Boat Tach on car

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Mike Stroz
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Boat Tach on car

Postby Mike Stroz » Thu Jan 18, 2018 2:57 pm

I know this is a pretty unusual question, but I've got a 1950's OMC accessory tach from the 50's that's just about the right look I want for my 1971 Mustang and I was wondering if it's possible to use it on there. I'm sure it's meant for a 2 cyl motor of some kind, and on the tag it says that one wire goes to ground and the other goes to the "non regulated output side of alternator". I can post some pictures if that helps
When in doubt, duct tape!

Jeff Register
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Re: Boat Tach on car

Postby Jeff Register » Thu Jan 18, 2018 3:04 pm

A belt driven instrument will not be accurate, Remember early Vettes used a drive from the genny for RPM? Two to one with 2 or 4 stroke,

Mike Stroz
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Location: Massachusetts

Re: Boat Tach on car

Postby Mike Stroz » Thu Jan 18, 2018 3:06 pm

I know it probably won't be too accurate, it's mainly for the look and to get an idea of the rpm on the highway. I'm just wondering if I can get it close enough
When in doubt, duct tape!

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58FourFifty
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Re: Boat Tach on car

Postby 58FourFifty » Thu Jan 18, 2018 4:29 pm

Hook up a dwell/tach to your Mustang so it reads the correct rpm, then jump some wires from your boat tach and compare readings.
simple!
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FrankR
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Re: Boat Tach on car

Postby FrankR » Thu Jan 18, 2018 4:47 pm

You obviously have a tach that receives an AC signal from the flywheel alternator. It counts those AC peaks. It certainly isn't going to give proper readings on your belt driven alternator, which runs at way higher RPM than the engine.

Mike Stroz
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Location: Massachusetts

Re: Boat Tach on car

Postby Mike Stroz » Thu Jan 18, 2018 6:49 pm

I'll definitely test it out using another tach, but if it's meant to be put on an outboard with the alternator connected to the flywheel, I would think the pulley ratio would be fairly similar. On a car, the alternator pulley isn't that much smaller than the drive pulley on the end of the crankshaft. I'm thinking the outboard might spin the alternator faster because of the pulley or gear ratio
When in doubt, duct tape!

FrankR
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Re: Boat Tach on car

Postby FrankR » Thu Jan 18, 2018 8:08 pm

Beside all that, how many poles does the car alternator have?

Mike Stroz
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Location: Massachusetts

Re: Boat Tach on car

Postby Mike Stroz » Thu Jan 18, 2018 9:03 pm

I might be wrong, but I was under the impression that these tachs took a signal from just one pole so it doesn't matter how many, it'll just get a signal once every rotation
When in doubt, duct tape!

Dave Bernard
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Re: Boat Tach on car

Postby Dave Bernard » Thu Jan 18, 2018 11:14 pm

yes 1 pole per revolution but on a car the alt turns more than 1 turn per rev with the belt.

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Garry in Tampa
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Re: Boat Tach on car

Postby Garry in Tampa » Fri Jan 19, 2018 12:17 am

My neighbor at Houghton Lake was a test engineer for Oldsmobile. One of his jobs was to calibrate the instruments. Using a "fifth wheel" he would drive at a certain speed and then mark on a blank face plate where the indicator needle was at that speed. I have to say, some of the best (fastest) cars they made never saw the light of day. That could be a way to calibrate your tack if the face plate is not too hard to remove. . . :lol:
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