Hot Rods Harley Tach on a Car?

Can I use my Harley Davidson tachometer on my car? The Harley is a 2 cylinder and my car (ok its a 1951 Hudson Hornet) is a 6 cylinder. I am replacing my Hudson clock with a tachometer and I want it to look Hudson, so I purchased a second Hudson speedometer and took it apart and I want to put a tachometer behind the face of the second Hudson speedometer (the clock numbers are in the wrong places for a tach). The Harley tach had a similar spacing of numbers as the Hudson speedo so I am trying to use but now that I think about it I remember that the electronic tachometers use coil sparks to generate the signal and if the Harley is a 2 cyl and my Hudson is a 6 cyl that that will not work. Am I right or wrong. Please help.

 

that's right, you need a tach for a 6 cyl engine.

 

There is a way to recalibrate an electronic tach, because I've seen it done (12 cylinder down to 8, 6 up to 8). There should be a resistor going to the tach movement, changing it's value will change the calibration. This may help....
http://www.jag-lovers.org/lumps/x/tachomet.htm
Your problem will be that you'll need to find the existing resistor, determine its value, then figure the change needed to correct it. In your case, you need a higher resistance but how high may be trial and error....
There are also electronic 'conversion boxes' out there, but I haven't seen one that does 2 up to 6 cylinder. Not saying they don't exist, just haven't seen one.

 

Try contacting Dakota Digital. They make signal converters for electronic speedometers and tachometers, and should be able to fix you up with something.

 

With a simple electronic circuit using components for a few dollars you should be able to turn just about any electric instrument gauges into a tachometer. Perhaps the electronic speedometer from a 1980/1990s Volvo 240/740, or maybe a fuel or temperature gauge? I've got the electronics, still looking for a (free) suitable gauge to convert just to see how it turns out.
http://www.555-timer-circuits.com/car-tachometer.html

 

The Harley tach will work great, you just have to bolt the handlebars to the dash.

 

most harley's fire a tdc on both revolutions, so it would be similar to a 4cyl tach

 

send it to Willamsons in Chester,Arkansas and let them convert it for you...They do it everyday and are reasonable priced

 

Did I miss something here or are you changing faces and such? If so, a generic 6 cyl tach and the harley face. Too easy?

 

I think he wants a 4-pulse Harley tach with a Hudson speedo face. If it were mine, I'd simply connect tach as-is and see what happens. Might read a bit too high but the OP seems to be looking for 'looks' not accuracy. Jack E/NJ

 

Comes out odd fire.. 0°-30°-330°...4cyl would be avery 180°..

 

Very bad idea. If you install a Harley tach, your car will vibrate badly, parts will continually fall off and you will never get it to run right.

Along with that, you will need a ratty old van driven by a greasy long hair, filled with parts and tools to follow you anywhere you go

 

Also, upper-management loafer-wearing yuppies who couldn't tell you where the oil goes into their Road Glide or how an internal combustion engine works if you held a shotgun up to their temple will pull up and ride along side of you, as if you share a brotherhood. That wouldn't be so bad if later that same night they wouldn't drive right by you in their new Mercedes and splash you with a puddle as you're broken down on the side of the road trying to change a tire.

 

On the auction site there is a guy selling tach converters that adapt any electronic tach to any engine
or type of ignition. $49.00 if I remember right......Al

 

And if you don't want the car to run at all, especially in the rain, put a Lucas tach in it.


Sent from my iPhone using The H.A.M.B. mobile app

 

A '53 Hudson is not going to wind enough to make looking at the tach worth the trouble. If it's just looks you want, why worry about it?

 

Some electronic tachos use a common circuit board (our eastern asian friends at work), and if you open up the tacho, you may see some solder pads marked "2", "4" "6", etc. (no. of cylinders). Have a look if there is a link at "2", and move it to "6". With generic tachs this used to be connected to a switch on the back where you could select the number of pots.

 

I thought they were a 45° Vee twin . the 0°-30°-330° odd fire would explain the bad vibrations
A Harley fires at TDC every revolution [not every 2nd revolution ] so every 2 revolutions would have 4x pulses the same as a 4 cylinder engine
A tach doesn't count the degrees of timing , just the number of pulses per minute. It would take a really trick tack to twitch the needle at 3000+ RPM because of uneven firing

The OP really needs to find a Suitable tach for his engine and instument panel.
Then get a computer geek to "shop" a period correct face and print it on some self adhasive paper/vinyl.
It would be easier to change the face than recalibrate a tach [I did this years ago on a Smiths Tach for my Lotus Cortina]

 

Brain fart yes 45°..Surprised that pluses don't have to be equal..0nly mixed 4-6-8 with equal firing degrees..

 

No, you only have one spark per crankshaft revolution; Harley (like many motorcycles and distributorless cars) uses a 'wasted spark' ignition system, firing one coil feeding both plugs. Remember, the camshaft is at 1/2 the speed of the crank; the crank has to turn twice to turn the ignition rotor once.

 

Technically speaking, neither cylinder fires at "TDC", they fire sometime before TDC spending on RPM and the advance curve. As for the Firing sequence, they are an Odd Fire engine,
The rear piston fires.
Then the front piston fires 405 degrees later.
There is a 315-degree gap.
Then the rear fires again.
Then the front fires at 405 degrees.
There is a 315-degree gap.
And the cycle continues.
Older Harley's had a wasted spark ignition where each time a cylinder fired on the compression stroke, the other one also fired even though it was no where near compression and there was no mixture to fire - this is referred to as Dual Fire. Newer Harley's, and many performance Ignitions do away with the wasted spark and only fire the cylinder that is actually going to turn it into HP - this is known as single fire.

 

Hell I wouldn't fight with it, there are plenty of yard sale or swap meet tachs for 5 or 10 bucks that you could pull the works out of and use with your speedo face for what you want to do that have a switch to set them on 6 cylinders and done.

 

Duh! exactly what I said......
Quote " you only have one spark per crankshaft revolution" = 2 sparks per 4 stroke cycle [x 2 cylinders = 4 sparks per 2 revolutions [exactly the same as a 4 cylinder with a distributor]

 

A Harley doesn't vibrate. Your hands, feet, and butt go to sleep from the beautiful music. If you change a HD part and don't contact the lawyers first for a trade mark infringement they will find you...

 

Lots of complicated answers here, lets make it simple. it won't work.

You are looking for a 20 dollar cure to a 10 cent problem. Just find a used 6 cylinder tach and use it.

@RichFox no need for a tach on a Harley either. Any Harley I have ever owned if it had a tach it had too many instruments, I just tossed 'em.

 

Think I'm going to have to disagree on two fronts. First due to the odd firing order the Harley motor does not fire on every revolution - there is 405° of revolution between the rear cylinder firing and the front cylinder firing, that is more than one complete revolution of the crank. Second there are not "Two Sparks" per, there is One spark that occurs in both cylinders - which is not the same thing - there is only one pulse generated per fire. The tach gets it's impulse from the coil output side of the ignition and it doesn't know if that signal is going to one cylinder or a hundred.
It is also Not like a four cylinder in that a four cylinder fires evenly every 180° for 4 times over 720°, 2 crank shaft revolutions. The Harley fires at 315° and 405° Unevenly for two spark pulses per 720° rotation - the wasted spark is Not a separate firing. Look at is this way, if a four cylinder had a wasted spark design, it would not "Fire" 16 times, it would still only be 4 firing events.
I can guaranty you that if you hook a Harley Tach to a four cylinder engine it is going to read double the actual RPM

 

So what you're saying is...... When the rear Cylinder is firing at 30 degrees BTDC on the compression stroke the front cylinder gets a "wasted spark" at 75 degrees BTDC on the exhaust stroke.
So what happens 405 degrees later?
Does the front Cylinder is fire at 30 degrees BTDC on the compression stroke while the rear cylinder gets a "wasted spark" at 15 degrees ATDC on the intake stroke [back firing through the carb]

Also how does the crankshaft change timing from 405 degrees to 315 degrees every alternative revolution???? [so it can fire each cylinder at the same BTDC]

 

You have to remember that this a a 45° V Twin with a Single crankpin. The connecting rods are knife and fork, they rotate around each other on a common bearing! This doesn't show the wasted spark occurring, but you can see how the motor operates and see when it would happen.

 

That's an accurate description... One reason Harley 'single fire' ignitions are popular is it does improve idle quality, as that 'wasted spark' at the rear cylinder does occasionally cause a backfire when tipping in more throttle and sometimes kills the motor. If you've ever heard a Harley sneeze and then die when given throttle, this is why.

As to the timing, the ignition rotor is ran at camshaft speed (1/2 crankshaft) and has two cutouts (for electronic ignition) that the pickups sense. They are not 180 degrees apart, but are offset by 45 degrees to account for the uneven firing. I would assume that the early points ignitions had two points lobes similarly offset.

 

Thought this was a post about a Tach....not the firing order etc. of a Harley