Automotive Tach. on an outboard.

[joelyons50023]

I know that i have searched this and seen people say no it wont work but I found this.
http://www.instructables.com/id/Automobile-Tachometer-on-an-outboard/

I was wondering if others would think that this would work. I think that all he is doing is making it to where it is able to read the signal of a 12cyl 4stroke or a 3 cyl 2 stroke. My motor is a 1975 Johnson 70hp sea horse. thanks any help is always great.

 

[Silvertip]

Re: Automotive Tach. on an outboard.

Your engine has three cylinders, each with it's own coil. Since an automobile engine with a distributor has a single coil serving all cylinders, the input to that tach can only be connected to one of the outboard coils. The number of pulses from that one coil will be whatever the engine rpm is since a two stroke fires every revolution. So at 6000 rpm there will be 6000 pulses/min. On a 4 cylinder car engine there are 2 pulses/revolution since it is a four cycle. So at 6000 rpm the tach would be seeing 12000 pulses. A six cylinder engine would produce 3 pulses/revolution so at 6000 rpm the tach would see 18000 pulses and for and eight cylinder engine there would be 24000 pulses at 6000 rpm. If you used the tach sense wire on the outboard, there are 6 pulses per revolution regardless of the number of cylinders so at 6000 rpm the tach sees 36000 pulses.

As you can see unless you do some conversion, the auto tach will not work due to incompatible calibration. If you use the tach sense line from the outboard, you could set the 4-6-8 cylinder switch on the auto tach to the 6 cylinder position but you would need to "divide" the reading by 2 to get the correct rpm If you used one of the ignition coils to feed the tach you would need to "multiply" the tach reading by 2 to get a correct reading.

 

[EricJRW]

Re: Automotive Tach. on an outboard.

I saw that article too. It certainly sounds feasible, and it even covers Silvertip's concern about the conversion. My only concern would be calibration. I still would like some way to double-check the accuracy of my tach. RPMs seem to be a pretty big deal when it comes to keeping engines healthy.

 

[NYBo]

Re: Automotive Tach. on an outboard.

Maybe I'm missing something. Why jump through all those hoops when a proper marine tach is inexpensive and essentially "wire and play"?

 

[Silvertip]

Re: Automotive Tach. on an outboard.

And less expensive than a similar quality auto tach that will rust up solid in short order because they are not sealed.

 

[joelyons50023]

Re: Automotive Tach. on an outboard.

not many options for 30 bucks and I use a sunpro on my motorcycle and have rode through the rain quite a few times with no problems. I have a 1975 glasstron with a 1975 johnson money is a bit of a challange and concern or i would have a boat that had all of the bells and whistles that i would like. I was just seeing if anyone had tried something like this or another alternative.

 

[Silvertip]

Re: Automotive Tach. on an outboard.

Go here. There are 229 tachs to choose from. It took about 10 seconds to find one for $39. Why in heck would you bother with a cobble job auto tach that simply will not last.

http://greatlakesskipper.com/category-1/56_76_80-gauges-outboard-only-tachs.html