Lotus Elan

Electric fuel pump

PostPost by: rgh0 » Tue Jan 05, 2021 10:51 am

I had problems with the engine misfiring at the end of long straights when at full throttle and maximum revs and maximum power for a significant period. Dyno work and use of a similar pressure gauge attachment showed it was due to the Facet low pressure cylindrical pump and standard fuel line failing to delivery the volume at enough pressure to match the well developed engines consumption after its most recent changes a few years ago when I went from Avgas to unleaded pump fuel due to a regulation change for my class. Probably not a problem on a less developed engine but another data point if you're pushing the top end of twin cam power

Based on my Dyno guys recommendation ( he really knows his Weber's). He demonstrated the change during the dyno runs by hooking up his fuel supply trolley using his recommended set up to replace mine as he comes across this issue often. Based on this I installed a new larger fuel line and a Carter 4389 fuel pump with 6 psi output and a Malpassi "petrol king" regulator to control the pressure at the Carbs to the 2.5 to 3 PSI target at all flows. Probably overkill but all the problems went away once this was done :D

Carter P 4389 pump - long link but you can find it on Amazon

https://www.amazon.com.au/Carter-P4389- ... XAMW3Y79Y9

Malpassi petrol king regulator - again a long link but you will find on Ebay

https://www.ebay.com.au/i/402556886567? ... 2abd7b8a46

photos of my installation below

IMG_9639(1).jpg and


IMG_9643(1).jpg and


cheers
Rohan
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PostPost by: alan.barker » Tue Jan 05, 2021 11:15 am

Hi Rohan,
so what Flow Rate are we talking about. Would 130 Litres be enough.
Alan
Sorry just looked 72 Gallons per hour. I take it that's American Gallons so 1 US Gallon is 4 Litres.
So that's 288 Litres/hour, that's a big Flow rate. Like you say maybe overkill :shock:
Last edited by alan.barker on Tue Jan 05, 2021 11:21 am, edited 1 time in total.
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PostPost by: 2cams70 » Tue Jan 05, 2021 11:18 am

I was thinking about using one of those Huco pumps too. How is the pressure regulated though? To me it always seems a bit unnecessary having two pressure regulators - one in the pump where the pump has a higher pressure rating than what is required and another regulating it down further to what you actually require. I'm sure Colin is looking down from above the clouds and nodding his head! If the pump itself has a high enough maximum flow rate and also the correct pressure rating that should be all that is required.
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PostPost by: rgh0 » Tue Jan 05, 2021 11:20 am

I think the pump is rated at 62 US gallons per hour free flow ( about 220 l/h). I use about a litre per minute or 60 litres per hour on the track on average so maybe 2 to 3 times that at peak consumption

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PostPost by: alan.barker » Tue Jan 05, 2021 11:23 am

So at 60 Litres/hour the "Huco" at 130 Litres/hour is enough.
Alan
info 1 US Gallon is 3.8 Litres approx
1 UK Gallon 4.5 Litres approx.
Last edited by alan.barker on Tue Jan 05, 2021 11:28 am, edited 1 time in total.
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PostPost by: innesw » Tue Jan 05, 2021 11:27 am

alan.barker wrote:So at 60 Litres/hour the "Huco" at 130 Litres/hour is enough.
Alan
info 1 US Gallon is 4 Litres.
1 UK Gallon 4.5 Litres approx.



Not if you occassionally need peak flow of 180 l/hr as Rohan suggests above. (Wide open throttle, long straight)
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PostPost by: rgh0 » Tue Jan 05, 2021 11:31 am

2cams70 wrote:I was thinking about using one of those Huco pumps too. How is the pressure regulated though? To me it always seems a bit unnecessary having two pressure regulators - one in the pump where the pump has a higher pressure rating than what is required and another regulating it down further to what you actually require. I'm sure Colin is looking down from above the clouds and nodding his head! If the pump itself has a high enough maximum flow rate and also the correct pressure rating that should be all that is required.



The issue is the pressure drops at the pump discharge and through the fuel line to the carbs as the flow rate increases. The Carter pump is a vane pump and its output delivery is regulated to 6 PSI over a range of flows but like all mechanical simple proportional regulators you have "droop" so the pressure the pump actually supplies drops as flow increases. The same thing occurs with the pressure regulator in the engine bay but it also compensates for the drop in pressure from the pump and pressure drop through the fuel line as flow increases so in the end you get a good consistent pressure at the carb over the full fuel flow range.

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PostPost by: alan.barker » Tue Jan 05, 2021 11:33 am

innesw wrote:
alan.barker wrote:So at 60 Litres/hour the "Huco" at 130 Litres/hour is enough.
Alan
info 1 US Gallon is 4 Litres.
1 UK Gallon 4.5 Litres approx.



Not if you occassionally need peak flow of 180 l/hr as Rohan suggests above. (Wide open throttle, long straight)

Sorry but i can't find that 180 L/Hr. Where did he say that :?
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PostPost by: pharriso » Tue Jan 05, 2021 11:34 am

rgh0 wrote:I think the pump is rated at 62 US gallons per hour free flow ( about 220 l/h). I use about a litre per minute or 60 litres per hour on the track on average so maybe 2 to 3 times that at peak consumption

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PostPost by: Andy8421 » Tue Jan 05, 2021 11:37 am

2cams70 wrote:I was thinking about using one of those Huco pumps too. How is the pressure regulated though? To me it always seems a bit unnecessary having two pressure regulators - one in the pump where the pump has a higher pressure rating than what is required and another regulating it down further to what you actually require. I'm sure Colin is looking down from above the clouds and nodding his head! If the pump itself has a high enough maximum flow rate and also the correct pressure rating that should be all that is required.


If the pump is in the boot, then there is a column of fuel in the pipe that needs to be accelerated and decelerated as the car brakes and accelerates.

My maths may be adrift, but as a quick back of the envelope calculation:

Lets say that the car can manage 1g of acceleration and braking force (I have no idea what the Elan can do, but there are road cars than can achieve this) then that is equivalent to standing the fuel pipe upright. 1 psi = 27" of water column, so again assuming water and fuel have the same density (they don't) then every 1ft of fuel pipe length adds or subtracts 0.5psi of fuel pressure at 1g. Given the relatively low pressure of the supply to the Webers, if the pressure regulator was in the boot, it would be possible to have the carbs starve under acceleration, and flood under braking.

Having a higher pressure pump in the boot, but regulating the pressure close to the carbs avoids this problem.

edit: - just read Rohan's contribution - which is of course correct. The two effects would be additive.
Last edited by Andy8421 on Tue Jan 05, 2021 11:41 am, edited 1 time in total.
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PostPost by: alan.barker » Tue Jan 05, 2021 11:40 am

Thanks Phil,
ok 2 or 3 times at peak.
Sorry boys i was reading faster than my brain could take it in :oops:
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PostPost by: rgh0 » Tue Jan 05, 2021 11:44 am

Andy8421 wrote:
2cams70 wrote:I was thinking about using one of those Huco pumps too. How is the pressure regulated though? To me it always seems a bit unnecessary having two pressure regulators - one in the pump where the pump has a higher pressure rating than what is required and another regulating it down further to what you actually require. I'm sure Colin is looking down from above the clouds and nodding his head! If the pump itself has a high enough maximum flow rate and also the correct pressure rating that should be all that is required.


If the pump is in the boot, then there is a column of fuel in the pipe that needs to be accelerated and decelerated as the car brakes and accelerates.

My maths may be adrift, but as a quick back of the envelope calculation:

Lets say that the car can manage 1g of acceleration and braking force (I have no idea what the Elan can do, but there are road cars than can achieve this) then that is equivalent to standing the fuel pipe upright. 1 psi = 27" of water column, so again assuming water and fuel have the same density (they don't) then every 1ft of fuel pipe length adds or subtracts 0.5psi of fuel pressure at 1g. Given the relatively low pressure of the supply to the Webers, if the pressure regulator was in the boot, it would be possible to have the carbs starve under acceleration, and flood under braking.

Having a higher pressure pump in the boot, but regulating the pressure close to the carbs avoids this problem.

edit: - just read Rohan's contribution - which is of course correct. The two effects would be additive.



I had never thought about this but yes the acceleration / deceleration also affects the pressure at the carbs with a rear tank mounted pump and a regulator in the engine bay counteracts that also.

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PostPost by: alan.barker » Tue Jan 05, 2021 11:45 am

Cheers Andy,
that's what i've got on my 1976 TVR 3000M.
High pressure "Huco" Pump in Boot and Regulator just before the Carb in Engine Compartment.
Alan
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PostPost by: 2cams70 » Tue Jan 05, 2021 12:03 pm

Andy8421 wrote:
2cams70 wrote:I was thinking about using one of those Huco pumps too. How is the pressure regulated though? To me it always seems a bit unnecessary having two pressure regulators - one in the pump where the pump has a higher pressure rating than what is required and another regulating it down further to what you actually require. I'm sure Colin is looking down from above the clouds and nodding his head! If the pump itself has a high enough maximum flow rate and also the correct pressure rating that should be all that is required.


If the pump is in the boot, then there is a column of fuel in the pipe that needs to be accelerated and decelerated as the car brakes and accelerates.

My maths may be adrift, but as a quick back of the envelope calculation:

Lets say that the car can manage 1g of acceleration and braking force (I have no idea what the Elan can do, but there are road cars than can achieve this) then that is equivalent to standing the fuel pipe upright. 1 psi = 27" of water column, so again assuming water and fuel have the same density (they don't) then every 1ft of fuel pipe length adds or subtracts 0.5psi of fuel pressure at 1g. Given the relatively low pressure of the supply to the Webers, if the pressure regulator was in the boot, it would be possible to have the carbs starve under acceleration, and flood under braking.

Having a higher pressure pump in the boot, but regulating the pressure close to the carbs avoids this problem.

edit: - just read Rohan's contribution - which is of course correct. The two effects would be additive.


Building on that logic the best system would appear to be the same as that typically used on EFI cars even for carbs. - i.e an unregulated pump with pressure regulator in the engine bay and a return pipe from the pressure regulator back to the tank for fuel that is excess to the engine's requirements.
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PostPost by: Craven » Tue Jan 05, 2021 12:07 pm

I would have thought the fuel reservoir in the carbs would more than offset any effect of slight pressure variation. Surely its purpose is fuel stability.
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