Dwell setting for a Brass Era car

[29 Chandler]

I know there is not manufacture setting for dwell on my 1914 Chandler Light Weight Six. The thread about low idle reminded me to get my old dwell/tach meter out and hook it up to my car. Currently I have it dialed in at 400 rpm.

 

My car has a Bosch DU6 magneto and the manual for that specifies a point gap of .40 mm. At this gap I have a dwell of 52. Increasing the gap to .635 mm gives me a dwell of 50. 

 

I am also running a 6V coil (came with the car) so I have been told I can run a larger gap for the points. Is there an ideal dwell setting I should be striving for?

[Bloo]

There is no standard for a brass era car. There are too many kinds of ignition systems. If your car was newer enough to have a Kettering style (points and condenser) ignition we could make a really good guess.

 

If this were a battery powered points and condenser system, with the same number of lobes as cylinders, and the shaft turning at half crank speed, you typically want a little over 50% dwell. So, in a 6 cylinder engine 60% is all there is so maybe 35% would be good. Really you want all you can get. The limit is how narrow you can gap the points without them arcing over, running badly, and burning up. Anyhow, I doubt this applies to a magneto.

 

Also if I am not mistaken a DU-6 is a high tension magneto, so probably doesn't use a separate coil. What is the 6v coil for? Starting? Is this dual ignition or something? A magneto generates the voltage itself, so there is no battery sitting there that you can get the maximum "on" time (dwell) from. If there was, going wider is the wrong way. Whole systems involving dual points and sometimes two coils came in later years with the intention of getting more dwell. You can only set the gap so narrow before you have to do something radical to get any more dwell.

 

50 sounds impossible, are you sure you are reading the right scale? Maybe it is just that dwell doesn't necessarily make sense on a magneto. This DU manual says 0.4mm gap and I believe that is how I would set it.

 

https://oldcroak.com/du1-du2-du4-ad-nauseum-catalog-50/

 

 

 

[29 Chandler]

11 hours ago, Bloo said:

There is no standard for a brass era car. There are too many kinds of ignition systems. If your car was newer enough to have a Kettering style (points and condenser) ignition we could make a really good guess.

 

If this were a battery powered points and condenser system, with the same number of lobes as cylinders, and the shaft turning at half crank speed, you typically want a little over 50% dwell. So, in a 6 cylinder engine 60% is all there is so maybe 35% would be good. Really you want all you can get. The limit is how narrow you can gap the points without them arcing over, running badly, and burning up. Anyhow, I doubt this applies to a magneto.

 

Also if I am not mistaken a DU-6 is a high tension magneto, so probably doesn't use a separate coil. What is the 6v coil for? Starting? Is this dual ignition or something? A magneto generates the voltage itself, so there is no battery sitting there that you can get the maximum "on" time (dwell) from. If there was, going wider is the wrong way. Whole systems involving dual points and sometimes two coils came in later years with the intention of getting more dwell. You can only set the gap so narrow before you have to do something radical to get any more dwell.

 

50 sounds impossible, are you sure you are reading the right scale? Maybe it is just that dwell doesn't necessarily make sense on a magneto. This DU manual says 0.4mm gap and I believe that is how I would set it.

 

https://oldcroak.com/du1-du2-du4-ad-nauseum-catalog-50/

 

 

 

Hi Boo,

 

It seems sometime in the 60's the lead from the magneto was disconnected and a 6V coil was installed on my car. So at this point the magneto is running just as a distributor. Power for the spark comes from the battery to the coil and then off to the mag to direct the spark to the correct spark plug. This has worked well for years.

 

I have now checked the dwell several times and am confident I am looking at the correct "6 cylinder" scale on the meter. I have the same Bosch manual that specifies 0.4mm gap and started with this setting. Still I get the high dwell reading.

[Oldtech]

Hmmm.  6 volt mags are typically made so the armature runs 1.5 crank speed and points open twice per revolution so the dwell reading is different than a distributor.  Can't tell what it should be, but if the gap is correct ( .016)  I'd go with it.  But not familiar with a DU-6

Edited August 11 by Oldtech (see edit history)

[29 Chandler]

21 minutes ago, Oldtech said:

Hmmm.  6 volt mags are typically made so the armature runs 1.5 crank speed and points open twice per revolution so the dwell reading is different than a distributor.  Can't tell what it should be, but if the gap is correct ( .016)  I'd go with it.  But not familiar with a DU-6

So if I take my dwell setting and divide by 1.5 I get a dwell reading of 33. That sounds much better and consistent with other readings I have seen for a 6 cylinder.

[Oldtech]

Sounds reasonable. but at 2 bumps per rotor revolution I think it could be 50.  Surmising as I never put a dwell meter on a mag!.  I may have to try it. 

[Hemi Joel]

The rate of lift on the ramps of the cam that opens and closes the points determines the dwell angle in relation to the gap setting. this varies greatly from manufacturer to manufacturer. Just look at the gap/dwell charts for various makes. The relationship between gap and dwell is all over the place.   In light of this, and lacking a factory recommended dwell setting, I would only use the recommended gap, forget about the dwell, and call it good. 50 degrees of dwell should not be a problem on a 6.