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  #51  
Old 03-13-2016, 12:59 PM
Mike S's Avatar
Mike S Mike S is offline
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Default toothed wheel

OK, gotta ask---------is the toothed wheel a one off item, or something commercially available for purchase?



Is there any reason the pickups cant be mounted at the case split line, thus removing all the heavy bar stock used to offset the pickups. Of course if the pickups are mounted at case center, the skip tooth must be offset to compensate.

Considering all the stuff in the prior pages about the drive speed of a six cyl mag/rotor assembly, I suspect your mag hole trigger may not be a viable option for me???

As I read these, your setup is actually running twice the need RPM at the pickup--------and most likely wasting a spark in the exhaust stroke??
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  #52  
Old 03-13-2016, 10:34 PM
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Quote:
Originally Posted by Mike S View Post
OK, gotta ask---------is the toothed wheel a one off item, or something commercially available for purchase?
I can send a .dwg file, and you can take to a waterjet shop. Same thing really, and for you, it's free...the drawing, that is.

Quote:
Is there any reason the pickups cant be mounted at the case split line, thus removing all the heavy bar stock used to offset the pickups. Of course if the pickups are mounted at case center, the skip tooth must be offset to compensate.
Mount 'em anywhere you like, as long as they are 60 degrees (for a 6-cyl) after the skip when the crank is at TDC. Don't discount the need for a stiff bracket.

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Considering all the stuff in the prior pages about the drive speed of a six cyl mag/rotor assembly, I suspect your mag hole trigger may not be a viable option for me???
Not viable given a 540's mag drive ratio. You need a trigger wheel turning at crankshaft speed.

No particular need to use the big wheel. Heck, do something like the Electroair crankshaft wheel, in 36-1 teeth rather than 60-1. It clamps on the shaft just behind the prop hub.

Looks like this:

http://www.electroair.net/5C_installation.html

There are a great many VR's available, auto and industrial. I don't think the EDIS unit cares as long as the signal is a nice sharp sine wave.

http://www.phoenixamerica.com/techno...e/default.html

http://www.spectecsensors.com/html_p...ic_sensors.htm

Quote:
As I read these, your setup is actually running twice the need RPM at the pickup--------and most likely wasting a spark in the exhaust stroke??
It is a very conventional wasted spark system.
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Last edited by DanH : 12-05-2018 at 08:14 AM.
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  #53  
Old 03-14-2016, 09:15 AM
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Originally Posted by DanH View Post
I can send a .dwg file, and you can take to a waterjet shop. Same thing really, and for you, it's free...the drawing, that is.


Please do, appreciate it.
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VAF 909

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"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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  #54  
Old 03-14-2016, 10:15 AM
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A few words about reluctor polarity, for those who want to experiment.

It's not unusual for a new EDIS/Megajolt installation to make no sparks on the initial try at cranking. The first thing to check is reluctor polarity; swapping the two reluctor wires often cures the problem.

Back in the 90's we learned that an ND pickup with reversed polarity would trigger a GM module, but the result was (1) imprecise timing (the timing marks would look real fuzzy when viewed with a strobe), and (2) timing that retarded with increasing RPM

The reluctor choice, tooth shape, and RPM combine to shape the waveform. In general, all the VR-triggered ignitions fire the plug when the output sine wave crosses zero volts, moving positive to negative.

Here's the cranking-speed output from an ND reluctor wired correctly. At low speed the negative-to-positive slope is gentle, while the positive-to-negative is steep. Because the slope is steep, the time range around zero is short, so there is little variation in timing from event to event. The slopes will steepen as RPM rises (more RPM means more cycles in the same time period), a happy property which improves timing precision. (Note to geeks; about the o-scope display, yeah, it was a quick snapshot, and I forgot to center the wave on the scope's zero line. Here the 5-line will just have to substitute for the zero line, ok?)



Just to illustrate, next I'll flip the same photo to simulate what the waveform would look like if the reluctor were wired backwards. Now the negative-to-positive is steep, and the positive-to-negative is flattened, thus the time near zero crossing is a much wider range. As a result, spark delivery becomes imprecise at low RPM, and will move as the slope steepens with increasing RPM.



UPDATE: Stumbled across a good explanation (in the Bowling and Grippo Megasquit manual) of why the EDIS often refuses to fire if the pickup polarity is reversed:

Note that you must get the sensor polarity correct for the missing tooth code. This is even more important than with regular triggers, because if you get the signal polarity reversed you will get two (½n+1)*t missing tooth 'gaps', instead of the desired one (n+1)*t gap (and the engine might not run at all).

Go here, near the bottom of the page: http://www.megamanual.com/ms2/wheel.htm

Excellent stuff on reluctors and Hall effect sensors here: http://www.megamanual.com/ms2/pickups.htm

If you're a long time Contact subscriber, you'll find the ND experiments written up by Paul Messinger, circa....ummm, 1996 or 97.

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Last edited by DanH : 12-05-2018 at 08:18 AM.
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  #55  
Old 03-14-2016, 12:00 PM
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Quote:
Originally Posted by DanH View Post

Not viable given a 540's mag drive ratio. You need a trigger wheel turning at crankshaft speed.

Quote:
Originally Posted by DanH View Post
It is a very conventional wasted spark system.
OK, just a bit of mental gymnastics here.

Does the system really need a trigger wheel turning at crankshaft speed, or does it simply need to see the correct number of pulses per rev??

A 24 tooth wheel turning 1.5 crank speed gives the required pulse count.

As long as the wasted spark is not happening during something important, this should work---------I hope.
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VAF 909

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Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."

Last edited by Mike S : 03-14-2016 at 01:27 PM.
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  #56  
Old 03-14-2016, 12:13 PM
rmeyers rmeyers is offline
 
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In the course of this thread there have been several mentions of toothed wheels in 36-1 and 60-1 teeth. With all due respect, I would like to point out that the 60-1 tooth wheels are in fact 60-2 tooth wheels.

Just didn't want somebody making one for themselves and then not have it work.
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  #57  
Old 03-14-2016, 12:52 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Mike S View Post
OK, just a bit of mental gymnastics here.

Does the system really need a trigger wheel turning at crankshaft speed, or does it simply need to see the correct number of pulses per rev??

A 48 tooth wheel turning 1.5 crank speed gives the required pulse count.

As long as the wasted spark is not happening during something important, this should work---------I hope.
pulses per rev is a software operation separate from determining the revolution and a revolution is 360 degrees

If the software is set up for crank speed, it needs cranks speed. Some software can be set up to run at cam speed or .5X crankspeed, but then need a crank synch of some sort. In my auto experience, I know of none that operate off a timing signal that is 1.5 X crankspeed though the software can do most anything if designed for it. I would guess that anyone doing a mag-based trigger on a 540 would be doing it in software, though it could also be done via gearing.

It is not the number of teeth that is the issue, it is the missing tooth/teeth. The missing tooth tells the software when you are at TDC or technically at some unique position. As Dan mentioned, the s/w uses an offset value to relate this to TDC. The s/w expects to see it once per revolution. The teeth themselves help to provide a more stable RPM indication (more teeth = more reliable RPM indication) and allow the software to determine the actual crank position at any point in time (s/w can then fire sparks at specific crank angles). The software will also count the teeth to predict TDC if it somehow loses the missing tooth position. The software re-synchronizes the cycle at the missing tooth each revolution. It also compares against the prior revolution to look for anomalies so it can post errors when appropriate.

Larry
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Last edited by lr172 : 03-14-2016 at 01:13 PM.
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  #58  
Old 03-14-2016, 01:39 PM
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Originally Posted by lr172 View Post
pulses per rev is a software operation separate from determining the revolution and a revolution is 360 degrees
Yep, understand that.

The MegaJolt is designed to use a 36-1 so that is the formula I was trying to achieve when dealing with the 1.5 drive ratio. Ten degrees of crank rotation per tooth, to make the existing software happy.

By the way, I had a bit of a brain fart earlier, it should be a 24 tooth wheel, not a 48-----------at least that is what my brain now seems to think. And, yes i did not mention the missing tooth, figured that was a given in all this.

And, I suspect there is yet another factor to be considered-------that is tooth size/shape. Too small a tooth, or too narrow of a tooth would most likely not trigger the pickup correctly.
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Mike Starkey
VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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  #59  
Old 03-14-2016, 01:53 PM
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DaleB DaleB is offline
 
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Quote:
Originally Posted by Mike S View Post
By the way, I had a bit of a brain fart earlier, it should be a 24 tooth wheel, not a 48-----------at least that is what my brain now seems to think. And, yes i did not mention the missing tooth, figured that was a given in all this.
Problem is, no matter what you do the index pulse will not occur once per revolution of the crank.
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  #60  
Old 03-14-2016, 02:34 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Mike S View Post
Yep, understand that.

The MegaJolt is designed to use a 36-1 so that is the formula I was trying to achieve when dealing with the 1.5 drive ratio. Ten degrees of crank rotation per tooth, to make the existing software happy.

By the way, I had a bit of a brain fart earlier, it should be a 24 tooth wheel, not a 48-----------at least that is what my brain now seems to think. And, yes i did not mention the missing tooth, figured that was a given in all this.

And, I suspect there is yet another factor to be considered-------that is tooth size/shape. Too small a tooth, or too narrow of a tooth would most likely not trigger the pickup correctly.

It has nothing to do with the number of teeth, only the rotation synch, which is achieved with the missing teeth . The software is designed to see a missing tooth once per crank revolution. It is expecting a one to one relationship between the synch event and one full engine revolution. Everything else if based upon this, including the # of degrees between spark events.

If I crank up the voltage to your clock and make it spin twice as fast, the big hand will do a full revolution in 30 minutes. You now need to re-wire your brain to convert that full cycle to mean something different (48 hours in a day). This is the same concept of teaching the EI computer to process differently.

I just don't think you can get there from here with your current idea.

Larry
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Last edited by lr172 : 03-14-2016 at 02:38 PM.
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