[Healeys] Dwell angle for standard BN1 100/4

Michael Salter michaelsalter at gmail.com
Fri Oct 24 17:04:58 MDT 2014


Sorry I mentioned the "High Lift Cam" issue with regard to Lucas
distributors but is an effort to pour some oil on troubled waters let me
see if I can explain.
Lucas experimented a lot with various cam profiles in their contact breaker
type distributors.
>From what I can remember from courses many many years ago this was the
sequence of events.
Initially Lucas had "Symetric" distributor cams. These were designed to
open and close points with the available points springs and rubbing blocks
of the day up to the maximum distributor RPM without "points bounce" or
significant rubbing block wear.
Points bounce is the speed at which the rubbing block fails to accurately
follow the cam and the timing becomes unreliable. (This issue was addressed
by Lucas and other manufacturers by development of twin point distributors
but that is a different subject).
 These points used a gap of 0.011" and a dwell of 45 degrees. I am pretty
sure they were only used on pre-war vehicles.
Research proved that producing an "Asymetric" distributor cam profile
provided better control of the closing points and allowed higher RPM with
the available points springs and rubbing blocks. These cams still used a
0.011" gap but the dwell angle was increased to 49 degrees
Further development of points design produced a better type of rubbing
block material which allowed stronger springs so a "High Lift" cam design
could be developed which allowed larger dwell angles and distributor RPM
and it is these cams that are used in all Austin Healey distributors. These
use a 0.015" gap and a dwell angle of 60 degrees.
Hope that helps.

Michael S
BN1 #174



On Fri, Oct 24, 2014 at 11:23 AM, Oudesluys <coudesluijs at chello.nl> wrote:

> Could it be that the hight of the cam lift has something to do with the
> speed of contact breaking and thus the strength of the spark?
> Kees Oudesluijs
>
>
> Bob Spidell schreef op 24-10-2014 16:19:
>
>> OK, I'll bite. What does distributor dwell have to do with the lift of
>> the cam? My understanding is dwell angle is basically optimized to provide
>> an optimal time period to charge the primary circuit in the coil to an
>> optimum value to produce the best spark from the secondary circuit (at
>> least, within a fairly narrow range of engine speed). Dwell, in time
>> duration, may be too long at slow speeds--which is why coils tend to heat
>> up if idling for too long--and too short at high RPMs (which is one of the
>> primary advantages of some electronic ignitions--they can keep a constant
>> time duration throughout the range of engine speeds).
>>
>> What am I missing?
>>
>> Bob
>>
>> ----- Original Message -----
>>
>>
>>
>> Damn... I am losing it.. so, now the 38 degree thing is (in typical
>> British fashion-opposite) the degrees that the points are open.
>> Finally, if anyone has an easy way to determine the difference between
>> a standard or high lift cam visually I'm all ears. Would also like to
>> know the reasons for fitting different cams... I'm out of reading time
>> for the day..
>> dave
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