I just bought and installed a C&H ignition system with Syncro Spark. With this system the ignition fires at set point at full throttle (about 28 to 32 degrees BTDC) and is then retarded as it senses lower RPM. The setup instructions say to time the engine by slowly turning the pro hub and seeing at what degree the spark plug fires. My question is, how does the module know that you are timing the engine for full throttle when you are only turning the prop hug slowly and how does the module know that when you actually start the engine that the first spark should be retarded? Does it take two flipping revolutions for the module to know to retard spark? Anyone know how this works??????????????

With a CH Ignition, and many others. The timing module is designed to "count" the number of revolutions of the crank in a given amount of time and adjust accordingly which makes for a small delay... almost unnoticable... At startup the module is preset to a given setting, i'm sure it varies from one ignition to another...

When you are setting up the ignition at installation you are not setting the timing for FT... just for TDC... the module then can adjust the spark for FT automaticaly

Hope this helps

If the module is preset to fire at about 4 degrees or so ATDC at startup (slow RPM), then when you initally set up the timing by turning the prop hub slowly, how does it know if your trying to set it up for full throttle at about 30 degree BTDC instead of just trying to start the engine?? What am I missing here??

I guess i'm just not fully understanding your question?

Does your setup include a bellcrank?
It's starting to sound like thats what your describing?

No, he has a very good question and I don't know the answer either...

If you slowly rotate the engine to time it at let's say 28 degrees, how does the module know that it is being static timed?? How does it not think that a slow revolution to time it is not an idle revolution? Does the module need to count several revolutions, or revolutions faster than say one per minute before it activates the automatic timing??

AV8TOR

Oh OK... gotcha...

Wow, well in that case i'm not sure either...

I would guess that it defaults to static timing untill it reaches a given RPM, but I'd ask [link=http://www.rcuniverse.com/community/profile.cfm?section=profile&forum=1&memid=24760]TKG[/link] (Terry works at CH Ignitions... just send him a PM)

I've had the same question on my mind for a while, and this is what I figure (I'm open to correction, of course).

I picked numbers that I could do in my head, without too much strain... ;-)

Assume that our example ignition varies the timing from 30 degrees advance at max RPM, to 0 advance at a 1200 RPM idle. The magnet will be set to pass the hall sensor at the maximum advance of 30 degrees (static timing).

At idle, that equates to about a 4 millisecond delay from when the magnet passes the hall sensor to when the ignition fires. So, regardless how low the RPM is, the maximum delay or pause between hall sensor and firing of the spark plug will be 4 milliseconds. When you are static timing the ignition, you are turning the crankshaft slow enough that this delay is insignificant.

Now, assume that when you flip the prop to start the engine, the equivalent RPM when the hall sensor fires is 600 RPM. This 4 millisecond delay will then equate to about a 15 degree delay (from the 30 degree max advance) before firing the spark plug; This will be equal to 15 degrees of advance at your "starting RPM". Perhaps not the optimum firing angle for starting, but certainly better than a magneto that is set to 28 degrees! By the time that crankshaft has made a couple of revolutions, the timing will be much closer to the optimum 0 degrees advance.


Hope that made sense.
Bill

Hi,

Then, I'm assuming, once the counter senses higher revs, it takes out the delay right?

Well that's an interesting explanation, and just might be the answer. It's all in the "timing". (Sorry, couldn't resist.)

I expect Terry will jump in here pretty soon and straighten us out if that's not it.

AV8TOR

[link=http://www.rcuniverse.com/forum/m_3187936/anchors_3187936/mpage_1/key_fundamental/anchor/tm.htm#3187936]I too, was a bit baffled[/link]

I think you've got it !! After reading some of the links to previous answers to this question, I like your theory the best. C&H says that their system doesn't measure RPM but the dwell angle timing (time that the magnetic switch is closed) to determine firing delay. There are a few variables here, the width of the magnet and pickup, and how far out from the centerline of the prop shaft (radius) that the magnet and sensor are mounted. Both of these would influence the dwell time. I assume that both of these factors have to be within certain limits. This explains how Syncro Spark knows to delay spark at low RPM on the very first flip. When setting up the initial timing, even though the system is saying to delay firing to zero degrees when you are slowly revolving the prop shaft, the delay time is irelavant because you are moving the prop hub so slowly. When you are flipping the prop to start (600 RPM or so), the delay starts to become relavant. This makes sense to me. Thanks

"C&H says that their system doesn't measure RPM but the dwell angle timing (time that the magnetic switch is closed) to determine firing delay. "

I'm havibg a REAL hard time swallowing that..... considering the gap will affect the width, and The magnet and sensor can be mounted in Sooooo many ways.... nope.... I ain't buying it....

[:-]

According to tkg Terry who works at C & H Ignitions, the dwell angle is indeed used to determine the engine RPM. Look at some of the previous post from tkg. He gives a good explaination on how their system works. RE:Electronic Ignition Theory 11/3/2004

I have to comment on the dwell subject....

Somone mentioned that distance from the prop hub center will effect dwell times? I'm sorry but this is not correct... regardless of how far from the center of the prop hub it is, it will take EXACTLY the same amount of time to make one full revolution... I think what you are thinking about is the speed of the magnet travels slower the closer it is to center and faster the farther out from center it is... but... it will still complete one full revolution in EXACTLY the same amount of time....

So... regardless of how large your prop hub is, dwell time is an accurate way to time the module... I can see why they used it... I could also see using a "counter" to measure rpm's to set the timing... but evidently they chose dwell as the better choice for them....

stephensackro, I would like to read that thread by TKG that you quoted, but I can't find it. (Slow dial up connection; "search" and I don't get along well here, but I tried.) Could you please provide a link to that thread??

Thanks!
AV8TOR

Here is the link to the forum that (I presume) Stephens is talking about: http://www.rcuniverse.com/forum/m_23...tm.htm#2311892
Excellent explanation on the CH Ignition. Looks like this system CAN in fact determine the RPM on the first flip, right down to 200 RPM. Even the wimpiest flip should be able to get that!

As Fence and others have previously stated, the size of the magnet and distance from the centerline of the crank would affect the dwell angle, and therefore affect the RPM "reading" on that first flip. I'm not sure how the CH unit would deal with that. tkg states "The Syncro spark is "calibrated" for a 3/16" dia magnet and will work OK with 1/8"-1/4" magnets"; though this does not address the issue of distance of the magnet from the centerline of the crank. The instructions on the CH site do not talk about this, but perhaps the written instructions that would come with a new ignition unit would.

Ace: sorry- I'll have to disagree on that. Yes - one revolution will take exactly the same time regardless of the distance from center; but the time that the magnet is under the hall sensor (dwell time) will be less for a larger diameter.

Again, easy numbers that I can work with in my head...

If we have a 1/4" magnet that is 1" from the center, it would be under the sensor for almost 1/25th of a revolution (just under 15 degrees or 2 milliseconds at 1200 RPM). This would be a "dwell angle" of 15 degrees. At 2" from the center, the dwell would be about 1/50th of a revolution (just under 1 millisecond).

I don't know if "THIS IS ROCKET SCIENCE" as tkg states, but it is obvious that some pretty smart people have done a lot of work to design & build these units for us.[8D]

Cheers
Bill

Been out of town for awhile...
OK the syncro spark DOES measure dwell to determine RPM on the first turn. The "magic " number is 180 rpm. Below that its fully advanced to allow proper set up, above that it is fully retarded.
It is "calabrated" to a 3/16" magnet on a 2" hub. Useable magnet is 1/8"-1/4". If the magnet is to large the dwell time is increased and the SS thinks your at a low RPM and stays advanced. SSSSoooo it won't work with magneto sized magnets. As the Dia increases the dwell time decreases so you are safe. NOTE the rpm is the same, but dwell decreases... think of a wedge of pie. The number of degrees in the wedge stay constant but each degree covers more distance. so the dwell of the same sized magnet gets smaller the larger the dia.

It is true that one revolution equals one revolution no matter how far out the magnet is from the center of the prop shaft, but the dwell time will not be the same. This is because, as you said, the magnet will travel faster over the sensor pickup the farther out from the shaft centerline that it is. The chip inside the ignition module has some logic that says a dwell time (time it takes for the magnet to pass by the sensor) of 15 microseconds equals so many RPM. In your example, if your magnets were farther out from the center of the shaft, then for the same RPM, the travel time over the sensor would be smaller. Lets say 10 microseconds. Therefore the logic in the chip would incorrectly calculate the RPM. That is why I think that the sensor distance from the center line of the prop shaft is a factor.

Terry: great info - thanks!

I like the pie analogy. Except that now I'm hungry!

Bill

Gotcha... I guess I missunderstood again... I'm good at that... LOL

I guess I thought that the "dwell" was the amount of time measured between the magnet triggers, at initialization, not including the time spent closed..

also, since a hall-sensor is a very "analog" device, as you increase it's distance from the magnet, the field becomes less precisely defined, it's actually round-ish, so as you move away from the magnet, without even thinking of motion, the field becomes narrower.... your pie has been made in a smaller diameter pie-pan !!

the experiments with gasoline running on full-time-powered glow plugs is getting more and more attractive...

[:-]

The hall switch generates a square wave. When the magnet approches it arms, goes high. It triggers when the magnet leaves, goes low. The duration of the square wave is the dwell time..

Terry, just one more mental exercise here then I'm done. As you pointed out the chip logic goes to full advance under the "magic" number of 180 RPM so that you can time the engine for set up. " TR8s-n-planes" also had a good point in stating that the maximium delay time at idle RPM is about 4 milliseconds. Since you are turning the engine over very slowly when you initally set the timing, the delay time, even when it is in effect is irrelavant. You would probably only be off a few hundreds or thousants of a degree. Close enough for government work !! Therefore you probably would'nt need the under 180 RPM=full advance programed into the chip logic. It might even be safer if it weren't there since if you tried to start the engine by flipping the prop very very slowy (under 180 RPM) then you would be trying to start the engine at full advance. This probably isn't a problem since that is a very slow flip. But the older I get, the slower I flip'em !
P.S. Great ignition products Don't know how you get all of that good stuff in the small iginition module box??

The clock speed on the CPU is 4mhz., 4 milliseconds is a LONG time.

Not compared to the 1 degree per second or so time when you are turning the prop hub very slowly to set initial timing.

Stephens: Terry can correct me if I'm wrong...

I suspect this is why they "cancel" the delay below 180 RPM:

The CH ignition does not use a fixed delay as I guessed in my first example. Using simple numbers again, the delay between sensor and firing at 1200 RPM would be 4 milliseconds (nice idle RPM), at 600 RPM - 8 milliseconds (a fairly healthy starting flip), at 300 - 16 milliseconds (a 'less healthy' flip), and so on. You have draw the line somewhere, and they chose 180 RPM. Below that, the system figures that you're not trying to start the engine, so it cancels the delay completely so that you can set the timing. If I understood Terry correctly in one of his other posts, the system only uses dwell timing for starting (up to 800(??)RPM), and then reverts to measuring between hall pulses (much more accurate).

VERY ingenious way to design an ignition system!!!