Dwell angle

[KenM]

Can anybody clarify for me the dwell angle, not that I think I have a problem but I wanted to check what mine is just because you do, anyway I understand dwell to be the period of time that the ignition points are closed for any given contact. The manual says that the dwell angle is not adjustable but what about the point gap adjustment, doesn't this alter the dwell if the point gap is smaller? Is there a spec for what the the point gap should be?

Thanks,

Ken

[oscar]

Points gap is so much easier to change than dwell IMO.  So far as if you're changing dwell angles via a MM, you're guessing the points gap, putting the cap back on, starting the car, reading MM, turn car off, take dizzy cap off, guess adjust points gap again and so on.  If a manual says the points gap is 0.8mm for example, well you just set and forget.

Anyhow, what year is your 450 Ken?  It might have the full transistorized non point ignition with a trigger wheel.  Hall effect thingy I know nothing about the system, let alone the correct terminologies involved, except there's no points to worry about.

Your summation of dwell is what I understand it to be.  Furthermore, I've always thought the "angle" refers to how many degrees of distributor cam rotation the point stays closed for.

[KenM]

Hi Oscar, yes I understand the angle reference to be the degrees of rotation as well, I have a June 75 D Jet, so one of the last if that makes a difference. I've always understood the point gap thing, bigger gap, fatter spark within limits etc but I can't see how adjusting the points gap will have no effect on the dwell angle, if you close the points up then they stay closed for longer and the dwell angle is increased no?
Please don't start with Hall Effect, never even heard of that I've got enough to be going on with already. What do you set your points gap to?

Cheers,

[koan]

If a manual says the points gap is 0.8mm for example, well you just set and forget.

Setting with a feeler gauge assumes perfectly flat and parallel contact surfaces. What usually happens with points in a conventional, non transistor system is a little pip growing on one contact and a corresponding depression in the other due to metal transfer, not those bloody ions again I hope . Using a feeler gauge under these circumstances results in a bigger actual gap than the feeler gauge and reduced dwell. Adjusting the gap to a dwell angle gets it right.

Its a variable reluctance sensor, just a coil of wire and bits of metal whirring round. Don't think hall effect sensors were around at 116 time. Crank sensor is the same.

koan

[KenM]

If a manual says the points gap is 0.8mm for example, well you just set and forget.

Setting with a feeler gauge assumes perfectly flat and parallel contact surfaces. What usually happens with points in a conventional, non transistor system is a little pip growing on one contact and a corresponding depression in the other due to metal transfer, not those bloody ions again I hope . Using a feeler gauge under these circumstances results in a bigger actual gap than the feeler gauge and reduced dwell. Adjusting the gap to a dwell angle gets it right.

Its a variable reluctance sensor, just a coil of wire and bits of metal whirring round. Don't think hall effect sensors were around at 116 time. Crank sensor is the same.

koan

So why does the manual say the dwell is not adjustable? Are you meant to understand that this means 'whilst the car is in operation' or something of that nature?

[oscar]

koan- Ah yes, those ions.  I remember being taught to file down points in other cars before setting the gap.   With w116s, it's only necessary for the 280S that doesn't have an ignition module.  Although you'd do it to clean up the points rather than set the gap because the manual still quotes a dwell angle rather than a gap.  For d-jets with ignition module the points don't carry a great load so you don't getting pitting or deposits (or so I've read).

KenM, I've thought bigger gap = shorter dwell, more intense spark over less time.  (same as what you've said).
As a bit of an experiment a while back I set my dwell at 11deg and noticed a power gain.  It was pleasant surprise.  Unsure if such a move would harm something I changed it back to around 28deg.  Manual says 30-34deg.

But where did it say it's not adjustable?  Its definitely adjustable and it's just like any other conventional ignition point.  There's a screw behind the contacts to loosen and indents in the base plate plus notch in the point so you can adjust with a flat blade screw driver.  I can take a labelled photo if you like

[KenM]

It's under job 15-562 in the engine manual Oscar, sounds like I'm making a mountain out of a molehill, I understand that changing the point gap will achieve the required results so it's certainly adjustable as you say. I'll play around with it a bit just for the sake of experimenting.

[oscar]

Ok, I think I see the prob.  Section 15 deals with breakerless ignition only.

Have a look at section 7.5 - 500

[KenM]

Ah, yes. 
Ok well I understand the basic theory then so it's pretty simple after all.

Thanks for the help,

[oscar]

Bugger, I'm up for a new rotor and cap.  I went and had a quick look under my rotor cap this morning just to make sure there was nothing fancy about the workings but didn't put the cap back on properly.  The Mrs went for a drive up town and came back to say how sluggish it was, full throttle, couldn't get any speed plus stalled at the shops. 

I got in and had a look and there's brass shavings and plastic in the dizzy.

Moral of the story, don't let the Mrs drive your w116.

[koan]

I've thought bigger gap = shorter dwell, more intense spark over less time.  (same as what you've said).

As a bit of an experiment a while back I set my dwell at 11deg and noticed a power gain.  It was pleasant surprise.  Unsure if such a move would harm something I changed it back to around 28deg.  Manual says 30-34deg.

The thing with standard ignition systems is the spark happens when the current stops flowing through the coil not when the current starts flowing through it.

By standard I mean points switching the current, or points switching  transistors that switch the current and variable reluctance sensors switching transistors. Same thing even with magneto ignition on old bikes and lawn mower engines, it's when the current stops the spark occurs.

The ignition coil is coil of wire with inductance. When Voltage is applied to an inductance current builds up slowly from zero to a maximum determined by the coils DC resistance, maximum current flow is not instantaneous. The points have to be closed long enough (dwell) to let the current reach maximum to get the biggest spark when the current stops. The rising current is storing energy in the magnetic field around the coil, when the current stops the field collapses and energy stored in the field is released giving the spark.

Sufficient dwell is important at high RPMS when the time to build up the field is minimum 

CDI or capacitor discharge ignition is different in that a high voltage, 400 Volts in one I had years ago, is stored in a capacitor and dumped into the ignition coil, in this case it is the flow of current that makes the spark.   

Now, getting to the point (pun), oscar did you adjust the ignition timing after reducing the dwell? I suspect increased performance may have come from a change in advance that would be a result of reducing dwell rather than the shorter dwell.

Bit of a downer on rotor and cap "oscar", something I always worry about on the first start when I've had the cap and rotor off.

koan

[oscar]

Fristly, well explained, gives me more depth of knowledge reagarding dwell, particularly why no one has points anymore.  Thanks! 

Now, getting to the point (pun), oscar did you adjust the ignition timing after reducing the dwell? I suspect increased performance may have come from a change in advance that would be a result of reducing dwell rather than the shorter dwell.

To answer your question... how embarrassing if I advanced timing at the same time   I'm fairly certain that wasn't the case, however, the extra power I think was perceived only at low revs.  At take off for example.  I don't remember going out on the highway and I didn't keep the dwell at 11 for long.  I changed it later that day. 

[koan]

The ignition timing changes as dwell is adjusted.

If you set the dwell to the correct degrees and timing to what it should be then reduce the dwell angle the ignition will be advanced by some amount. The points are closed for less time and therefore open earlier and close later.

koan

[KenM]

Thanks Koan, that fleshes it out a bit, I didn't realise that the dwell had an effect on the advance and retard mechanism also, a timing light is one of the things on my to get list. Took a little spin up to Noosa and back today, I'm getting about 19 or 20 MPG so things must be working pretty well, I don't imagine it would get a lot better than that.

Cheers,

[koan]

Thanks Koan, that fleshes it out a bit, I didn't realise that the dwell had an effect on the advance and retard mechanism also, a timing light is one of the things on my to get list. Took a little spin up to Noosa and back today, I'm getting about 19 or 20 MPG so things must be working pretty well, I don't imagine it would get a lot better than that.

I've got a Hella timing light with a dwell and volt meter built in. I have no trouble seeing timing marks, it's bright, best I've had. Looks like a torch except the light comes out the little end and the round dwell/voltmeter is the big end. Think the models have changed since I bought it though. Came from Repco or Burson, about $50 or $60.

Don't thing you could expect any better MPG than you are getting.

koan