Lotus Elan

light pods up impact starting ability??

PostPost by: tedtaylor » Sat Jan 28, 2017 4:12 pm

Question: Do the light pods up (after sitting some time) make it hard to start the car?? In other words, does the vacuum void (having to pull the lights down automatically when engine starts) impact negatively the ability of the car to fire up??
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PostPost by: RichC » Sat Jan 28, 2017 4:25 pm

You mean u think you are getting extra air into inlet manifold thru the vacuum pipe?
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PostPost by: tedtaylor » Sat Jan 28, 2017 6:17 pm

so I guess the installed "one-way" valve prevents just what i'm suggesting then??
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PostPost by: billwill » Sun Jan 29, 2017 1:07 am

tedtaylor wrote:so I guess the installed "one-way" valve prevents just what i'm suggesting then??



Not that. The non-return valve should be permitted to suck air INTO the manifold but not let air rush back FROM the manifiold to the headlight system.


~~~



No what stops it being a starting problem is that it is a closed air system, once it has sucked the air out there is no more air there to upset the mixture. You do get a fresh set of air in the headlight system when you release the vacuum with the up/down 'switch'. that is when you lower the pods on a UK car or when you raise the pods on a USA (fail-safe) car. However this doesn't seem to upset starting to any significant extent, probably because it can only affect the front cylinder anyway and the other 3 cylinders cope with the starting.
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PostPost by: tedtaylor » Sun Jan 29, 2017 3:44 am

i followed you up until the end when you suggest that it only affects the one cylinder, so it's obvious you're referencing a Weber intake manifold which has individual throats for each cylinder, but in my Federal Stromberg setup, the headlight vacuum source is the front "two" cylinders which are paired, but furthermore, the emissions balance plenum shares that with the back two cylinders.
So i'm back to the start wondering. The federal "fail safe" car has lost its vacuum after a day or so and with the light switch off (pod normally down position), the pods are UP due to the new startup, so while cranking to start the car, I'm pulling air out of the light system as opposed to pulling air out through the carb body fed with fuel. Not until the car starts with difficulty (much cranking), does it fire off and then the light pods are pulled down.

Am i making sense or confusing everybody here?

In contrast, with a "later in the day" startup (with light pods down utilizing the existing vacuum), then upon cranking over the car, it pulls air immediately from the fuel fed carbs and fires up right away. There is no alternative open source of air (the example of a cold start days later when vacuum is gone).
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PostPost by: Spyder fan » Sun Jan 29, 2017 9:31 am

a few thoughts
is it a true vacuum in the reservoir or just a reduced pressure?

The volume in the crossmember/vacuum tank doesn't seem to be that big.

On start up, no 1 & 2 cylinder on a stromberg head car would see a weak mixture from the carburettor due to it being diluted by plain air from the vacuum tank. Can we calculate how many inlet strokes are required to supply the vacuum tank and the percentage quantity that is taken from each inlet cycle.

On my failsafe single pod system the headlamps would be raised if the car wasn't used for a few days, I noticed that the engine needed to be running before they would close, even if you cranked the engine for a long time they would stay up. Once running they would close pretty quickly. I always assumed this was down to some characteristic of the non return valve. They do seem to have quite a restricted flow.
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PostPost by: AHM » Sun Jan 29, 2017 11:55 am

Spyder fan wrote: Can we calculate how many inlet strokes are required to supply the vacuum tank and the percentage quantity that is taken from each inlet cycle.

Yes you could calculate it but you would need to know a lot of other information.

In answer to the op question - The calculation would show the partial vacuum bleed to be insignificant. Otherwise we would all have a problem and they would have changed the design.

There is a simple way of checking - block off the vac pipe and see if there is any difference.
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PostPost by: john.p.clegg » Sun Jan 29, 2017 2:14 pm

[quote="Spyder fan"]a few thoughts
is it a true vacuum in the reservoir or just a reduced pressure?

The volume in the crossmember/vacuum tank doesn't seem to be that big.


Firstly how to prepare this state in practice is another matter. There are both practical and theoretical difficulties. First the question of matter quantum fields. The key idea here is that regions of space only evacuate through diffusion: we don't have any magic means of calling up all the particles in a region and saying "hey guys, could you all please shift to one side of this chamber now?" You simply place the region of space you want to evacuate in contact with another region of space with a lower particle density and let particles freely diffuse back and forth between the two. We can prepare low particle densities through piston apparatus and the like, and one could imagine in theory using pistons with non return valves over many cycles to achieve a high probability of no matter particles in a region (somewhat like the old joke about homeopathic procedures diluting medicine to below single molecule in the bottle concentrations), but in practice this procedure is hard to make perfect. Some of the practical difficulties (outgassing and so forth) are talked about in the Wikipedia article "Ultra-high Vacuum". Moreover, even if you did get rid of all matter particles, then your chamber walls are at nonzero thermodynamic temperature, so now the inside of the chamber contains blackbody radiation. This you cannot get rid of unless the chamber walls are at truly absolute zero thermodynamic temperature.

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PostPost by: Spyder fan » Sun Jan 29, 2017 2:35 pm

Sorry I asked :lol:
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PostPost by: billwill » Sun Jan 29, 2017 3:19 pm

Well Ted, being an engineer, rather than a theoretical physicist, by now I would have tried starting with the headlight 'switch' in both positions to see if I could observe any difference and I would probably have also tried with the vacuum intake blocked off.

~~~

There ain't no such thing as absolute vacuum even in intersolar space, there's only reduced pressure. Even one hydrogen molecule per cubic mile can be considered a pressure :roll:
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