Dual VR Porsche 951

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Joined: Thu Apr 22, 2010 8:35 am

Dual VR Porsche 951

Post by Delectable » Thu Apr 22, 2010 10:06 am


I am working with the VR sensors in my porsche 951. The stumbling point has been trying to use the two stock VR sensors. One for the speed of the motor and one that is the top dead center reference. In the 944 world they are just referred to as the speed and reference sensors. The issues with using the inputs from these sensors directly to either a megasquirt or a microsquirt seems to be two fold. (and probably related)

Problem #1: The speed sensor reads from a 132 tooth wheel. This high tooth count seems to overwhelm the MS.
Problem #2: The output voltage from the VR sensor may be getting to high as rpms increase and again overwhelms MS.

What I don't know is if it is one or both of the above problems that are causing the problem for the MS units. The signal start at 2v when ideling and from what I under from other posts is that the signal can rise to 10~12v at 6800 RPM.

What I am wondering is would your Duel VR board be able to condition the output from these two sensors and turn it in to a form usable to the megasquirt for engine speed and top dead center reference? I'm planning on using MS2 / MS2 extra

Of course any progress that I make will of course will be posted back.

All the best from the Netherlands,

Posts: 3655
Joined: Sat Oct 03, 2009 12:24 pm
Location: Quebec, Canada

Re: Dual VR Porsche 951

Post by jbelanger » Thu Apr 22, 2010 12:04 pm

The problem with the 132-tooth wheel is actually more than just the large number. MS2/Extra requires a number of teeth that produces an integer number of tenths of degrees. With 132 teeth, you have 2.727 degrees per tooth and to have something usable you need to divide 132 by 11 to get 12 virtual teeth.

I do have a board with VR conditioners and a CPU that does divide the number of teeth and generates an emulated missing tooth wheel. It does work with the 135-tooth Audi wheel and produces a 45-1 virtual wheel with a divide-by-3 logic. I also assume it does use the same or similar sensors so it is possible to handle them with some tweaking of the resistors.

It would be possible to change the firmware to do a divide-by-11. It would create a 12-1 wheel which would be less accurate but still more than acceptable. If you want more information have a look at this post on msextra: http://msextra.com/forums/viewtopic.php?f=101&t=35409.


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