The Stovebolt Site Forums The Shop Area Engine Shop Dwell Angle

My new Dwell/Tach/Volt meter says the dwell is 39 on my 1953 216 Chevy 3100. What does this mean and is it within limits? Thanks

Dwell angle means the time the points stay closed. I think the book calls for 35°. 39° means your points are too close which also retards the timing. I like to set them around 31° to 34°. That way the rubbing block can wear a little and they will be in spec.

Thanks...great info..and simple too. I'll get this done over the weekend and get back with you. Thanks again.

The term "dwell" is very accurate, when the information is broken down into understandable terms. There are 360 degrees in a circle, and when the points are closed, they "dwell together". If 360 degrees of distributor rotation is divided by the number of cylinders, there's a possible dwell of 60 degrees for a 6-cylinder, 45 for a V-8, 90 for a 4-cylinder, etc. The magnetic saturation of a coil is best at about 66% dwell, which would be 30 degrees for a V-8, 40 degrees for a six, etc. A lot of the oscilloscopes I grew up using for engine diagnostics rated dwell in percentage instead of degrees. If a dwell specification or point gap isn't available, the 66% rule works very well. BTW, changing dwell one degree results in a 2 degree change in ignition timing, since the crankshaft turns twice the speed of the distributor. Close point gap (long dwell) results in late timing.
Jerry

Thanks for the great info Jerry!
I pretty much understood dwell to start with, but the way you explained it is a very easy to understand and I'm sure to remember it for a long time to come.

Grigg

41 years in the classroom- - - - -I can't count the number of kids I've presented with that info- - - -and some of them actually got the message! In the past three months, I've encountered at least six of my former students who are working somewhere in the automotive trade, from shop owners down to parts counter guys. One guy from 25 years ago keeps coming back for more! His latest request was for a reference to an employer. He's applying for a job with a speed and machine shop that specializes in offshore powerboat race engines!
Jerry

So how does an electronic ignition setup like Petronix deal with the issue of dwell? Is it simply a constant which is fixed within the system?

electronically of course

Bill

I don't know about Pertronix, because it would be hard to run fast enough to give me one, but the GM HEI system uses a sensor circuit in the module that monitors magnetic saturation of the coil. When the coil reaches full saturation, a current limiter kicks in. As speed increases, the dwell time lengthens by changing the power transistor turn-on point which would equate to "points close" on a mechanical system. The turn-on point can actually back up into the induced primary voltage oscillations from the previous cylinder. That's why a regular HEI starts to spark-scatter at around 5500 RPM unless the reluctor is modified. By cutting half the teeth off the reluctor, the induced voltage from the pickup coil is reduced, and the distributors I modify that way go to 7,000+ RPM. They're stock-appearing unless a tech inspector is really on his toes. That change, along with a few other "Sneaky Pete" tricks is why my engines are very much in demand on the "claimer" cars. It's a race class where a racer can offer to buy a competitor's engine for $1,000.00, and he has to sell or forfeit credit for the race.
Jerry

Hotrod Lincoln, Just wanted to pick your brain on the various uses of the 'vacuum gauge' with regards to tune up? With all those years in that classroom you may know more virtues of the vacuum ga. than the next guy. I've used mine for setting timing, valve adjusting and carb adjustments, got any more???
Thanks
Jim

There's lots of opinions about how to interpret vacuum gauge readings, and I'm sure all the usual trolls will pick my answers to pieces, but here goes- - - - - -:

Manifold vacuum is a function of the pistons creating a low pressure in the cylinders on the intake stroke, and that lower-than-atmospheric pressure is allowed to get into the intake manifold by the fact that some of the intake valves are open and the throttle plate in the carburetor is closed, at least partway except for full-throttle operation. The closer to closed the throttle can be kept, the higher the vacuum will be. Vacuum is measured in "inches of mercury", literally, the height in inches a column of mercury in a tube can be supported by the vacuum. A complete vacuum, which would be nearly impossible to achieve, would support a column of mercury approximately 30 inches tall. What is actually supporting the column is the pressure of the atmosphere pushing down on the reservoir of mercury the tube is inserted into. No matter how much suction is applied, it is impossible to lift mercury higher than that distance at sea level, and the higher in altitude one goes, the less mercury the atmospheric pressure will support. Very low vacuum is measured in "inches of water" instead of mercury. A perfect vacuum would support approximately 34 FEET of water.

An engine with the right idle mixture and ignition timing, plus good mechanical condition of the rings, valves, and gaskets should have a reasonably steady manifold vacuum around 18 inches of mercury or more. The timing and lift of the camshaft will have a big influence on how high and/or steady the vacuum will get from tune-up adjustments. A single-cylinder miss, whether caused by bad ignition or incorrect fuel, will cause the vacuum to drop every time that cylinder fires, or tries to. The same sort of quick, steady vacuum drop can be caused by a leaky intake valve, or one that's adjusted too tight, letting compression get back into the intake on a regular basis. A steady, but very low vacuum reading can be caused by late cam timing, such as a timing chain or gear installed with the crank a tooth ahead of the cam. A VERY badly worn timing chain can cause similar low vacuum- - - -but I've never seen a gear drive cam get that bad without losing a tooth or two. Sloppy distributor bushings can cause erratic dwell and corresponding ignition timing changes, and that shows up as erratic, unpredictable vacuum fluctuations because of the random effect on ignition from one cylinder to another. An engine that lacks hill-climbing power and has extremely low manifold vacuum with even slight throttle opening under load is exhibiting symptoms of an exhaust restriction, like a plugged-up muffler or a catalytic converter that's clogged with carbon from an oil-burning engine. Obviously, low, erratic vacuum can be caused by badly worn piston rings. This symptom is usually accompanied by lots of blow-by from the crankcase vent system and blue exhaust smoke, as well as excessive oil consumption. In the days before electronic diagnostics, a vacuum gauge, dwell meter, and a compression gauge were all the tools an experienced mechanic needed to troubleshoot an engine. The same techniques work today!
Jerry

Thanks Jerry, I've been tuning on this '55 265 V8 that I've just aquired, vac is 21" Hg with compression at 125 on all eight. So I'm hoping this motor will last me a while. Previous owner may have changed the oil once in the past 20 years but that's about the extent of his caring for this car. I've done quite a lot of tune up on this car plus complete brake job and lots of lube and tweaking.
Thanks for the tutorial.
Jim

If you want to know more about the HEI, you can add this to what Hotrod lincoln gave you.
HEI module
The MC3334 is part of what is inside the HEI (High Energy Ignition) Module. The pickup coil is on the left in Fig.1 and the primary side of the ignition coil is on the top right.

I agree with Hotrod's opinion of the pertronix

Good info, Richard! I'm glad to see that an aftermarket device has been developed that includes a current limiter- - - -GM used to insist that nothing but OEM modules be used for service parts because of overcurrent problems with aftermarket replacement modules. One economy measure the replacement parts people took was to eliminate the current limiter in their designs.

Here's another bit of trivia about dwell meters- - - -they're really nothing but voltmeters! When the points are open, the voltage at the distributor side of the coil, where a dwell meter is attached, and ground (where the other lead goes) is always going to be applied voltage because there's no current flowing. With the points closed, the voltage will be zero, because the points provide a ground for the coil primary. The dwell meter is reading an average of the voltage between points open and points closed, and the longer the points remain closed, the lower the indicated voltage will be. The "voltmeter" scale is simply calibrated in degrees of dwell, rather than volts. That's why some dwell meters need to have a selector for 4,6,or 8 cylinder measurement- - - -the meter has to discriminate between the number of times the points open and close for a particular engine. Other meters are self-calibrating.

"point resistance" is also important for good dwell adjustment. If the points are dirty or burned, the meter will not see a true "zero volts" reading at the points closed position, because a little voltage is consumed pushing the primary current through the points and the dwell indication will be inaccurate. Always be sure the points are new, or at least clean, before setting the dwell. Some meters include a "primary resistance" feature which involves checking the meter reading with the points closed and the ignition turned on before doing an adjustment.
Jerry

I probably didn't explain that very well. The Motorola IC(Integrated Circuit) is part of what is inside the GM HEI. The drawing below that I found on the web is a quick and dirty depiction of the HEI module. The Motorola portion is what they are calling a signal converter. The npn transistor is actually a Darlington power transistor labeled MJ10012 in the block diagram in my previous link.

http://www.jetav8r.com/Vision/Ignition/hei.gif

I have had very good results with the Pertronix units I have installed. I hadn't heard bad about them either until I read here where a couple of folks don't like them, or think well of them.

I sell these at the parts store where I work for farmers with old tractors and old grain trucks that they get tired of replacing points all the time. So far the track record with them has been good. In the 14 years I've been selling them, only one has come back.

Roy

There's an RPM-related sine wave voltage developed by the reluctor that serves to turn the module on and off when it's applied to the integrated circuit. On the GM system, there's as many high spots on the rotating shaft as there are cylinders, ditto for the pickup coil. Each time they align, they generate the positive part of the sine wave. As RPM increases, the amplitude of the voltage gets high enough that the signal tries to turn the power transistor on before the induction in the primary winding dissipates from the previous spark. That creates a momentary impasse that effectively shuts the ignition down, or causes a stutter in the performance known as "spark scatter". In plain terms, the engine quits accelerating somewhere between 5,500 RPM and 6,000 and starts missing and backfiring. If every other point of the rotating magnet is ground off, the remaining ones still align every time a spark is needed, but the amplitude of the induced voltage is cut in half. This allows for at least 1,000 extra RPM, sometimes more, without the stutter coming in. That's enough to win races, when the competition is limited to stock, unmodified components. (It's only cheating if you get caught!)
Jerry

That's all well and good for those who are racing their engines. But for old beaters like mine the Pertonix works quite well. I'm not trying to get anywhere close to 5,000 rpm with my old 261.

The ignition module failed years ago on an old AMC Pacer that I had. Another one at the time was outrageous on price. I put an older point style distributor in it with a Pertronix unit in place of the points and drove it for years without any trouble.

They are like anything else, they can be a reliable tool when used correctly.

But I also understand your point as well.

Thanks

I'm sure the Pertronix is a viable substitute for points, the same as the custom-fashioned war club was a suitable replacement for a tree branch to my Cherokee ancestors. It was even a giant leap forward in technology. The problem I have with Pertronix is the sloppy fit of the trigger wheel under the rotor on the ones I've seen, resulting in unpredictable timing from one cylinder to another, regardless of speed, and enough anecdotal accounts of sudden failures that the conventional wisdom here on the forum is "carry a set of points and condenser in the glove box" in case of a breakdown. The price is also a turnoff. It's possible to fashion a better, less expensive system using off-the-shelf GM or Chrysler parts, and in case of a breakdown, repair parts are as close as the nearest big-box FLAPS. Pertronix seems to be better than the original point-type system, but not much better.

The race modification was just a heads-up for people who do hotrod the GM system. It's quick, reliable, and very hard to spot at the head of the tech inspection line. There's also a couple of other go-fast tricks I use that are very effective in the low-budget racing classes that require "stock-appearing" parts.
Jerry

Maybe my experiences with them must be the exceptions rather than the rule.

I've removed my distribitor for cleaning and inspection and to replace the vacuum advance module with a new one. What is the small piece on the side of the distributor that contains what appears to be heavy grease. Do I need to clean this and replace with fresh grease?

It's something called "cup grease" which was supposed to lubricate the upper bushing of the distributor. I doubt if it's even sold anymore. It's too thick to be pumped through a pneumatic grease gun- - - -we got a drum of it by mistake at a service station where I worked in the early 1960's and it clogged up the whole lube system. If you clean it out, try using disc brake wheel bearing grease as a substitute. Fill the cup about halfway, and screw it onto the fitting, after cleaning out the passageway all the way to the distributor shaft with a small drill bit. Once the passage is filled all the way to the shaft, top it off and tighten the fitting until it's snug. Give the cap about 1/4 to 1/2 turn at every oil change.
Jerry

There is a little spring and rubbing block of some material(?) in there. Don't lose it when you dismantle to clean.

Thanks Drew and Jerry...I plan to break it down to clean later today. I'm sure I'll be back with more questions. Thanks again.