MGB & GT Forum

I'm fiddling with the points the other day and got to wondering why dwell on the MG calls for 60 +/- 3 degrees when other 4 cylinder engines, like my VW's were, call for 47 +/-3 degrees.

I'm thinking there would have to be enough of a difference in coil construction to have a drastic effect on coil saturation times between the MG and the VW, Bosch Blue coils work on both, but in thinking back to all of the coil specs I've looked at over the years, and buying "hotter" coils for "increased performance", I don't remember that much difference in the specs between coils except for the difference in primary resistance between internal and external ballasted coils and that some are oil filled and some epoxy filled.

In my mind as soon as the primary winding has all the available voltage flowing through it the length of time that it is flowing is immaterial. Yes, no, maybe?

This isn't keeping me up at night, yet, but inquiring minds want to know so if you can shed some light on this subject I'm all eyes.

Joe

Hi Joe,

Firstly, to clear up a slight misconception, it is current that flows, not voltage, which is electrical pressure.

You are right, once the current has reached its peak it doesn't make any difference for how long it flows, but there is the catch. Current in an inductor (coil) cannot change instantaneously, it takes time to build up. This time is set by the dwell of the points. Unfortunately the points with their fixed dwell are a compromise, since the charge time, which is fixed for a given coil, varies with engine revs. This means that the coil is seldom operating at optimum. At idle the coil is well and truly fully charged but at high revs there isn't enough time to fully charge it.

I guess that the VW's dwell is set to optimise at lower engine revs, than the MGB, which after all is a sports car, and is expected to run higher revs.

To further confuse the issue, my Wolseley, a sedate sedan, has the points gap soecifies as 14 to 16 thou, the same as the MGB.

Maybe the VW's OEM coil has different characteristics to the Lucas coils.

Herb

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Thanks Herb, good thoughts.
Joe

In further checking I see that that the L16, L18, and L20 series of Datsun/Nissan engines used a dwell around 50 degrees and to my mind were certainly more of a performance engine than the B motor so I feel the whole story hasn't revealed itself yet.

Joe

I think there are a few design differences at play that determine what the specified dwell will be. I think it's probably more to do with what the manufacturer thought would be a good match, IE coil to distributor. The Lucas dizzys that I have seen have very steep ramps on the points cam. Very steep. Good for low rpm as the points open very quickly, but bad for high rpm operation where points bounce will be greater unless really high spring pressures are used. So why did they specify such long dwell angles when at low rpm they are not needed and only results in excessive coil heating once coil 'saturation' is reached? Maybe to compensate for all the bounce at high rpm! Our man Jeff should be able to provide the answer as he has mapped all of this. And for anyone that thinks bounce doesn't occur until high rpm, you are wrong. Points are undamped and bounce starts to become noticeable just off idle. The Bosch ramps on VWs and my old Volvos are much milder but not nearly as mild as a dual point Mallory distributor (my favourite). In practice the dwell angle the Bosch calls for works great with their coils and most probably a Bosch coil would work great with the Lucas dizzy too. I have found that dwell takes a back seat to points gap which to me is the more important parameter. Once the dwell is set, then check gap. If the gap is too narrow, adjust it. Wider is better in most cases. Scott will chime in here I'm sure. In any case, manufacturers do goofy things sometimes and the reason will never be known. Every design is a compromise or multiple compromises. Fred

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Used to be that a typical dwell for a 4-banger was 60 degrees, 45 for a 6 cyl and 30 for an 8 cyl engine...I guess old rules don't apply any more!

For clarity's sake, I'm going to state the obvious. Dwell is the amount of time your points stay closed.
There are multiple cam designs used to lift points. That design dictates what dwell works the most effectively. Personally, I prefer designs that allow you to run a higer dwell - like the original 25D (not the aftermarket ones that use a 45D shaft assembly.)

As for the Datsun design, they simply run a huge point gap, which decreases dwell. They run perfectly fine set at 60 degrees dwell at .016" point gap versus .020" point gap. We run that distributor at 10K+ rpm at bonneville with great success - gapped at .018". The points bouncing must be terrible, because we made in excess of 190 RWHP at 9000 rpm. The dyno operator was afraid to go higher.

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jeff@advanceddistributors.com

As Fred said: a series of compromises. The real truth for optimum can be found looking at the current waveform of the primary side.

The usual design goal is to reach about 98% primary current at idle as not to overheat the coil sitting there. Less current means less energy stored and all related issues of weak ignition. This is a TIME based value, not a degree value. A BIG difference is in idle RPM. So add cam profile as Jeff mentions, gap size, coil inductance, RPM, left rear caster, and phase of the moon, you get different values. FWIW, into a stock Lucas 3 Ohm coil, early dizzy, SPICE simulation shows 98% current @ 800 RPM with 60 degree dwell assuming a clean brake and no significant bounce. Just maybe MG and Lucas knew what they were doing! 60 @ 800 RPM is a good place to start. Then testing will find optimal.

Do be aware, DWELL meters are accurate at only one RPM, usually 1000. This may be some of the inaccuracies some have complained about.

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Cogito ergo sum periculoso

Assuming we are talking about an early MGB using the 3 ohm OEM coil p/n DLB101, it is perfectly happy running nearly 1/2 the factory stated dwell. The same coil is used on V8 used in the Daimler V8 with 2x the number of spark events as the I4 MGB. Dwell and Gap are relatively unimportant in our cyl cars.


More important is that you set the timing after servicing the points as the timing will change with dwell/point gap.

Thank you all for some great input! I won't say it gives me a clear picture of the differences in dwell between cars with the same number of cylinders but the picture is becoming less foggy.

Chuck did mention something that has came into my mind as more members posted and that is a V8 engine, that usually calls for a dwell in the 30 - 35 degree range, basically uses the same coil as a 4 cylinder but still gets the job done. Blows my mind.

Joe

Posts like this make me appreciate the low maintenance and great performance of my re-curved Lucas CEI and 1.1 Ohm E-Core even more.

++ on the CEI.

I disagree completely with Chucks comments. It's just physics. If the points remain closed for longer than it takes to reach maximum primary current, you will overheat the coil and it will fail. For all the time above idle when you never reach maximum current, the ignition is getting weaker and weaker. It is a fortunate feature that start and idle are about the hardest things to do so getting weak with increase in RPM actually works. That is until it is too weal at high RPM. Then you cheat a little more with a lower inductance "Sport " coil giving up a little of the idle in favor of a few more RPMs.

Again, it is about TIME. We just measure it indirectly with DWELL or gap.

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Cogito ergo sum periculoso

Answer me this Scott, If a coil is supposed to just barely reach max current at idle, how can a V8 at 750 rpm idle reach maximum current if the same coil just barely reaches max current in a 4 cyl at 750 rpm idle?

The coil does not reach "saturation". It reaches peak current for the restive load. The point where inductance is no longer inhibiting current. This is a common terminology error but we are talking about the same thing. Saturation refers to the core.

No, they are not designed to be so abused. They are designed to be use within their parameters. Overkill costs money.

Go calculate it your self using coil primary specs. You will find 60 degrees into a Lucas 101 @ 800 RPM with 14.2V is in fact, 98% peak current. ( +/- manufacturing tolerances) Even the Prince of Darkness could do his math.

Why 98% some may ask? Not 100%? It has to do with how much time it would take to get the last 2%. Not worth it. We are talking about the current being like a sine wave at the peak, very little slope.

All things being perfect and under optimal conditions, yea there is is a wide range one can make an engine run. Tell me about specs though on a cold damp dark night a long way from home with plugs that should have been changed, points more than a week old ( about when they are no longer good), a bit of wear on the engine... They designed the cars to be able to be reliable under a very wide set of conditions. Getting away with it under optimal conditions is not a good basis for decision making. No more valid than what you get away with on a race car.

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Cogito ergo sum periculoso

What utter bull crap...

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