Lotus Elan and Lotus Elan Plus 2 Forums

Craven wrote: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.

The float chambers in the carb are there to provide a constant head of fuel to the jets. While the chambers may also act as reservoirs, they don't hold much fuel (the floats take up a lot of space in the chamber) and any reduction of fuel height because demand has outstripped supply will result in a rapid weakening of the mixture.

Not an issue if you are pootling to the shops and back, but with Rohan's heavy right foot on the straight at Albert Park (if that is where he races) then fuel starvation / engine damage is a real possibility.

The issue with Webers is the needle valve can only close against a relatively low pressure - I recall 3.5psi is the max recommended pressure or the carb will flood. This doesn't give you much headroom to play with given the effects listed above.

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.

hmm, don't want to split hairs but the typical condition when maximal flow is required for a extended period of time is not the same as the conditions where maximum acceleration (defavorable case) takes place I would think ... plus if the boot regulator is set at 0.5psi it is clearly set too low to begin with. Yet I agree, it makes more sense to me to have the regulator in the engine bay for several reasons (including bypassing the pressure losses inline)

For corner of the enveloppe calculations, there is a correspondance between power and fuel consumption, so one may use that to pick the right pump for his engine, including the safety factor of his choosing.

nmauduit wrote:hmm, don't want to split hairs but the typical condition when maximal flow is required for a extended period of time is not the same as the conditions where maximum acceleration (defavorable case) takes place I would think ... plus if the boot regulator is set at 0.5psi it is clearly set too low to begin with. Yet I agree, it makes more sense to me to have the regulator in the engine bay for several reasons (including bypassing the pressure losses inline)

For corner of the enveloppe calculations, there is a correspondance between power and fuel consumption, so one may use that to pick the right pump for his engine, including the safety factor of his choosing.

Fair point. My calculations were ball park at best, but I may not have been clear about the pressure drop due to acceleration. My 0.5 psi at 1g was per foot, the Elan has an 8ft wheelbase, so lets say it is 8ft from the pump in the boot to the front carb. At 1g that could be a 4 psi drop, so it would be possible that under these conditions there would be no fuel flow at all.

It is unlikely that you would get anywhere near 1g acceleration in the Elan, but my point was to indicate that this is a real effect to be considered - particularly if you are racing.

Using an online calculator, it would require the car to do 0-60mph in about 2.75 seconds to achieve 1g acceleration. The Elan is about a 7 second 0-60 car, stock, so something like .39g

But I'm thinking the fuel in the tank and lines is accelerated to vehicle speed in a very short amount of time, so any pressure change would be so brief as to be undetectable.

And Elans have 7' wheelbases, btw.

My Elan will get to 60 mph in about 4.5 seconds. On a long straight the acceleration steadily slows as the car goes faster up through the gears. However even at 0.1g acceleration the pressure loss in the line due to acceleration is about 1/2 a PSI which when combined with fuel line losses would start to limit flow if you only had a low pressure pump in the boot.

This pressure drop is continuous as long as you are accelerating as the fuel in the line is being accelerated with the car and the effective static head of fuel in the line causes the pressure drop continuously under acceleration. Under hard braking you have the reverse and G force will be close to 1 G so the pressure increase will be around 4 PSI but that tends to be only for short periods so the increase of supply pressure above what's needed may not be noticed but it may cause over filling of the fuel bowls and an engine hesitation as you accelerate out of a corner with engine going over rich

The Pegasus Racing web site in its Facet pumps section says an engine needs 1 US gallon per hour for every 10 HP produced ( around 2.6 HP produced for every Litre per hour) That says I need for my Elan and 180 hp around 70 litres per hour which seems low compared to my actual average track use of 60 litres per hour which includes significant time off the throttle ? Whatever is required the pump I have keeps up !

cheers
Rohan

bitsobrits wrote:
And Elans have 7' wheelbases, btw.

You are quite correct, I was looking at a Plus2 chassis drawing where the wheelbase is 8ft.

My maths was only a ballpark guess, I am sure there are other mistakes there as well. My point was to illustrate why having a higher pressure pump in the boot and fuel regulator near the carbs was a good idea.

Andy.