PULS/REV tach setting?

[13brv3]

Greetings,

I may answer this question myself at the airport today, but as verification, what factor is required for a 4-cyl Lyclone with standard mags, both L and R hooked to the EMS inputs?

I would expect that this should be 2, since that's the number of cylinders that fire for each rev, but the instructions say this is typically 1/2 to 1/4 the number of cylinders. That would make it either 2 or 1 for a four cyl engine. I just don't understand how it could be 1 for a 4 cyl engine.

Also, since both are hooked up, which one does it use when the key is set to both? Does it actually use both of the inputs, or just pick one?

Bottom line is that my tach is clearly reading low. It might be half of what it's supposed to be, but I'm not sure it's quite that far off. I'll check the PULS/REV setting first off today, and hope that I find it at 3 or 4, so I can reduce it to 2 which seems logical. If it's already at 2, then I'll try 1, but I won't have a good feeling about it unless I can understand how this works. I'll be updating the firmware on my D-180 as well, so I'll check to see what gets loaded into the setting after the update.

Thanks,
Rusty

 

[13brv3]

I'm talking to myself, but figured it might save Dynon some typing. I used an optical tach to check the RPM today, and the D-180 was reading half what it was supposed to be. The PULS/REV setting was 2, and I changed it to 1. Now the optical tach, and Dynon agree. I don't understand why one pulse per rev is a correct setting, but I learned a long time ago that you can't explain everything

Cheers,
Rusty

 

[dynonsupport]

Rusty,

If you hook to a mechanical mag, it's almost always 1/2 as many pulses per rev as you have cylinders, because it's based on the spark firing.

If you have an electronic ignition, they can design it to output anything they want, which is why you could have a 1/4 ratio. It's based on some guy programming a microprocessor which can output at a different time than the spark firing.

If you use a sender, it can also be anything the designer wants. A sender is just a wheel that spins and has something on it that makes pulses. They could put one on it per revolution or 100. They're not all the same so we can't tell you what to use.

Anytime your RPM seems off by a lot, it's almost always a PPR setting problem.

If you have the EMS hooked to two RPM inputs, the EMS just picks one that isn't zero. When the one you're on goes to zero (like during a mag check) it switches to the other one. If they are both zero it cycles until it sees one start going.

 

[13brv3]

Rusty,

If you hook to a mechanical mag, it's almost always 1/2 as many pulses per rev as you have cylinders, because it's based on the spark firing.

Thanks for the comments. This is exactly what I would have expected, and was the default setting (PPR=2 for an O-360).

Unfortunately, my tach read half what it was supposed to read. When I changed the setting from 2 to 1, the tach is now correct. I will eventually add a Lightspeed ignition, and REALLY like the latest ability to set each tach input as needed. At the moment though, I have two Slick mags, and hooked both mag inputs up at the keyswitch via the resistors supplied.

FWIW, both L and R positions on the keyswitch give the same RPM readings, and both are stable and correct. I don't have any problem using them like this, but I'd feel even better if it didn't seem like I was using the wrong setting

Cheers,
Rusty

 

[meljordan]

I know from experience that on a 4 cyl lyc the correct setting is "1". I believe that the reason for this is due to the nature of how a mag produces it's energy and the way the Dynon counts pulses. The spark that a mag produces alternate in polarity. The first spark goes positive and the following spark goes negative. This is the reason that when you clean spark plugs you need to follow a rotation pattern so that each plug gets approximately the same time running positive and negative to balance electrode wear. That being the case, the pulse output on the mag "P" lead also goes positve on one spark and negative on the next. I speculate that the Dynon only counts positive pulses, so it then only sees one event per revolution, not the two one would normally expect. Thus there is nothing wrong with a setting of "1". This is my theory and I am sticking to it until told it is hogwash ;D

Best Regards,
Mel Jordan