Dwell meter recommendation

[71 V12]

I am still running on a points ignition system as I find it utterly reliable and requiring very little maintenance. I usually set the points gap with feeler gauges but I can see a more accurate setting would be achieved with the dwell accurately measured.

Is anyone on the forum using a dwell meter? If so which brand and from where?

Cheers 

Kevin

[John Morrison]

Yep, an old Gunsons one, a basically idiot proof system to get the gap, ,Spot on,!

John.

[71 V12]

Thanks John, but it looks like Gunsons stopped making that meter some years ago.

Any suggestions of a reliable currently available?

Many thanks

Kevin

[Steves_TR6]

I used one of the gunsons testers when i first tinkered with cars back in the 1980s!

these days sealey seem to be the make to go to for automotive testors, they have quite a range including this :

 

 

[71 V12]

Hi Steve,

That Sealey analyser looks like an excellent option - many thanks

[Eddie Cairns]

I have yet to see any power or fuel consumption increase from any tiny difference in dwell angle setting.

Can anyone post a link to a third party independent test what shows any improvement brought about by such minor dwell angle changes.

Assuming the distributor bearings are in good condition, the rubbing surfaces of the rotor is not worn and the points heel in in good shape I have never found any issues here.

[RobH]

In can't link to any test Eddie but there is a reason you have seen little difference.

Provided the minimum dwell time is sufficient to establish magnetic saturation of the coil core, there will be full spark energy available. If the coil doesn't become fully saturated the spark energy will be reduced. It's only going to make a difference at high revs so for normal driving it is unlikely to be reflected in mpg, though it could possibly affect peak power. 

At 6000 rpm on a 4 cylinder engine the dizzy is rotating at 3000rpm so there are 20 milliseconds per revolution, and for each cylinder there is 5 mS available. Each lobe takes up 90 degrees of rotation so a dwell angle of 50 degrees would be 5/9ths of 5ms = about 2.8mS which is the time available for the coil to 'charge'.  The strength of the magnetic field is ampere-turns and this relates directly to the spark energy.  Obviously at lower revs there is ample time for the core to be fully saturated.

Charging current v time  is not linear but follows a curve which is asymptotic to the value set by the DC resistance of the winding.  Minor variations in dwell time as shown by the double arrow will make small differences in the current because they affect only the flatter portion of the curve, so at max revs a 10% shorter dwell will make only a much smaller percentage change in spark energy. Whether that is reflected directly in engine power will depend on the characteristics of the engine.  (don't take any notice of the numbers on the graph, they are for illustration only).

[Waldi]

Thanks for this Rob.I learnt something today.

Waldi

[Steves_TR6]

Excellent explanation Rob, thanks !

additionally , once the coil is saturated the remaining time that the points are closed creates heat in the coil instead of useful spark energy.

hence at low revs the coil gets hot, and if you use a coil with a lower impedence, ie a ballast coil on a non-ballast system, then it gets extra hot and fails prematurely

some electronic ignition systems vary the dwell angle with revs, and this is why.

steve

[Andy303]

Try looking on Ebay. There are plenty of "vintage" ones for sale. Seems to be less demand for them nowadays.....

[Steves_TR6]

Here’s a ‘classic’ one :

 

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F303346513764