Electronic geniuses.. Dingos tach?

[used red MN]

Just for fun and possible solution this gentleman wants to put a tach on his VAC. I have asked him if it is 6 or 12 volts. If it is on 6 it would probably make this more difficult. It is an 8 cylinder tach and his engine is a 4. Could a person devise a magnet and pick up on his front pulley to provide the the switching pulses needed? If I am thinking correctly he needs 8 pulses for every two crank revolutions. So a single mag pickup with 4 magnets 90 degrees apart would provide that, is that agreed? What else is required circuit wise to provide a pulse signal like this. I hope FBH44 sees this, he is the one that has fuel injection on his Massey, do I have that right? I am attaching a link to the post I am referring to.
Poke to see post

 

[Dingo3]

Red, thanks for the reply. It has been converted to 12v. I know that a 8 cylinder tach will see half of pulses from a 4 cylinder engine. I have found that its pretty easy to convert the opposite- 4cyl tach to 8cyl engine but could find nothing on my situation. I did find a company last night that makes a small converter that will work. Costs about $40. My other option is to save the tach for another project and buy a new tach which is also about $45. Just trying to use what I have and save a little cash. Thanks again

 

[BarnyardEngineering]

Years ago I bought a cheap digital tachometer that you could easily switch between 4-6-8 cylinders.

 

[Janicholson]

The crank turns two times per one combustion. Combustion always happens when two pistons are at the top, and two are at the bottom. So 2 magnets are required only. Jim

 

[used red MN]

I thought you wanted this for the ..bling.. factor? Are you going into the tractor pulling realm and over revving your pour old 4 popper? Otherwise, these tractors have governors that take of that. Sort of like an early computer. ;>) Now that you put that reply here no one will try very hard to answer or comment on my proposal.

 

[used red MN]

Jim, maybe you missed it but the tach being for a 8 cylinder required it to get a reading of 8 breaks of the points in two revolutions. So by that 4 magnets on the crank pulley would provide 4 pulses per revolution, 8 in two. I can hardly see how any other possibility could be correct.

 

[Janicholson]

You win. Jim

 

[js305]

I had one of those. The pickup was a wire that wrapped around the number one plug wire, or if that didn't work it
was wrapped around the coil wire. It had a switch to change the number of pulses it needed for a 4-6-8 cylinder.

 

[rustyfarmall]

There are many, many 4 cylinder cars and trucks out there equipped with tachometers. Why can't one of those be adapted?

 

[Dingo3]

Yes I liked the look of the old tach. I was into using something I already have also. I would just like to know what its spinning at during different situations. I have tachs in all my old cars even the automatics.

 

[Dingo3]

They all can Im sure, just trying to use something
cool looking and already in my possession.

 

[sotxbill]

CCan you put a generator or alternator on it with a tach drive. They are common on some fords and masseys.