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MG MGA - low dwell angle

Adjusting my points recently I checked the dwell and found it was low at 52 degrees. I closed the points gap down from 0.014" to 0.012" and it pushed the dwell up to about 53 degrees but no more. My dwell does not seem to respond particularly to the points and I'm not sure if I should keep reducing the gap. I wondered if I'm doing anything wrong?
J H Cole

Could it be that you have a spike on one contact and a pit on the other, so your gap is really bigger than you think? Another reason might be a worn dizzy bearing. Why worry if you have a good enough spark?
Art Pearse

Years ago all Toyota and Datsun points were set to 49-55 degrees of dwell. I could find no spec other than point gap for my MGA. I would set the points by gap and then check the dwell. This appeared to me to correspond well. So for years I have used that as my spec. Setting to 50 and resetting or replacing the points when the dwell reached 55.
Point gap is not really an accurate way to set points. Dwell is. Cranking dwell and running dwell may not be the same due to wear in the ditributor. Set the points so that at 1500 RPM the dwell is between 50 and 55 and your car will run fine.
Dwell is a measurment of what the coil sees. Point gap is a Hope for the same thing.
R J Brown

Dwell sets how long the coil has to reach peak primary current, so it also depends on rpm, far more than a few degrees.
Art Pearse

I am about to commit heresy. Follow with me please.

The actual dwell is "relatively" unimportant in the MGA. The coil will have enough of a field built up to fire efficiently and effectivly even down near 25 deg dwell. OK, I said it.

As it turns out in general, most engine dwell specs are close, give or take a few degrees, to 30% of the available time between ignition events. So for about 30% of the ignition cycle, the coil is charging. Since the number of ignition events per crank revolution are different for 4, 6 and 8 cylinder cars, the dwell is spec'd differently. For an 8-cyl this works out to .3*90deg (or 30 deg). 6cyl engines are .3*120 deg (36 deg) and 4-cyl cars are .3*180 deg (54 deg) The coils are designed to fully charge in the actual time, milli-seconds, it takes to build the field. This is affected by the application (4 6 or 8 cyl) and the RPM the engine is spec'd to run at. Most coils "of the era" were 12V or 6V and either internally ballasted or externally ballasted. The same coil was used across entier engine offereings with no respect to 4,6 or 8 cyl operation. The MGA uses an internally ballasted 12V coil.

So, back to the MGA. The std coil is the Lucas DLB101. This same coil was/is used on MANY 4 and 6 cyl British cars of the era. Maybe some 8 cyl, but I'm not sure here. So safely, if the dwell in the MGA is down to, say 36deg (the above mentioned 6 cyl spec), there should still be plenty of spark left to run at full RPM.

Minimum point gap not specified but is important as the gap must be adequate to hold off around 250-300 volts at the points during the ignition events themselves. I'd guess the minimum should be around .008", well below anything remotely close to specification. Also, the points must be open a minimum amount of time for the coil to allow a full spark before the next charging cycle. A small gap (and dwell) would prevent that from happening.

That being said, neither point dwell nor gap are critical settings, if the ignition timing is always set after adjusting the points. This is the important part. Always check timing after adjusting the points. Monitoring the dwell is not a bad idea. A change in dwell over time will infer a change in timing. This will certainly effect the drivability of the car. So it is one way of monitoring point wear, either of the cam follower block or the buildup of the point contacts themselves.

Back to JH Cole's question, and supporting Art's reply, if the car runs good at 52deg dwell, then there is no technical reason to try to get anything different.
Chuck Schaefer

Chuck

Interesting and informative post. Thanks.

Steve
Steve Gyles

Dwell is a strange thing to measure and I assume was developed as simply a way of making sure the points were set somewhere near their optmum setting with the engine running, thus no need to remove he dissy cap.

I am only guessing as I have never seen the need to set dwell angle when it is extremely easy to set the points to the FACTORY setting. I suspect that due to the shape of the distributor cam a dwell setting of well below 50 and well above 55 would be impossible The cam is a base circle with 4 very steep peaks and as you have noticed John a change of quite a large points gap makes very little difference to dwell.

Naturally the time when the points are closed is all the time that is available for the coil to charge. However belive me with some engines having 8 cylinders and revving at 8000 plus RPM you can rest assured that even with an extra wide points gap the MGA ignition system will have more than enough time to charge up even with the engine revving at a staggering 6000 rpm LOL

As Chuck described the timing is the key issue and if it was set with a points gap of 15 thou then each time the points are adjusted the gap needs to remain the same. Any deviation will change the timing figure. IMO the easiest and most accurate way to ensure that is by setting the points gap and not dwell.

But I ould be wrong.
Robert (Bob) Midget Turbo

Chuck, Nice explanation and, just for future reference, I believe the Daimler V8 used the same coil so "4,6 and 8 cylinder cars of the era"
Neil McGurk

AP, I will inspect the points this weekend for any spikes - difficult to see without taking them off.
RJ I thought the dwell for a B series engine was 60 degrees +or- 3 degrees ie about 2/3 of 90 degrees and I've always tried to get about this figure. Failure to do so indicates wear or poor adjustment somewhere? I also thought that dwell does not vary much with revs, perhaps 1 or 2 degrees max. This is just from reading
stuff and I could easily be wrong here.
CS, why are you quoting a fraction of 1/3 of the ignition event? My understanding is that the points should be closed for 2/3 of the ignition event so for a 4 cylinder engine you get 2/3 of 90 = 60 degrees? I understand that the coil can recover in much less time than 60 degrees but if we are talking about 'optimum' performance should this still not be in the region of 60 degrees? The fact that an engine will run with a wide range of settings does not validate their use.
Robert, I follow your reasoning except that dwell is most often said to be a better indicator of engine set up than the points gap. If this were not the case why do we (or some of us at least) bother with it? My engine used to have a dwell of about 58 degrees with points of 0.013" but for some reason it has changed. I expect I will end fitting new points to see if there's any change.
J H Cole

As mentioned in other posts here, it is not the dwellangle (or pointgap) on it's own that's important. It's the relation it has with the timing.

Imagine the (distributor)timing set at TDC, with normal pointgap. If you then set the pointgap larger, the points will open earlier thus giving the spark earlier (advanced). With a smaller pointgap the spark will come later (retarded).

Striving for the correct timing is more important than striving for the "perfect" pointgap/dwellangle.

JH: double check if your points pivot freely on it's "axle", if it drags it could influence your dwellangle with the correct pointgap
Willem vd Veer

John

The dwell angle on B-series engines would appear to vary according to the type of distributor fitted.

As mentioned already, there does not appear to be data in the manual for the MGA DM2 type. However, the later MGB B-series quotes 60+/-3 for the 25D4 distributor and 51+/-5 for the 45D4 and 45DE4. May be that's where you got your higher figures from?

Steve
Steve Gyles

Steve, Moss give the dwell for a 1500 engine as 60 +/-3 degrees. I think the weight of written material (not as you say from the manufacturer) suggests its the 60 figure. WV - this is new to me. I've now gone back to the manual page B5 and found exactly what your saying, to quote '..the moving arm must be free on its pivot. If its sluggish, remove the arm and polish the pivot with a strip of fine cloth ....apply a spot of clean engine oil to the top of the pivot....contact breaker spring tension should be between 20 and 24 oz'. I will check and report back. I'm assuming the other post that takes the lead from the capacitor with the nut on top can be screwed down tight and rotation of the spring not needed here.
J H Cole

John

I have picked up some information while trawling the net that suggests that from 1961, the DM2 in the MGA had its centre shaft and advance plate replaced with that from the 25D4. It would seem, therefore, that if this information is correct then, depending on which version of DM2 you have, the dwell angle could be either 50 or 60.

Apparently "You can tell the difference by removing the breaker plate. There's a stamped steel plate under the advance mechanism - the good one (25D type) is square, the bad one (old DM2 style) is round."

This was the link (second post). I am not saying it's correct. May be others may like to comment.

http://www.mgexperience.net/archive/dm2_distributor_on_1800_question/888692

Steve
Steve Gyles

My applogies again for the long post.

JH Cole said:

“CS, why are you quoting a fraction of 1/3 of the ignition event? My understanding is that the points should be closed for 2/3 of the ignition event so for a 4 cylinder engine you get 2/3 of 90 = 60 degrees? I understand that the coil can recover in much less time than 60 degrees but if we are talking about 'optimum' performance should this still not be in the region of 60 degrees? The fact that an engine will run with a wide range of settings does not validate their use.”

Perhaps I stated things a bit incompletely, although the principles remain the same. I tend to look at the engine as a whole rather than just the distributor. I know that the dwell is actually a disty-specific specification. What I was trying to convey was related to crankshaft. So the fraction of 1/3 vs. 2/3 works in that context. More specifically:

In a 4 cylinder engine, the crank rotates 2x to get all 4 complete combustion cycles. Therefore there are 2 ignition events per crank revolution; i.e. 1 every 180 degrees crank rotation. IF, the dwell specification is 60 degrees, then that is 33% of the crank rotation, close to the 30% I used as a rule of thumb.

Likewise, in a 6 cyl engine there are 3 ignition events every crank rotation; hence, one spark for every 120 degrees of the crank. Dwell specifications I have found for typical 6 cyl engines run around 36 deg. 36/120 = 28.33%, very close to the 30% I stated.

The 8 cylinder engine has 4 sparks per full crank revolution, or one every 90 degrees of the crank. Dwell specs I have found for a typical engine is around 28.5deg. 28.5/90=31.66, again very close to my 30% ROT number

This may be an unconventional way of looking at it; however, it does work. Nobody ever accused me of being conventional. If you would like the more traditional method, then this works too using your 2/3 charge time rule of thumb:

A 4 cylinder engine has 4 lobes on the distributor cam, each being at 90 degrees (360/4). The points are closed for about 2/3 of each ignition cycle so .67*90deg = 60 degrees dwell dwell.

A 6 cylinder engine has 6 lobes on the distributor cam, each being at 60 degrees (360/6). The points are closed for about 2/3 of each ignition cycle so .67*60deg = 40 degrees dwell.

An 8 cylinder engine has 8 lobes on the distributor cam, each being at 45 degrees (360/8). The points are closed for about 2/3 of each ignition cycle so .67*45deg = 30 degrees dwell.

Each of these dwell numbers are slightly higher than the actual dwell specs I looked up earlier. But not to worry, we are talking in generalities. You see, the math works either way. It is the same thing just looking at it from different perspectives. If the traditional method is easier, then use it. It doesn’t change the fact that the DLB101 coil will work easily and properly down below 30 degree dwell or less at 6000 RPM. As Neil suggested, I have looked up the coil used on the Daimler V8 and indeed it is the same DLB101 so….On to more math.

The Daimler SP250 V8 was rated Power - 140 bhp / 105 kW @ 5800 rpm
I don’t know what the redline on this engine is so we will use that 5800 rpm. 5800/60 = 96.66revs per second. Since the disty runs at ½ the crank speed, that relates to 48.33 revolutions of the distributor shaft per second. Invert that and we get 20.68mS. per rev of the distributor shaft. At 30 degrees dwell, we see that 30/360*20.68mS= 1.72ms charge time of the coil.

Now compare that to the MGA engine @ 6000 RPM. 6000/60=100 Revs per second or 50 revs of the distributor shaft. 1/50 = 20 mS per rev of the disty cam. At 60 degrees dwell, 60/360*20mS=3.33mS charge time of the coil, 92% longer than when used in the V8 engine.

Now, both engines use the same coil. So how can the charge period be optimum in the MGA’s 4 yet still be optimum in the Daimler V8 engines? The simple answer is that it isn’t. Once the coil is fully magnetically saturated, then any more time spent charging is just is a loss. The same energy is stored in the coil and the same spark energy is delivered in both cars. In fact, if you recalculate the V8 engine at 3000 RPM, you’ll find that the charge period is identical to the MGA engine at 6000 RPM. Can the coil charge time be optimum for 3000 RPM and not for 6000? The simple answer is no. There is enough spark for all RPMs and up to 8 cyl engines. Charge time varies with RPM and with the number of spark events per rev (either disty or crank, your choice).

Some references I have seen refer to a nominal dwell spec for a typical 4 cyl to be 60 degrees while others are as low as 45 deg. I suspect that the different specs may relate to the shape of a specific cam lobe. Here the designers use the contact spring pressure, cam follower block and cam lobe design in an attempt to optimize such things as contact opening rate, contact bounce, to improve the wear rate of the rub block etc. It would not surprise me to find that one design of a disty has a slightly different shape cam lobe than the next. I’m just guessing here.

Now, cam lobes themselves eventually wear down too. What was an original factory shape when new may not be quite the same shape after 50 years of use, at least in the area where the rub block runs. The high points of the cam lobe will wear down from years of wear. If you have ever seen a cam lobe where the vertical edges at the high points of the cam are not quite straight, this is an example. We always grease the cam lobe when installing new points don’t we? I actually had this problem on a Ford Pinto I had back in the mid 70’s. The cam lobe needed to be reground by a local shop. Yes, there were places that did this sort of thing back in the day.

As stated earlier in the above posts, if you set the timing an a car after adjusting the points to one specific parameter (point gap or dwell) then later change out the points and adjust them for the same parameter, you will get very close to the same timing advance without readjusting the timing. It just works that way. I don’t recommend it. Most cars I have worked on have recommended setting the point gap. You may just let it set at that. In fact, I have never measured the dwell of my MGA engines. If you are of such a mind, you would then check dwell and then readjust the points and retest the dwell, ad inifinitum. Older Chevys had an access port in the disty housing where you could dynamically adjust dwell with the engine running. IF you were a student of the “dwell school of ignition systems”, then that system was ideal.

JH: I just thought that, rather than just say ”Don’t worry about the 52 deg dwell, just ignore it. Everything will be just fine” I would share a personal viewpoint as to where those dwell numbers come from and how they relate to point gap; and why neither dwell nor point gap are important specifications to get “just right” in the MGA. Based on what I see, these numbers appear to have no real engineering basis. I maintain that one should get the points adjusted to "somewhere close", set the timing and get out and drive the car. That last one is the important part.

As has already been stated, there is no recommended dwell spec in the Factory Service Manual for the MGA, only point gap of 0.014” to 0.016”. I wonder were Moss is getting their info? If, for some reason, your interest is in meeting some dwell specification, then by all means go ahead and do so. It certainly won't hurt. If you don't agree with my analysis and reasoning, that is fine. But I would have to ask, why would you choose to meet some undocumented dwell spec while disregarding the Workshop Manual spec for point gap? I think that you would have to believe that the point gap specification is not necessary, or at least a secondary consideration.

I mean no personal attack on you. I am just using this to make a point. The numbers, neither dwell nor point gap are absolute. Anything approaching "close" will work just fine, even optimally. I hope this post clears up my first post and your subsequent questions. I really do hope you find some acceptable conclusion on your dilemma.

Disclaimer: Other ignition designs such as high voltage sport coils, dual point distributors, CD ignitions, ECU controlled ignition, Coil-on plug, Waste spark ignition etc…. are not considered in this discussion. Just the quaint, antiquated, basic ignition system as used in the MGA and millions of other vehicles built up thru the 60's

Cheers,
Chuck
Chuck Schaefer

Chuck, thank you for taking so much trouble in explaining things, I had not been thinking of crankshaft rotation. I now understand a little more about the process and find that your point that the manual only specifies the points gap persuasive. I'm still going to check out the points spring etc but will probably be more relaxed over the dwell.
I suppose those with electronic ignition can be comforted that these issues are not for them!
John, with thanks. (not at all offended by your post)
J H Cole

Don't "dwell" too long on the issue!
Art Pearse

John, those with electronic ignition don't have the pleasure of discussing basic principles (at length) in order to understand their cars better; they are to be pitied.

I bet someone in Canada who knows the Art of Pearsing a good thread has electronic ignition? ;-)
Willem vd Veer

Once upon a time there were cars but no dwell meters. The tachs were mechanical and timing was done with a test light not a strobe light.
You would use your matchbook cover to set the point gap. Then you would turn the engine to the base point for timing. For example the notch on the pulley is lined up with the pointer on the timing cover. Then using a test light attached to ground and the point touching the distributor primary lead you check for the position the points open and cinch down the distributor hold down.
Later in progression of car technology someone discovered that a meter could be used to check the percentage of time that the points stay closed. This was a more accurate way of checking point gap and all the car makers adopted it.
I know that we as confirmed Luddites should never use a technology newer than the cars we drive. And as such never touch those new fangled dwell meter things.
But as I stated earlier dwell is more accurate because it takes wear and other anomalies into account when setting points. Point gaps does not take into account those other variables.
JHCole, To answer your question. When I was a young mechanic working for a Datsun dealership I could not find a dwell spec for an MGA. I simply co-opted the spec from all the points I was setting at work and applied it to my MGA. It worked and I never changed how I did it.
R J Brown

RJ I think you could be right about the accuracy of using dwell to set the points. I couldn,t say for sure however because I have never had difficulty setting using feeler gauges.

The point is however that as described above that so long as the timing is set to the points gap used it doesn't matter a hoot.

If a points gap of 10 thou was set and the timing set accordingly the engine would probably run perfect. Equally if the gap was set at 20 thou with timing set to suit I suspect the engine would still run fine.

Never tested hat theory however but can't see any reason why it wouldn't work as described LOL. :)
Robert (Bob) Midget Turbo

Chuck, Both of your posts were very well written and explained dwell correctly. Thanks for the posts. Fred
FC Winterburn

I'm not an MGA type, but still trawl through to see what interesting threads I can find. Problems with old British cars seem to spread over a wide range of makes ad models as they are pretty similar in many ways.

Chuck, your explanation is very informative but please allow me to throw a spanner in. The Daimler V8 has a dual point dissy to extend the dwell.

Cheers
Tony
Tony Oliver

Chuckle. In my MGA I has a Mallory Dual points dizzy. The two sets of points motion is 8 degrees out of phase, one opening and closing before the other. One point set by itself has 32 degrees dwell. Two point sets together have 40 degrees dwell. Yes, dual points does have more dwell than single, but in this case it is a lot less than the Lucas 60 degree dwell. The short of it is, dual points is not used to increase dwell, but it has other reasons to exist. The reduced dwell means that power to the coil is disconnected more than half the time, so the coil should run cooler. For the tech behind it see here:
http://mgaguru.com/mgtech/ignition/ig200.htm
Barney Gaylord

Thanks Chuck, I really should have described it as a threshold voltage that doubles as compression doubles more or less linearly. Thanks for the info on plastic rubbing blocks. I'll bear that in mind and warn others not to use them with low resistance coils. Take care, Fred
FC Winterburn

I started this thread asking why I had a low dwell angle despite adjusting the points etc. This is just to say that in the end I put in a new set of points and condenser and the dwell has gone back to around 58 degrees. Can't explain why this made the difference since the old set up looked okay but thanks for all the suggestions.
J H Cole

Dwell is the amount of time the points stay closed. For the Lucas distributor it will be roughly 60 degrees out of each 90 degrees rotation of the distributor shaft, ie. 60 degrees closed and 30 degrees open. The dwell and spark timing changes if the points gap changes. When points gap goes smaller points open later and close sooner, yielding longer dwell. When points gap goes larger points open earlier and close later, yielding shorter dwell. If you had low dwell angle the points may have been adjusted too wide.
Barney Gaylord

This thread was discussed between 08/02/2011 and 30/03/2011

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