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MG MGA - Mallory Dual-Point distributor – dwell time and co

Just finish a 2 1/2 years total nut and bolt restoration of a dry Nevada MGA. – Hurray!
I have chosen to install a Mallory dual-point distributor, the old Lucas was worn-out. One of the advantages with a dual point distributor is that the dwell angel in a four cylinder engine is 73° which give the ignition coil more time to recharge for the next spark.
See article: http://www.chicagolandmgclub.com/techtips/general/590.html
I have some problem understanding why not I can get the 73° at my dwell meter. I set the dwell angel at 32 and combined dwell at 41 following the instructions on centuryperformance.com:
http://www.centuryperformance.com/point_settings.asp
Yes I know that 32+41=73°, but if the ignition coil receive that dwell angel why do I only get 41° combined at my dwell meter?

The next, which have disappointed me, is that its looks like the distributor is “eating” condensers. The fist was gone after only 1 hour of tune in, and it took hours (days) to diagnose that is was the condenser to blame. The next was gone after only 1K mils of driving, but now I was able to diagnose the condenser immediately, - but I took hours to get the car home.
I have installed a Lucas sports ignition coil, but I don’t think that it is the reason for the early condenser failure?


JF Hedetoft

The points overlap. The # of degrees that they overlap is not added twice.The timeline is: point 1 opens, 9 degrees later point 2 opens, for the next 24 degrees both are open, at 32 degrees point one closes, point 2 remains open for 9 more degrees, now at 41 degrees both are closed. Each set of points is open for 32 degrees but together they are open for 41 degrees. These numbers are referenced by your second link.
With the second set of points disabled you set the first set to 32 degrees. Leaving the first set hooked up you hook up the second set and set the total dwell setting to 41 degrees by adjusting only the second set. Now you are done set the timming and go racing.
Randy
R J Brown

Being webmaster for CMGC, I had to look at that article again, and I can say it has aome errors, and some slight misrepresentation. I'm glad I didn't write it, but now I may have to get in there to revise it. Yhe first error is the statement that first points open gives spark. Actually, you don't get the spark intil the second set of points opens to break the ground contact.

Degrees of dwell expressed in that article would be apparently at the crankshaft, while common notation gives dwell angle at the distributor (half as much). What you measure with a dwell meter is dwell angle at the distribuitor. Cranking open the points gives less dwell, not more. Changing the dwell does not screw up ignition timeing, if you just set the timing after setting the dwell. Enough about that article.

I have a Mallory Dual Points (non-vacuum) distributor in my car, and I have the spec sheet in my hand. For the 4-cylinder model dwell for each set of points is 32 degrees. There is 8 degrees offset between the two sets of points, so the total effective dwell is 40 degrees (at the distributor). You can double those numbers for dwell angle at the crankshaft.

For comparison, the Mallory Single Points 4-cylinder model has a dwell angle of 35 degrees. There is not a lot of functional difference between 35 or 40 degrees dwell. I have seen race engines running up to 9000 rpm with a single points distributor and never miss a spark. I can remove one set of points in my Dual Points unit, reverting to 22 dgrees dwell, keeping ignition timeing the same, and run up to 7500 rpm with no noticable difference in performance. I am certain you would have to have the car on a dynomometer to notice any power difference due to the dwell angle (assuming the spark timing is the same).

For a street engine the dual points has very little advantage over single points, and vacuum type is generally better than non-vacuun type for low end performance and throttle response. A new distributor can of course be better than an old worn out one. Perhaps the best selling point of the Mallory Dual Points distributor is the wow factor in the name. Second best selling point is constructive pricing, where the initial purchase price is lower than a new Lucas distributor (bue the long term maintenance parts are more expensive).

You can seemy 160,000 mile user report here: http://mgaguru.com/mgtech/ignition/ig201.htm

If I had it to do over, I think I'd pay extra for the stock Lucas distributor and save more money long term on tune up parts. But some people are not terribly concerned about cost.
Barney Gaylord

My description was correct but I had it backwards. Reverse the words closed and open in my description to make it correct. (What was I thinking?)
Dwell is the number of degrees that the points are closed not open.
The definition of dwell is the number of Degrees of Distributor rotation that the points are Closed.
On a 4 cylinder ditributor there are 4 lobes on the distributor cam each lobe is 90 degrees of the total 360 of the distributor. The dwell angle spec on a MGA is 60 +/- 3 degrees. Because points close as they wear and cause the dwell angle to increase the best place to set the dwell is at 57 to 58 degrees.
The first link you presented is wrong disregard it, the second one is correct.
I hope this makes it clear, if not ask again.
Randy
R J Brown

I understand that the dwell angle is the time where the ignition coil has a connection to the condenser, normally in a single pointed distributor, the period when the pointer is closed which again is ~60° of the 90°.
In a dual-point distributor the period without connection to the condenser (both pointer open) is from 9° to 32° = 24° which give a dwell period of 66° (90°-24°=66°).
The dual-point distributor is then only superior with 6° compared with a single pointed Lucas distributor?
Not only the article quoted from CMGC, but also MossMotors claim that the dwell angle with a Mallory dual-point distributor is 72° - please see:
http://www.mossmotors.com/MemberServices/TechTips/TechTipsArticle.aspx?TechTipID=177&ModelID=30000&SubcompCode=P100

Back to first part of my question: Why do I only get 41° combined dwell at my Dwell Meter when the correct is 66° - the combined dwell must be the time were both points NOT are open?

JF Hedetoft

My Mallory spec sheet says, DWELL TOTAL 39d-41d

Last time I installed new points was April '02, more that 25,000 miles ago. I probably re-adjusted them once since then, but certainly not this year. Assuming they may have worn slightly, I would guess the dwell may have increased a little.

So I just checked it. Points gap is close to the specifird .022" (maybe .020"). My dwell meter registers 44 degrees, rock solid, not a wiggle, which seems to agree with the Mallory spec sheet (nominally 41+/-2).

I have never seen any distributor spec'd with dwell greater than 60 degrees. 4-cylinder distributors might be nominally 60 or 45. 8-cylinder distributers might be 30 or 22.

I have no idea how a 4-cylinder engine could run with 72 degrees of dwell when the points must be closed every 90 degrees of rotation of the distributor. You may get that sort of condition with a single points distributor when the points are so badly out of adjustment so as to barely open a few thousanths of an inch. That's when the engine tends to sputter and die, because the points aren't open long enough to allow the coil to fully discharge.

Interesting that the tech articles on the CMGC and Moss Motors web sites are identical, word for word. The article on the CMGC web site pre-dates my tenure as webmaster. As with most of the tech stuff posted there prior to March 2000, I have no idea where it came from. In this case I suspect it may have been copied from the Moss Motors web site, or otherwise both articles were copied from the same source. (Moss wouldn't copy that from CMGC would they?). When you copy something which in incorrect, it is still incorrect afterward.

The Century Performance web site has the correct specifications here: http://www.centuryperformance.com/point_settings.asp
Barney Gaylord

Hi JF, I have the same distributor, it eat 2 condensers in the first year, however the third is 4 years old with 10,000 miles . I think they had a batch of low quality condensers.
w.g cook

Just when you thought this thread was dead, I'm back. This is primarily a response for J.F.Hedetoft, who started this thread, but others may find some benefit as well (especially with a new web page).

One point of clarification: For those of you who bought a Mallory Dual Points distributor to have longer dwell for a hotter spark, you have been bamboozled, or are misinformed. The Lucas 4-cylinder distributor has dwell of 60 degrees. Mallory Dual Points distributor has total dwell of 40 or 41 degrees (depending on which spec sheet). Mallory single points distributor has dwell of 35 degrees. Yes, dwell is the rotation angle when the points are closed (connection to ground). So if you want longer dwell, stick with the original Lucas distributor. This is NOT the reason why Mallory uses dual points (find that on my web page).

This large difference in dwell angle between various distributors is due to the the cam being much different. For various distributors dwell can be anything from 30 to 60 degrees, and open circuit can be anything from 60 down to 12 degrees. A V8 engine might run 6000 rpm with a single distributor making 400 sparks per second, and you still get a hot spark, so it's no big deal for a 4-cylinder to do 200 sparks per second (or more if you have a heavy foot).

So as neither dwell nor points open time is particularly important, then why would dual points have any advantage? The answer is accuracy, stability, long term durability, and a certain ease or convenience of adjustment. For lots more details on the how and why, I have just posted a new web page here: http://mgaguru.com/mgtech/ignition/ig200.htm There is more to Mallory than a name, but it is not longer dwell.

Just to set the record straight (from prior comment), the coil is always connected to the condenser, and the condenser is always connected to ground. Points are connected in parallel with the condenser, between the coil wire and ground. If the condensor goes bad as open circuit the spark will be very weak. If the condensor goes bad as short circuit you get no spark.
Barney Gaylord

Thanks to all for helping sort out my problems with the dwell angle, I now understand that the dwell is 41° and I have accepted it. But somehow I feel that I have been fooled - one of the reasons for buying a Mallory dual-point distributor was the bigger dwell angle.
Thanks to Barney Gaylord for the very competent instructions both on this site and especially on mgaguru.com

JF Hedetoft

As an MG Owner/Shady Tree Mechanic since 1962 and Electronics Technician since 1957, this thread was of great interest to me. The CONDENSER (Called a CAPACITOR in the Electronics World since the 1930s!) has to have the RIGHT CAPACITANCE in order to reduce POINT CONTACT "PITTING/MOUNDING" that can occur from the SPARK when the points open. This REQUIRED CAPACITANCE VALUE may be DIFFERENT if you have CHANGED your IGNITION SYSTEM TYPE, including a DIFFERENT COIL! Or maybe not. No "pitting/mounding"? IF IT AIN'T BROKE, DON'T FIX IT! EXCESSIVE "pitting/mounding" on you points? TRY a DIFFERENT CONDENSER (CAPACITANCE VALUE). You can MEASURE "CAPACITANCE" on some very inexpensive DIGITAL voltmeterS (DVM). Even the CHINESE "CHEAPIES" are surprisingly accurate. Incidently "LUCAS", "BOSCHE" and "MALLORY" make very good condensers, and I have NOT found EVEN ONE "BAD" (Shorted, Open, or Excessively Leaky Current) "LUCAS" condenser since 1962! THIS IS NOT TRUE FOR "DISCOUNT STORE" CONDENSERS SOME OF WHICH ARE LEAKING CURRENT RIGHT OUT OF THE BOX!
OLD BILL-67

This thread was discussed between 21/10/2005 and 10/11/2005

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