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  #1   IP: 174.67.229.161
Old 12-05-2013, 03:46 AM
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Ignition 101

I wrote this summary of (distributor/points) ignition, based on my recent exchanges here. Hopefully others will find it helpful.

How (old school) ignition works.
Or – what I learned from Dave Neptune, Neil Dutton, Hanley Clifford, John Cookson and others.

In a 4 stroke engine, the piston rises and falls twice for each explosion of fuel. Slightly simplistically, its like this -
TDC (Top Dead Center)
Inlet stroke – piston goes down, inlet valve open, gas in.
BDC
Compression stroke - piston goes up, valves closed
TDC - spark ignition of gas mixture
Power stroke - piston goes down, valves closed
BDC
Exhaust stroke - piston goes up, exhaust valve open.
(Both valves are open at the top of the exhaust stroke - referred to as "overlap".)

Battery voltage is supplied to the coil – intermittently- by the points, which are actuated by a cam on the distributor shaft. When the points close, power is supplied to the primary of the coil. The coil, an inductive device, produces a high voltage spike at the moment of the collapse of the induced magnetic field – called flyback voltage. Voltage in multiplied by the primary/secondary relationship.

Points gap is the maximum opening of the points, which is when the shoe rides on the highest point of the cam.

Dwell is the period of time the points are closed, measured in degrees of distributor shaft rotation. Ie this is the period during which the coil ‘charges up’.

The Condensor or capacitor. The primary winding and the capacitor form a ‘tuned circuit’. Stored energy oscillates between the inductor formed by the coil and the capacitor. The condenser, which is connected across the points, is also said to absorb back emf, protecting the points from arcing/burning.

The high voltage spike is passed to respective spark plug by the rotor of the Distributor. As the rotor rotates it contacts the contact for each spark plug lead in succession. The distributor rotates at 1/2 the speed of the crankshaft – thereby synchronizing with every second TDC of each piston!

The distributor is thus two separate machines which happen to be on the same shaft. The points supply power (12v) to the coil, the rotor supplies 20000 volts to the sparkplugs.

Timing refers to the moment of spark ignition with respect to piston TDC. Advanced timing is when ignition occurs before TDC. Retarded ignition when ignition occurs after TDC.

Centrifugal advance is caused by weights and springs under the plate in the distributor upon which the points and condenser are mounted.

There is a useful video at http://www.youtube.com/watch?v=W94iksaQwUo
This discussion is also helpful http://www.ratwell.com/mirror/users....r/ketterin.htm

Timing Adjustment
Timing can be adjusted by loosening the clamp which lock distributor body to block, turning it a few degrees - when the engine is running. Best position is maximum smooth revs for any throttle position.

The manual says - "Take the boat out for a trial run end after bringing engine to normal operating temperature and the boat running at top speed. Loosen the distributor clamp bolt and caefully advance the ignition timing by slowly rotating the distributor body counter-clockwise until the RPM begins to fall off. Then rotate the distributor body in the opposite direction to obtain the greatest number of RPMs without rough running of the engine. The timing is now set properly in the advanced position. The spark automatically adjusts as the engine speed changes."

Distributor Cam Wear
Dave says- The lobe of the cam that wears over the years as it also wears out the many sets of points contact shoes. As the lobe wears there is far less lift (opening) required for the points to come back together at the same spot on the cam lobe, so when setting the "gap" (lift) to "spec" the points will stay open much longer (more degrees of rotation) on what is left of the "cam lobe". This holds the points open longer and gives them far less degrees of rotation closed which is the dwell timing that saturates the coil. (With an EI the timing is fixed and once set does not ever need adjusting unless moved). Once you have set a set of points and set the timing the timing is constantly on the move as the cam shoe on the points and the contacts themselves wear, that is why in the old days if you really took care of your car or boat you constantly had the timing readjusted because it is constantly moving as it wears.

Hanley says- Just make sure that you set your points by means of a dwell meter without regard to point gap specification; then check the actual point gap that yields your required dwell and post that number for us. This will provide useful information about the condition of your distributor cam.

Testing Spark.
Romantic comedy says – Take (spark plug) lead and hold 1/4 inch away from a ground (block). Remember that the rubber cap that fits on the plug will hold the lead away from the ground. Many use a spark plug hooked up to a plug lead, and grounded. Any plug will do. Blue is the best spark. Nice and fat if you can get it.

You can also do a quick and dirty test to the coil. Just hook up 12 volts to the positive side. Then hook up a ground wire to the other side. (no other wires are connected) Take the ground wire and touch to ground and off ground. This will cause a spark. Maybe not the best spark, but a quick test.
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Old 12-05-2013, 09:58 AM
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That's about it. I would add the reason for advancing the timing. There is a time lag between the introduction of spark and the complete burning of fuel that delivers the driving force. The goal is to have the peak force present at TDC which means the spark needs to initiate combustion a little before TDC. As engine RPM increases the time lag remains the same so the timing needs to advance further to keep up, the reason for the advance weight system.

Ignition 102 might be all the things that can go wrong.
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Old 12-05-2013, 11:09 AM
hanleyclifford hanleyclifford is offline
 
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Ignition 103 would be another argument over the use of a strobe light to set timing to specification before doing the "tweak".
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Old 12-05-2013, 11:24 AM
Dave Neptune Dave Neptune is offline
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Talking Igniyion 103~S

Forget the trouble and go EI

Dave Neptune
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Old 12-05-2013, 11:48 AM
hanleyclifford hanleyclifford is offline
 
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Quote:
Originally Posted by Dave Neptune View Post
Forget the trouble and go EI

Dave Neptune
Amen to that!
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Old 12-05-2013, 11:50 AM
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Quote:
Originally Posted by hanleyclifford View Post
Ignition 103 would be another argument over the use of a strobe light to set timing to specification before doing the "tweak".
Why must there be an argument? Is there a downside to timing with a light?
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Old 12-05-2013, 12:09 PM
hanleyclifford hanleyclifford is offline
 
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Quote:
Originally Posted by ndutton View Post
Why must there be an argument? Is there a downside to timing with a light?
Not that I'm aware of.
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Old 12-05-2013, 12:15 PM
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Well then, there's the difference and why I would expect universal acceptance of timing with a light. Other issues we've "discussed" typically have trade-offs, while seemingly solving one problem they introduce others that often require further band-aid fixes with problems of their own.
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Last edited by ndutton; 12-05-2013 at 12:22 PM.
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Old 12-05-2013, 12:42 PM
JOHN COOKSON JOHN COOKSON is offline
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Quote:
Originally Posted by HalcyonS View Post
You can also do a quick and dirty test to the coil. Just hook up 12 volts to the positive side. Then hook up a ground wire to the other side. (no other wires are connected) Take the ground wire and touch to ground and off ground. This will cause a spark. Maybe not the best spark, but a quick test.
You can do the same test using the points.
Power up the coil with the points closed. Pull the wire out of the center of the distributor cap and hold it close to the engine to ground. Flick the points open and let them snap close while checking for an arc.
This procedure checks both the coil and points for proper function. If there is still no spark to the plugs the problem is after the coil. (Did you put the rotor in?)

TRUE GRIT
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Old 12-05-2013, 01:04 PM
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Just so that there is no misunderstanding, the addition of electronic ignition does "not" negate the need for accurate ignition timing. Of all of the A4's areas of "tuning speculation", ignition timing is at or near the top of the list, and for good reason. The practice of setting the timing "by ear" was brought on primerilly because of the extremely poor timing marks that Universal supplied on this engine. Add to that the fact that, in some installations like a Catalina 30, the factory marks are almost impossible to see. The main "potential" problem with "tuning by ear" or "power tuning", as I see it, is that we run the risk of putting the
"firing position" at or very near the detonation point. For the "engine" guys in the group, you will most likely agree that detonation will kill an engine faster than just about anything. Universal's rather conservative timing marks were established to accomodate a wide range of fuel octane ratings. If, for instance, you were to buy a tankful of "the good stuff", and then do your timing, things would probably be okay until you switched to a "lesser quality" gas sometime down the road. At this point, what was an acceptable
tune-up, now becomes a potentially "detonating" one.
Tom
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Old 12-05-2013, 03:16 PM
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thatch
please explain 'detonation'
thx
S
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Old 12-05-2013, 03:28 PM
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The A4 is such low compression I doubt it would be an issue.
What you get with the light is the ability to check the advance over various RPMs as well as set timing accurately. I am pretty sure a 6:1 A4 could run on 80 octane gas if you could still buy it.

From Wikipedia:
When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used), detonation may occur. Detonation is characterized by an instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the flame front. A local shockwave is created around each pocket and the cylinder pressure may rise sharply beyond its design limits.

If detonation is allowed to persist under extreme conditions or over many engine cycles, engine parts can be damaged or destroyed. The simplest deleterious effects are typically particle wear caused by moderate knocking, which may further ensue through the engine's oil system and cause wear on other parts before being trapped by the oil filter. Severe knocking can lead to catastrophic failure in the form of physical holes punched through the piston or cylinder head (i.e., rupture of the combustion chamber), either of which depressurizes the affected cylinder and introduces large metal fragments, fuel, and combustion products into the oil system. Hypereutectic pistons are known to break easily from such shock waves.[2]

Detonation can be prevented by any or all of the following techniques:

the use of a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate;
enriching the air-fuel ratio which alters the chemical reactions during combustion, reduces the combustion temperature and increases the margin above detonation;
reducing peak cylinder pressure by decreasing the engine revolutions (e.g., shifting to a higher gear, there is also evidence that knock occurs more easily at high rpm than low regardless of other factors);
decreasing the manifold pressure by reducing the throttle opening, boost pressure or
reducing the load on the engine.
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Old 12-06-2013, 12:31 PM
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Thank's Joe for posting "Wiki's" version of detonation, however, in the last paragraph they neglected to mention one of the most commonly recommended "fixes" which is to "retard" the timing. In fact most modern day autos have "knock sensors" which sense detonation and retard the timing automatically. In "cutting to the chase" on how detonation or very aggressive timing may be affecting our A4's, I feel that we have to look at the crankshaft and main and rod bearings. As we know the A4 has only two main bearings which means that there is no support next to the #2 and #3 rods. "If", under extreme loads, the crankshaft is flexing (as I suspect) this will definately lead to some bearing wear issues. Recently, during his engine rebuild, Hanley remarked about excessive rod side clearance which may be the result of this "crank flex". A couple of years ago, I recall some photos of some A4 main bearings that showed some excessive wear on the inner half of the crankshaft main cap which I feel also points in the direction of "crank flex".
Tom
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Old 12-06-2013, 01:42 PM
Dave Neptune Dave Neptune is offline
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Thumbs up Crank flex II

Tom, nice post on the detonation and flex. I'd like to add a bit to it. The flexing of this type of crank is a two fold issue. First as Tom stated under hi-loads. The cranks will "flex" more due to cylinder loads related to RPM's or "lugging" as it is often referred to. This lugging is low RPM and low vacuum settings. This flex is more from a hard push overloading the time on the power stroke against the crank, more RPM's mean more "short time" pushes making the same power, meaning lower loads.
However detonation is far more load on the crank shaft. If it occurs before TDC it is like hitting the top of the piston with a hammer, very harsh. If it is after TDC IE the "isolated explosive pocket of gasses" it is not quite so harsh but still problematic. Detonation after TDC in a low compression engine is very difficult to diagnose as it may not "ping" or pop enough to hear at all, the engine will just sound sort of loaded up to the seasoned ear.
This is why "total advanced timing" is so important. These low compression engines have a broader range of OK when POWER TIMING than their hi-compression cousins. To this point is why I always say as I was taught on duty rated engines that set like the A-4~swing the distributor to get your best and then back off a "TIC" (retard slightly). A degree or two off retarded will be almost negligible in loss of power and the engine will usually run smoother. If your go a few degrees to advanced the power gain will also be negligible as will the possible small detonations overworking the bearing and crank, vastly decreasing engine life.

Dave Neptune
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Old 12-06-2013, 09:32 PM
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Dave, Your expertise, particularly on tuning these little A4s, is always welcome. Several years ago, after acquiring my first A4, I phoned Don asking him to measure a connecting rod so that I could establish what the "rod ratio" of the A4 was. Rod ratio, for those who are unfamiliar, is the ratio of an engines rod length to it's crankshaft stroke. Engineers, long ago, figured out that the ideal ratio was about 1.9 to 1 for an internal combustion gas engine. Engines such as the early Olds, Cad and legendary Chrysler Hemi, had one thing in common, a rod ratio of about 1.9 to 1. This "long rod" technology has proven to be about the ideal combination to allow the piston to dwell at it's highest point, in the combustion stroke, allowing the air/fuel mixture to fully combust. The main benefit, for us A4 guys, is that our engines are very forgiving with a varity of fuel/air ratio and timing settings. What does all of this mean? Quite frankly I'm not completely sure, except to say that, " a wide varity of tuning setups seems to work well for these A4s".
Tom
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