Challenge to you CNC guys

[Matt Story]

Nice explanation Howard.  Thanks

The resolution of the teeth matters relative to the resolution of the sensor.  Scaling the wheel down results in the teeth have the same  width in degrees, and the rotation speed remains constant versus the traditional mounting location - the signal can look the same as read from a smaller wheel as long as the sensor resolution is suitable.

[Chuck in Indiana]

Yeah, but what about

inconveniencing a few more electrons

[rodekyll]

So scaling down the wheel to fit inside the distributor may not work because it will change the size of the teeth and thus the signals to the ECU? I was wondering about that.

I don't think so.  My analog is computer hard drives, which we used to cylinder, sector, and track, into theoretical Russian doll arrangement of 'cylinders' nested one inside the other.  The cross section of the cylinders was then divided into pie shapes and the wedges further processed to make a low-level format.

Why did I tell you that?  Because we're looking at what I think is the same physics here.  A disk has 360º.  At a given (1) RPM the disk is passing all 360º under the sensor in one minute.  It's doing 180º in 30sec, 90 in 15sec, etc.  AND IT DOES THAT ON EVERY PART OF THE DISK.  So if the disk has a diameter of 2 inches, the outside edge of the disk does it at the same time the corresponding point of the disk at 1 inch does it.

But the circumference of the disk at 2" is 6.28".  At one inch it's 3.14".  So to make the full 360º sweep, the outside edge of the disk needs to cover more distance in the same amount of time as the center.  To do that it needs to travel faster.

So the sensor picks up the same number of degrees of sweep in the same time for any point of the disk, and it takes the same time for the same degree sweep to occur at any point of the disk.  And from that I figure the diameter of the disk circle makes no difference, except to the sensitivity of the sensor.

Now this might be entirely bad logic -- I don't know.  It just seems logical to me.

[Matt Story]

An ideal solution would be to use the stock sensor.  If viable, it would only require 2 new pieces, a wheel, and a special distributor cover that would double as a mount for the sensor.  The cover part would be designed for 3D printing in ABS.  It would have to be a short enough cover so as not to interfere with fuel tanks..

A substantial problem here has to do with the size of the sensor field vs the wheel resolution.  Maybe this can be dealt with by altering the proportion of the vanes to the spaces.  I think if the teeth are narrowed by some degree, but still maintaining the same number, a bigger scale sensor might work with a smaller wheel.  The problem with a sensor with a disproportionately large field vs the target vanes is that the gaps spinning by may not be wide enough (long enough in time duration) to be detected.  By biasing the vane width, the gaps gain a longer duration while passing the sensor and the opposite with the vanes.  Per the theory Howard helped us with, I don't think the pulse widths are important (except the wider one), only the number of them per revolution.  I think the ECU is looking for a number of signal transitions or maybe peaks in the signal (46 per rev) without regard to the width (which would change depending on engine speed anyway).  Some bench testing with a cam position sensor would be required.

Here's a couple sketches.  There's no reason the sensor couldn't be mounted in the top of the cover looking down at the side of the wheel I guess.

[rodekyll]

That would solve the problem of what sort of signal it is.  Not being optical anymore, thin is no longer an advantage.  The disk needs to be chunky to have enough mass for the sensor.  And as long as we're hanging the sensor out the side again, we can get by with a larger wheel and offer it even more mass.  I might have a sensor in a tote out back.  If I do I can scope it and see what the signal looks like.