,

Martin has got a Cob replica (like me) noisy fuel pumps just aint an issue!!!

Martin, I'd just go for an eletric one they deliver the stated G/h regardless of the engine RPM.

Mine is setup with a switch so that it only pumps if there is oil pressure, I've a small push button that is an overide to prime the carb if the car has been sat for a day or so. I think that jeff has a good setup where the pressure switch is overidden if the starter motor is cranking.

It is worth doing one or the other, my car will nearly kill the battery before it starts if I don't prime the carb! You can not bother with any of this and just have the pump pumping all the time but that could be a bit dodgy if you crash.

Last thing, try and get a pump that has a high enough G/h rating but is still a low enough pressure so that you don't need a regulator, it just one less think to go wrong!

Pete

[martin_t]

Sorry guys, I'm sure its obvious buts is G/H gallons per hour? Also what pressure should I be looking for on an RV8? I guess if I put a regulator in I will need it fairly low volume low pressure for an RV8? Anyone got one and done it?

Thanks

Martin

[Darkspeed]

Get a Malpasi filter king filter regulator and a Facet red top pump kit, that will meet all your needs - it's the carb that the pump pressure is set for and power output that the flow rate is set for. Get the Filter with the gauge and you can have the pressure accurately set and a nice "bling" meter in the engine bay

Red top is an interuptor pump and although more expensive is much quieter than the solid state constant running ones. and will be good for up to 200BHP IIRC and Holleys are 3-4PSI I think but do check.

Cheers

Andrew

[sidecar]

Sorry guys, I'm sure its obvious buts is G/H gallons per hour? Also what pressure should I be looking for on an RV8? I guess if I put a regulator in I will need it fairly low volume low pressure for an RV8? Anyone got one and done it?

Thanks

Martin

Yes, G/H is gallons per hour

I have a book with a formula for working out how much fuel your lump needs, I think that it was 4.6xBHP=CC of fuel required per minute. (CC being cubic centimetres and BHP being best guess I suppose!).

Heres some more guess work:-

BHP =230 (if you've got a tuned 3.5)

CC= 4.6x230=1058cc (or 1.058 litres per minute)

so 1.058x60=63.48 litres per hour.

63.48/4.5=14 UK gallons per hour.

14 UK Gallons = 16 US gallons/hour (pumps are rated in US gallons).


This seems a bit low to me, I'll have to check my book when I get home! either way this is the absolute minimum that your pump must be able to flow. (Not free flow, but this must be the flow passed the float valves into the carb, or at least out of the end of the fuel pipe that connects to the carb)

As to the PSI it is usually something quite low for the holley like 3-5 psi but don't quote me on that.

Having a pump with way too much G/H won't be a problem, but too much PSI will deffinately be a problem.


Before anyone has a "go" about the 4.6 figure (if that is the figure in the book) this method can be used because all four stroke engines fall into a narrow band of efficiency in terms of turning the chemical energy in petrol into mechanical energy. The crappest engines are round 20% efficient whilst the best are only around 25% efficient! The other 75% is wasted.


Anyway I'd go for 25-30 gallons per hour to be safe!!

Regards,

Pete