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A question of dwell

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blumontag View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote blumontag Quote  Post ReplyReply Direct Link To This Post Posted: May/10/2019 at 10:23am
Thanks for all the input.
I'm borrowing a friends dwell meter to see how it reads.
The points I bought at NAPA are Echlin (not cheap) and are spec for the 232 and 258.
I will check that they are aligned properly.
I also have a new set of cheaper Standard Motor Products points I could try on there.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: May/10/2019 at 11:37am
Originally posted by blumontag blumontag wrote:

Thanks for all the input.
I'm borrowing a friends dwell meter to see how it reads.
The points I bought at NAPA are Echlin (not cheap) and are spec for the 232 and 258.
I will check that they are aligned properly.
I also have a new set of cheaper Standard Motor Products points I could try on there.


Its a great idea, and recommended... condensers are very much rated like capacitors, not only by farads, but also by tollarances to its value. I figure out of 10 new packaged condensers, many will give off different readings on their values when measured by a capacitance meter.

Given the values from the previous reply on engine types having specific condensers, a larger value will make the points act like they are delayed on timing, and the switching current will act closed slightly longer, when the points are actually opening. A lower value will make the the points more repsonsive and will cause premature burn up of the points at high RPMs. Thus the reason for dual points on very high performace engines, that run top end 90% of the time.


Not many check capacitance on new condensers any more, but back in the day i am sure many racers have done so, and probably kept it a garage secret, with many other odd tweaks and checks before assembly.
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: May/10/2019 at 10:02pm
Originally posted by 304-dude 304-dude wrote:

Its a great idea, and recommended... condensers are very much rated like capacitors, not only by farads, but also by tollarances to its value. I figure out of 10 new packaged condensers, many will give off different readings on their values when measured by a capacitance meter.
Not many check capacitance on new condensers any more, but back in the day i am sure many racers have done so, and probably kept it a garage secret, with many other odd tweaks and checks before assembly.


good ideas all.

the capacitor needs to "match" the coil. it's a series-resonant circuit, but ideally, in the instant after the points OPEN the magnetic energy built up in the coil during dwell folds back on itself ("collapses"), and transforms the energy stored over 5 to 15 milliseconds, into a huge brief (microseconds) voltage spike -- if all is well, that energy dissipates into the spark plug gap. if it doesn't, the coil "rings" (the series-resonant thing).

there's some particular capacitance value that provides a low-impedance path for the coil primary, but doesn't induce too much ringing after the plug dissipates the coil energy. there's probably a world of theory and practice tweaking the cap value for some characteristic or other. i bet there's a bunch of geezer racers taking that knowledge with them.

any idea what the criteria is?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: May/11/2019 at 6:03am
Originally posted by tomj tomj wrote:

Originally posted by 304-dude 304-dude wrote:

Its a great idea, and recommended... condensers are very much rated like capacitors, not only by farads, but also by tollarances to its value. I figure out of 10 new packaged condensers, many will give off different readings on their values when measured by a capacitance meter.
Not many check capacitance on new condensers any more, but back in the day i am sure many racers have done so, and probably kept it a garage secret, with many other odd tweaks and checks before assembly.


good ideas all.

the capacitor needs to "match" the coil. it's a series-resonant circuit, but ideally, in the instant after the points OPEN the magnetic energy built up in the coil during dwell folds back on itself ("collapses"), and transforms the energy stored over 5 to 15 milliseconds, into a huge brief (microseconds) voltage spike -- if all is well, that energy dissipates into the spark plug gap. if it doesn't, the coil "rings" (the series-resonant thing).

there's some particular capacitance value that provides a low-impedance path for the coil primary, but doesn't induce too much ringing after the plug dissipates the coil energy. there's probably a world of theory and practice tweaking the cap value for some characteristic or other. i bet there's a bunch of geezer racers taking that knowledge with them.

any idea what the criteria is?



Gosh, i gave up on the matching coil and condenser idea long ago when I made HEI my choice.

NOTE: To buy 20 oem condensers to match to your coil of choice would be like finding a needle in a haystack. So, will explain the procedure in broadening the variances by being selective enough to make a cost effective and highly effective option for matching.

Reason it is not so critical in grabbing two dozen of one type to find a few that will match, is that even with a standardized tollarance set in condenser values, you may only see 1 to 2uf variance with a budget handheld meter. Also some internal characteristics in the way they are manufactured, can allow for properties that effect high rpm use. As when testing, the data gathered will make sense as to how frequency effects dwell over points adjustment. Usually the optimum setting for daily driving will not be the optimum for performance. Since rpm ranges will increase beyond what is set for oem operation.

Part two of the equation is resistance... some coils require a resistor and most using points will need one inline with tye coil at tye battery side. In a recent AMCENTHUSIAST'S post on finding a resistor for his dual points and coil setup, he had to hand select a proper resistor, to make street driving better. Note, street driving RPM was not the goal at first when tuning the ignition.

Given there are a dozen after market coils and hundereds of OEM coils to pick from.

My option to make a static control, is to pre-setup before testing live.

You will need a 12v bench test with a small automotive or flashlight light bulb as a replacement for coil, and electric motor drive set at 1000 to 1500 rpm. This will assist in obtaining static dwell around factory spec adjustment of around .016" gap. Verify with billd's measuring guide, that was posted a wee earlier, as a reply. Dial in with the best value on the meter. You may find that you may not get optimum dwell value, even if you have moved points gap up and down the adjustment scale. It is critical the points adjustment are left are static (untouched), by not using kit with integral condenser, as each change requires removal of the points.

As for your small army of condensers... for the fun of it, you can go with a few oem replacements for your engine and mark them at the bottom flat, as you want to keep the body clean from anything that may limit electrical contact. To add with the fun, obtain some big three factory labeled variants from similar engines of known high performance builds.

Install distributor with your choice of OEM condenser made for your engine, back on your engine. You will want to properly time the engine, before testing. Once everything is set. You will need to collect dwell measurment data from idle, 1500, 3000, 5000, and 7000 for your performance engine. Dwell will vary as RPM increases. You may also note that low rpm dwell also changed from being on the bench from being on the engine.

Do the same test for each of the condensers, and validate which one performs best at keeping dwell within the RPM range intended.

If any further adjustment is required, measure the resistance wire or resistor to the coil. Make note, and if you need to bring more dwell into the high rpm operation a higher resistance will need to be changed out. If you get too much dwell in high rpm range, a smaller resistance is needed.

Here is the link to AMCenthusiast's coil and resistance findings, to add to this information and topic. You will need to select privious pages to get the full details, as the link opens to his reply about his coil and resistance choice.
http://www.theamcforum.com/forum/xrv8-gremlin_topic76151_page47.html

Edited by 304-dude - May/11/2019 at 6:13am
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amcenthusiast Quote  Post ReplyReply Direct Link To This Post Posted: May/11/2019 at 11:41am
Your dwell should be just about 'perfect' if you set the gap to specifications.

My Chilton's Repair Manual says your 6cyl point gap should be .016" and your dwell reading should be 33 (degrees)

My '71 AMC Technical Service Manual tells about installing a new set of points using a dwell meter method only... as follows:

" ...Turn car ignition 'on' and crank engine with remote control starter switch while observing the dwell meter reading. Adjust dwell angle by inserting a screwdriver blade in the adjusting slot of the breaker plate and moving the plate until the specified setting is indicated on the the dwell meter (33 degrees) Tighten retaining screw and recheck dwell angle. Install distributor rotor and cap, verify dwell angle with the engine running."

***NOTE: this is obviously done with the distributor cap and rotor taken off (so the car can't start) -so you can turn the engine over and adjust the points with the distributor spinning, while watching the dwell meter... 

----------------------------

If point gap or dwell meter reading is not close to specifications, this very likely indicates some type of mechanical error... could be wrong set of points and/or like points aren't sitting flat on the breaker plate, something bent ...only guessing now ...but there are always a thousand ways something can go wrong & this is called 'Murphy's Law'!

IMO, it is very-very doubtful the metal lobes on the distributor shaft mechanism are worn; the plastic rubbing block on the breaker point is made to wear down first.

----------------------------

Just keep trying.

It's not hard 'to master'; installing a new set of points.

We all have our 'good days' and 'bad days'.

Some days you have to try ten times harder, just to do 'something simple' on any other day.


Link to XRV8 Race Parts website: www.amcramblermarlin.1colony.com
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Post Options Post Options   Thanks (0) Thanks(0)   Quote blumontag Quote  Post ReplyReply Direct Link To This Post Posted: May/11/2019 at 3:43pm
Will check new points for straightness or other install error on my part tomorrow. Raining right now. We took the Gremlin out for a local car show earlier. "You don't see these things anymore, do ya?!!"
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: May/13/2019 at 6:25am
If it's a driver get a Pertronix and be done with all this. I'd only run points in a 100% restored car. Yes, points work just fine, and if you really like tuning every 3-6 months or just like them because they are a little anachronistic and you just like things like that, nothing wrong with it at all. All I can say is that if electronic ignitions were relatively cheaply available back then car makers would have used them. Higher performance and emissions (mostly emissions!) is why the bigger makers developed their own electronic ignitions in the 70s.

I always suggest updating a driver... really no point in not doing it (unintended pun there....). Better, more consistent spark = better efficiency and power, though you may not notice much of each if you keep the car well tuned.  The main thing you will notice is you don't have to get under the hood and tune as much as with points, and only have to replace the cap and rotor on occasion, unless something goes wrong. And of course spark plugs... though they will last longer due to the better spark.

A Pertronix can fail, nothing lasts forever or is failproof. Keep a set of points, a screwdriver, and a matchbook in the glove box in case the Pertronix does fail... though it's not likely. The only drawback is there is one extra wire that needs to be run to power the Pertronix unit. Use a black wire and it will only be noticeable on the closest inspection.


Edited by farna - May/13/2019 at 6:27am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote blumontag Quote  Post ReplyReply Direct Link To This Post Posted: May/13/2019 at 8:12am
This is a photo of the Pertronix 1162A, which I have read is recommended for the 258 in our Gremlin.
I printed out the install instructions. Second photo is my current coil setup. Would I just hook up the 2 Pertronix wires to the coil (leaving the wires currently attached...the brown and green), or is there some other wire involved? Sorry, but I don't have a strong grip on electronic stuff in cars! 





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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: May/13/2019 at 10:50pm
a good power transistor suitable for reliable ignition coil service, the transistor alone cost a modest dinner in 1970 dollars. that's why the good stuff (MSD, etc) was expensive and the cheap stuff was unreliable (Prest-No-Lite). today you can't even buy a transistor that crappy. for five bucks i'm buying an 80-ampere "smart" high side drivers that has built-in overload detection, overheat detection, load monitoring, you name it and acts like an "ideal" part. that was 2016. now i'm looking at one that has four drivers in one package, MORE monitoring and processing guts, and less money. and smaller. that's two years of change.

the revolution in electronics is hard to grasp. every field that electronics has touched has advanced more in the last 30 years than it did in the last 300 years, without exaggeration (eg. navigation, communications, ...)

here's a 1962 transistor price list. 2N174 and 2N176 were used in car radios (final audio). awful things, but so much better than tubes. https://worldpowersystems.com/ARCHIVE/solid-state-datasheets/Motorola/misc/images/price-1.jpg

a phone today has 2 billion transistors.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote blumontag Quote  Post ReplyReply Direct Link To This Post Posted: May/14/2019 at 8:30am
Borrowed a dwell/tach meter from a friend. It read 33, while mine read 28 (at about .015 gap). I reset gap to .016 and now have 31 dwell.  Bottom line , I believe my ancient dwell meter (a Craigslist buy) is off. Thanks for all the input...and I'm looking hard at the Pertronix as an option.
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