Subject: Re: [Healeys] Dwell angle for standard BN1 100/4

From: David Porter <[email-masked]>
Date: Thu, 23 Oct 2014 14:29:42 -0600

38 degrees
On 10/23/2014 1:07 PM, STEPHEN CUSS wrote:
> Does anybody know what the standard dwell angle should be for a unmodified BN
> 1
>
> Thanks
>
> Stephen
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-- 
Dave Porter Porter Custom Bicycles 2909 Arno St. NE Albuquerque, NM 
87107 505-352-1378 Go HERE: my world www.porterbikes.com/
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From: Michael Salter <[email-masked]>
Date: Thu, 23 Oct 2014 16:57:10 -0400

Don't like to disagree with Dave but I think the BN1 has a Lucas 40321A
Model DM2P4 distributor which has CCW rotation and uses a standard "High
Lift Cam", 0.015" contact breaker gap which will have a closed period
(dwell) of 60 degrees + or - 3 degrees.

Michael S
BN1 #174

On Thu, Oct 23, 2014 at 3:07 PM, STEPHEN CUSS <[email-masked]
> wrote:

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From: Oudesluys <[email-masked]>
Date: Thu, 23 Oct 2014 23:01:33 +0200

The dwell angle on a 4-cyl. engine is usually around 600.
Kees Oudesluijs

STEPHEN CUSS schreef op 23-10-2014 21:07:
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From: "Team.net" <[email-masked]>
Date: Thu, 23 Oct 2014 19:24:52 -0400

600?

> On Oct 23, 2014, at 5:01 PM, Oudesluys <[email-masked]> wrote:
>
chello.nl
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From: David Porter <[email-masked]>
Date: Thu, 23 Oct 2014 18:15:45 -0600

...what I posted is the cam angle.. This can vary a bit depending on 
what cam is fitted.. If Healey (for instance) used a "high lift cam" 
then the dwell would be closer to 60 degrees.
  Since the cam is what ever is fitted, then one must set the gap. The 
wider the gap, the less the dwell and vice versa..
  As a rule I tend to set the gap on the wide side to compensate for 
worn cams and/or bushings.. The number I posted is from the Lucas book..
dave
... on the other hand, using an old fashioned dwell meter, the number that c
comes to mind is in the 52 neighborhood.. but haven't used one for years..

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From: Oudesluys <[email-masked]>
Date: Fri, 24 Oct 2014 09:02:09 +0200

Apparently the sign for degrees turns into a 0 in firefox. So it should 
of course read 60 degrees.
Kees Oudesluijs

Team.net schreef op 24-10-2014 1:24:
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From: David Porter <[email-masked]>
Date: Fri, 24 Oct 2014 07:07:07 -0600

oh geeze.. hope this doesn't turn into a 'what oil' or 'what tires' 
thing.. anyway it has made me think and start reading (easy with 
computers these days). problem is separating machinist think from 
mechanic think, I think. ;~)
  Any way here is a short explanation for all of us directly from the 
Internet I found this morning:

When the distributor shaft is rotating, the contact-breaker points open 
as the heel of the moving point is pushed outwards by a lobe of the cam, 
and close while it is over the flat area between two lobes.

If, for example, the angle of rotation between the centres of the lobes 
on the cam is 90 degrees, the dwell angle - the period with the arm over 
the flats and the points closed - may be 52 degrees; the remaining 38 
degrees are taken up by the action of opening and closing. This would be 
a typical dwell angle for a four-cylinder engine 
<http://www.howacarworks.com/basics/the-engine>.

A dwell meter connected between the distributor or ignition coil and 
earth registers the dwell angle on a scale, and must remain steady at 
the prescribed figure while the engine is running. If the dwell angle is 
not the same for all cylinders, the result is rough running and poor 
fuel economy because the moment the spark-plug fires varies from 
cylinder to cylinder.

The dwell angle varies according to the make of car; check it in a 
service manual. There are two scales on the meter, one for four-cylinder 
engines and one for six-cylinder engines. Eight-cylinder vehicles are 
taken from the four-cylinder scale and halved.

  End of article... I liked this one best as they use the numbers I 
posted yesterday that let me validate (personally) that I haven't 
completely lost my mind yet..:-)
   BTY, I don't ever disagree with Michael or Richard, Kees sometimes..
dp
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From: David Porter <[email-masked]>
Date: Fri, 24 Oct 2014 07:17:12 -0600

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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From: Bob Spidell <[email-masked]>
Date: Fri, 24 Oct 2014 13:54:25 +0000 (UTC)

Well, the 'solution' is obvious: you just have to measure dwell at specific lift, like camshafts (usually 0.050" lift). 

Half kidding. 

Bob 

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From: Bob Spidell <[email-masked]>
Date: Fri, 24 Oct 2014 14:19:17 +0000 (UTC)

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 

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From: Oudesluys <[email-masked]>
Date: Fri, 24 Oct 2014 17:15:08 +0200

It seems that there are two dwell angles for 4-cyl. cars. ca. 60 degrees 
and ca. 52 degrees. 60 degrees is used most on EU cars with high lift 
cams in the distributor (whatever that means). On the Lucas 23/25D4 (on 
my cars) and 43/45D4 distributors it is 60 degrees.

Just as well David does not always agree with me. I do not pretend to be 
always right. I am only human. However there is often room for 
discussion and the truth can be both ways.

Cheers,
Kees Oudesluijs

David Porter schreef op 24-10-2014 15:07:
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From: Oudesluys <[email-masked]>
Date: Fri, 24 Oct 2014 17:23:46 +0200

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

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From: Michael Salter <[email-masked]>
Date: Fri, 24 Oct 2014 19:04:58 -0400

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 <[email-masked]> wrote:

>> 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
>>
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