Tachometer

[Motorhead]

I'm looking for the Tachometer ratio on a boxer made in 1973

the electric I repalced mine with didn't last so maybe if I find a ratio I can adapt something?....

[vanzen]

Relevant information is in the BMW manual ...
beyond that, displacement might be necessary criteria to answer the question, eh ?.

[Motorhead]

would not the geared ratio used on the /5 motor be the same for the Tachometer? 1969 to 1973
Motor orginal R75/5 1973

[Kurt in S.A.]

Tachometers didn't come with "ratios" that I know of. Speedometers did, though.

Kurt in S.A.

[Jean]

As far as I can tell from the BMW info I have, the mechanical tach counted crankshaft revs and the "meter" read in RPMs sorta like a speedo. All crankshafts had the same mechanical apparatus. (as in: ONE rev/crank = 1 RPM)
Electronic (older ones) takes signal from points side of a coil and counts impulses. Still does not involve any "ratios".

[Deleted User 72]

I believe Lyman is looking for the ratio between engine rpms and tachometer cable rpms so that he may fit some alien tachometer to his airhead.

[vanzen]

My mistake, motorhead.
I read "tach" but was thinking "speedo" because of the request for a "ratio".

As to after market tachs / rev-counters –
It might be difficult to find a mechanical (cable operated) replacement to fit.
But any electronic tach intended for 2 cylinders should work.
Some tachs / rev counters are designed specifically for 2 cylinders,
others are # of cylinders adjustable, but careful,
many of these will only calibrate to a minimum of 4 cylinders.

[Garnet]

The tach drives on all BMWs from 70 to 78 (or when the electric tachs arrived) is a 1 to 1 ratio, but it is driven off the CAMSHAFT. So you need a 2 to 1 ratio tach.

[Motorhead]

My mistake, motorhead.
I read "tach" but was thinking "speedo" because of the request for a "ratio".

As to after market tachs / rev-counters –
It might be difficult to find a mechanical (cable operated) replacement to fit.
But any electronic tach intended for 2 cylinders should work.
Some tachs / rev counters are designed specifically for 2 cylinders,
others are # of cylinders adjustable, but careful,
many of these will only calibrate to a minimum of 4 cylinders.

the 2 cylinder electric Tach I picked was a Harley aftermarket.
In thought I figured a Tach for a harley woud work great as less Vibration and less Voltage spikes
My BMW is Modfied almost too far in work/operation as a sperate switch keeps light and day run seprate but the Tach worked funny in start ups and thats what killed it



when I switch on the tach, it wound up and went to 0 rpm, I'd start the motor and when it was ready to go, I switch on the lights and the tach would wind again to zero then motor RPM again at least it worked to the end of my trip when I found my 500.00 heads were not as rebuilt as sold

Mostly from experence if a tach is crank run its 1:1 if cam then 1:2 if more cam's then ratio can be set to calbration conditions from what cam is used and what time between crank and that gear to map RPM

I can see the drawing on BMW book the cam has a worm with 4 teeth per quater so to the drive that matches had 10 teeth? I currenty have no part to inspect teeth..... so I guess I need to get off my arse and just stick something into the drive turn the motor 1 crank and see how many time the cable drive moves for ratio

so I think its 1:4 or 2:4 as Garnet points out its cam driven....... if you have sound Info lets hear it thanks

[Kurt in S.A.]

Somone on ADVRider installed this:

http://www.advrider.com/forums/showthre ... ht=ACEWell

Kurt in S.A.