Piston Slap: Dwelling on Ignition Timing


Hagerty Community Member Jeepcj5 writes:

I’ve successfully (at least in my mind) set point gaps and timing in various cars. The reason I say successful is because they seem to run well afterwards. I have a dwell meter that I’ve been told to use, but I just don’t understand it or what it’s even telling me.

How does dwell come into play? Should I not worry about it? If I do need to worry about it, can you explain it in a “Dwell for Dummies” sort of way?

Sajeev answers:

This applies to me, as I am clearly a dummy when it comes to dwell or any other aspect of mechanical ignition systems. I (poorly) define dwell time as the time it takes to make a good spark, but Denso properly defines it as “The period when the ignition system applies an electric current to the ignition coil’s primary winding.”

Dwell time and dwell angle is adjusted on mechanical ignition systems with hand tools, a feeler gauge, and dwell meter. You can’t do much on any vehicle after 1975(?), as electronic ignitions became standard fare. (Vehicles from the 1980s and newer can be tuned with a computer and software, but that’s irrelevant to this discussion.) No matter the ignition system, this concept is important because an ignition curve is crucial to efficient combustion.

Perhaps Jeepcj5′s experience proves that you don’t need to mess with dwell time (i.e. the part of the process that needs a dwell meter) very often, but that’s between an owner and their fuel economy calculations. And the sensitivity of their butt dyno, but now we’re really getting off the rails.

Ignoring dwell time and only using a feeler gauge for your points (i.e. dwell angle) is basically approximating your ignition timing. And since graphics and videos are better at this than my wordsmithing, let’s try a couple of selections from YouTube. Here’s a fun retro video on the concept of dwell angle.

And the video below is a solid example of how to set dwell angle and dwell time, among other bits of knowledge for analog ignition tune ups. I’ve started it at the part relevant to dwell time, but you might want to watch the whole thing. Stick around for the 10:15 mark, as Uncle Tony’s Garage rightly suggests, “It’s not rocket science, right?”

The only issue I have with analog ignition systems is the availability of quality-made points in our modern times. Condensers in the ignition system have the same issue but to a lesser extent, as they are generally more durable. If you can still get good quality points/condensers for your application, adjusting your dwell time can improve your vehicle’s performance over the long term. If you cannot, maybe it’s time to convert to electronic ignition.

So give it to me, Hagerty Community: What did I miss in this discussion of dwell time? I am far from an expert in this matter, and your feedback will only make this article better.

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.comgive us as much detail as possible so we can help! Keep in mind this is a weekly column, so if you need an expedited answer, please tell me in your email.



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    It has gotten to the place it would be best to go electronic ignition.

    You can upgrade to a higher voltage or you can convert the distributor you have to a lower voltage system.

    The ignition will remain set and no issues.

    Unless you really like playing with dwell.

    Back when Dad had a Chevy with that door in the side of the distributor cap, and I was just getting into tuning up the family cars (let’s say around ’63 or so. I was very young, and Dad was evidently not afraid of me screwing anything up), Dad asked me what tools I would need. I asked for a timing light, dwell meter and a Unisyn (’cause the other car wasn’t a Chevy, but a Mercedes 220 with a pair of Soles carbs). I also wanted a proper feeler gauge set, and some better sockets and ratchets.
    Dad complied, and I immediately went to the Chevy (a 283 4 speed Chevelle) and set the dwell and the timing. It ran perfectly well. That dwell meter proved useful on my Uncle’s 327/350 Corvette, the neighbor’s 215 Buick V8 and the Chevys and GTOs of my friends in high school.
    I also used the dwell meter on the dual point MoPar distributors on the family Barracuda and Mom’s Max Wedge Dart (we became a Mopar family for a while).
    And then, when I was up to my neck in British cars, the little screw in electronic triggers came out, and I jumped right in, installing Pertronix ignitors in everything that had points. The dwell meter has been on the shelf for decades… ‘Turns out, I have two of them.
    Electronic ignitions have made them redundant. Now, I set the timing once, and check it periodically, or if I have to remove the distributor for any service. It’s amazing how little tuning an engine takes once those little mechanical points are eliminated.
    So what has replaced the dwell meter? How about the “dial back advance” timing light, now with integrated tach and volt meter? I find this very useful in getting accurate timing settings.

    Ignition dwell is the time the points are closed, measured in degrees. When the points open the coil is saturated and the energy field collapses within the coil, creating the higher voltage needed to fire the spark plugs.
    Courtesy of my Mr. Luger, my teacher at Dix Hills High School auto shop, around 1973.

    I have a Factory Dual Point Distributor 1965 Police Interceptor 383 Mopar. I am ready to replace it with a
    a} Electronic Distributor and b} Multimeter. Also I have a 1974 454 Chev pick-up converted motor with 2000 Serpitien Belt system with air-conditioning and I can,t see timing marks on bottom crank pulley. Again a good multimeter might be the answer. . . Thanks

    Oldest car I own now is 1975 GM, so it has factory HEI distributor. Last time I used a dwell meter was over 50 years ago, but as I recall it was easy to adjust thru the distributor cap “window” on earlier GM cars.
    As far as points quality, I installed a new set a on front mount distributor Ford 8N three years ago. Would not start – no spark. I had to use a points file on the “new” points (that looked perfect right out of the box) to remove oxide before they would work.
    Granted, they did not have a “best if used by” date, but I never had a problem like that with new points before. Everything you can purchase now is imported.

    Hey Ken, you can even get an electronic ignition for your 8N now. Upgrade that and the coil and that old tractor will start better than a new one. No more crappy import points. And yes, I still have my dwell meter, circa ‘65- it looks cool on the shelf!

    Thanks Randy, but we sold the 8N 2 years ago after getting a new Kubota. For a 75 year old tractor it is still a dependable workhorse and I miss it for doing the road work around the compound. No way will the new tractor be around for 75 years.
    I will tell the guy that bought it about the ignition upgrade being available when I see him.
    Old school tools are great to display in the shop – young guys ask “what the heck is that for”.

    I extend my thanks to Randy Smith as well (and also to Ken_L) for the tip on electronic ignition swap for the 8N. Mine is still functioning (and still earning its keep) here on the ranch, but it is sorely in need of some major rebuilding. Been putting a few bucks into a Folger’s can to save up for part purchases, and will look into the swap idea. Working on those front mounted pieces on the 8N has always been a pain – would like to find the solution to that!

    I have been setting points with a matchbook forever. Get the gap right, and the dwell will be in the ballpark (that is basically what dwell amounts to). Judging from my experience, you do not have to be terribly precise to get good performance. Unless you have shoe polish numbers on your windows or a parachute on the back, I wouldn’t worry about dwell too much

    Matchbooks are getting harder to find than quality parts nowadays. I would suggest a feeler gauge, but I do agree, for most daily drivers, getting too involved with dwell is like sorting the peas on your plate by size before eating them. The other side of that, IMO, is that finding a decent dwell/tach machine and fiddling with it (learning how things work and what affects what when you make changes) can be extremely satisfying to the curious automotive minded person. And in some situations, one might even find a small bit of improvement if the cars has some performance mods. The question to answer is: will that get you and the grandkids to the ice cream parlor more quickly or with better mpg?

    And I forgot to say – if you’ve got a car with “some performance mods”, you’ve probably already done what hyperv6 suggests and converted to electronic ignition anyway.

    My matchbook these days is generally a tab ripped off of a cereal box. Much easier to find than my feeler gauges… where did I leave them this time?

    Dwell effects engine performance, but it also effects the life of the ignition coil, points, condensor, cap and rotor, ignition cables and spark plugs. Think of the coil as a bucket. Providing battery voltage to the primary coil charges the coil(fills the bucket). If the bucket runs over, it creates heat. If you fill the bucket completely full, the coil will have more energy when the points are opened, and that energy is transferred to the secondary side of the system. A coil capable of creating 50K Volts on the secondary side is great for supercharged engines making lots of boost, but wasted on a stock 265 in a ’56 Chevy. In fact, it will cause damage, sooner of later, to the ignition cables and spark plugs. The goal is not to charge the coil any more than the engine requires. Most spark plugs will live a long healthy life if the secondary volts are kept below 30KV, but even the best ceramic available will be damaged if continually exposed to 50KV. That is why selecting the right ignition coil, and dwelling it properly, for the application is so important. It’s kind of like putting a turbo on that ’56 Chevy, doubling the horsepower, and expecting the Powerglide to live behind it.

    There is only one adjustment on the points. A feeler gage always seemed like a more accurate way to set them since most dwell meters had iffy electronics essentially averaging the open time of the points with questionable accuracy.
    Timing is a totally different setting that needed to be done with the vacuum advance disconnected and the rpm set low enough to disallow centrifugal advance. Dyno shops could optimize timing curves. The rest of us just had factory specs.

    Good article on a seldom-discussed area, Sajeev.

    Like Ken_L, I remember how much easier it was, on those cars with the distributor cap “side-window”, which was simply a small sliding metal plate which allowed you to access an Allen-Key adjustment.

    The whole procedure could be done with the cap in place and the engine running!

    Also, set the dwell then check th ignition timing. If you set the timing first, then set the dwell, you will change the initial timing.

    Back in the day I had a ’58 Corvette with a built up small block. The cam and valvetrain could handle 8000 rpm but the single point mechanical distributor couldn’t. The stock points would bounce at high rpm causing ignition misfire. Tried some aftermarket contact points with a stiffer spring but all that did was increase wear on the plastic nub that rides against the distributor shaft, which meant more frequent point adjustments to compensate for wear. Switched to a dual point distributor that gave better coil saturation and a hotter spark at high rpm but still had the point bounce/wear issues to deal with. Final solution was to install an aftermarket electronic distributor. No more point bounce/wear issues after that!

    The intitial set up of points is with a feeler gauge. To get them fine tuned you adjust the dwell. Vacuum or any mechanical advance disconnected. The dwell sets the true discharge duration. As RPMs increase the points ten to float due to spring reaction delay. So the dwell ensures your points are opening at the ideal angle on the cam lobe inside the distributor. Most hand held dwell meters are too slow to do in car measurements. We used to have an old SUN machine with the scope inputs for timing. It was a relic but worked. Converting to a more stable and repeatable electronic ignition is a good investment. Especially if you drive it a lot. The days of spare points and match box in the glove box were not as glorious as some remember, especially on the side of the road at night during a storm.

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