[TR] TR4 - Dwell v Points Gap

[Michael Porter]

Jeff Scarbrough wrote:
> At 01:46 PM 6/2/2008, Jim Muller wrote:
>   
>> If it just measures on-time or off-time and
>> normalizes that against a firing interval then it would depend on how
>> it "knew" the firing interval.
>>     
>
> A dwell meter needs to know how many cylinders there are, either by 
> switch or by scale.  My Mac pro meter has separate scales for 4, 6 
> and 8 cylinders.
>
> The dwell angle is the amount of distributor rotation ~per cylinder~ 
> where the points are closed.  Typically, that number should be half 
> of the total rotation per cylinder.  Bentley sez for TR6, dwell is 
> 35*, open angle is 25* for a total of 60*.  60 x 6 = 360.  For the 
> Spitfire, dwell is 51*, open angle is 39* for a total of 90*  90 x 4 = 360.
>
> That's why I said that the spec of 60* dwell was suspicious...that 
> only leaves 30* or less for an open angle where the coil 
> charges.  Doesn't sound like enough to me...
>
>
>   

Umm, the distributor turns at one-half crankshaft speed, so it takes 720 
deg. of crank travel to complete one full round of the distributor. 
Measurements based on 360 deg. are in dist. degrees, not crank degrees, 
just for clarification.  Further, the description of dwell open and 
closed angles is backwards. The coil doesn't charge during the open 
period, it discharges.  When the points are closed (dwell), current is 
flowing through the coil.  More dwell than open is required to saturate 
the coil, as the figures suggest.  The actual time of discharge is quite 
a bit less than the 25-30 degrees that the points are open.  The points 
are kept open for that period to help them and the coil to cool.  So, 
it's a balance between how long it takes to saturate the coil (during 
which the points and the coil are heating up because current is passing 
them) and how long to cool them off. 

The ideal, of course, is to have as much dwell as possible to ensure 
coil saturation, without damaging the coil or the rest of the system. 
That's why many electronic ignitions today will pump dwell up to 70-75 
deg. on starting to make sure cold plugs fire, then drops it down to 
55-60 deg. or so.


Cheers.

-- 


Michael Porter
Roswell, NM


Never let anyone drive you crazy when you know it's within walking distance....