Lotus Elan

Ingintion closing angle

PostPost by: elan+2s » Wed May 27, 2015 12:16 pm

Hello,

I try to set precise ignition at my Elan +2 ( TC engine with Stromberg carburetors ) . To adjust the contact breaker I find in the repair instructions only a contact separation . From my other vehicles I know it so that the contact distance is only a clue to start the engine . For the precise adjustment of the breaker contact a closing angle is specified .

Is there such an indication the Twincam engine also ?
Can anyone tell the value?

Thanks and Greetings
Egbert
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PostPost by: billwill » Wed May 27, 2015 12:23 pm

Hi Egbert,

I think what you are describing is called the Dwell angle.

As far as I can tell it is directly related to the shape of the cam on the distributor shaft, so any particular dwell angle will have a particular contact gap at the widest point. So either method can be used to define the setting for any specific distributor.

Here is an article about Dwell angle and how to measure it.
http://www.howacarworks.com/ignition-sy ... well-angle

Miles Wilkins book on the Twin Cam engine on page 173 gives a specification of dwell angle as 60 degress plus or minus 3 and a points gap of .35 to .40 mm
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PostPost by: elan+2s » Wed May 27, 2015 12:54 pm

Hi Bill,
thanks for the quick response and apologize my bad english. It is correct, I meant the dwell angel.

I will set the dwell angle to 60 ?.

Egbert
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PostPost by: billwill » Wed May 27, 2015 1:29 pm

Notes:

The spark occurs when the points open. So the points must close before that in sufficient time to ensure that the magnetic field in the spark coil has achieved full strength. because that is the source of the spark energy.

But, with a simple distributor, the TIME for which the points are closed depends on the engine speed, at low RPM the closed time is much longer than it is at maximum engine RPM. So the Dwell angle has to be designed so that the contacts are closed for adequate time at the full speed of the engine. And on a 4 cylinder car only a quarter of a revolution of the distributor shaft is available for each spark plug.

We can calculate the closed time for a Twin Cam engine using Miles figure of 60 degrees.

Engine speed max 6500 RPM
therefore distributor speed 3250 RPM
divide by 60 to get 54.2 revs per second
invert to get seconds per rev = 0.0185 seconds
divide by 4 for 4 cylinders = 0.00461 seconds available for each spark plug
A quarter revolution is 90 degrees out of the full 360 degrees, so a
dwell angle of 60 degrees is 2/3 of the available time
So the contact-closed time is 2/3 of 0.00461 seconds which is 0.00308 seconds or 3.08 milliseconds

Doing the same calculation for an engine speed of 1000 RPM gives approx 20 milliseconds

So anyway, we have to presume that the ignition coil and distributor of the Twin-Cam have been chosen(designed) so that the coil will achieve full magnetic field in less than 3 milliseconds

If the points gap is too wide it implies that the points close too late and open too soon, reducing the dwell angle and reducing the contact-closed time. If the time falls below the charge time of the coil you will get weak sparks at maximum revs of the engine, but no bad effects at lower speeds.

~~~~~~~~~~~~~

Electronic ignition systems (other than the very simplest ones) are usually designed so that the electronics govern the coil current, so they are able to create a fixed TIME duration for the coil to develop its field. i.e fixed width pulses which get closer together as the engine speed increases. At some some speed (presumably above the maximum expected RPM) the pulses merge into one continuous 'pulse' so you won't get any sparks; the engine cannot run above this speed.
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PostPost by: billwill » Wed May 27, 2015 1:31 pm

Don't worry about your English. It is better than my German language; though I spent 7 years total in Germany in my youth.
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PostPost by: ricarbo » Wed May 27, 2015 4:00 pm

There is a problem with a worn distributor. If you set the gap correctly, you will get the wrong dwell angle. When the distributor is worn, you will have to set the gap smaller.
regards
Richard
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