Suggestions for a cheap rev counter

[Neil2]

Just looking for a 52mm dia rev counter, only needs to go to 4000 rpm, and I have a W connection on the alternator.

 

Have seen the Durite one for about £50 does anyone know of anything else?

[bigste]

Go on feebay and type in 52mm rev counter. There are hundreds for a lot less than half your £50. Some say petrol only so don't get the wrong one.

Edited December 3, 2014 by bigste

[by'eck]

Make sure it can be calibrated for your alternator pulley ratio.

[RLWP]

Some say petrol only so don't get the wrong one.

 

I wonder why that makes a difference?

 

Richard

[Guest User]

If you can get to a boat jumble you might be able to pick up a used one for a couple of pounds.

 

LC x

Edited December 3, 2014 by Lady Cassandra

[magnetman]

Or ebay maybe ?

 

I wonder why that makes a difference?

 

Richard

 

I'm intrigued that you managed to post the pre-edited version of a post which was in fact edited almost 3 hours before you added your post to the thread

(post no.2 Edited by bigste, Today, 05:30 PM.)

[RLWP]

Go on feebay and type in 52mm rev counter. There are hundreds for a lot less than half your £50. Some say petrol only so don't get the wrong one.

 

 

Or ebay maybe ?

 

I'm intrigued that you managed to post the pre-edited version of a post which was in fact edited almost 3 hours before you added your post to the thread

 

Where's the edit?

 

Richard

[magnetman]

sorry brains gone

Edited December 3, 2014 by magnetman

[RLWP]

You mean:

 

 

 

Edited by bigste, Today, 04:30 PM.

 

Are you living in some kind of temporal anomaly?

 

Richard

[OldGoat]

sorry brains gone

What's new.....

[magnetman]

sorry about that

[Alan Saunders]

bigste, on 03 Dec 2014 - 4:15 PM, said:

Some say petrol only so don't get the wrong one.

I wonder why that makes a difference?

Richard

My best guess is that the 'petrol only' ones are fed by the contact breaker or electronic ignition. Even then many are calibrated for a four cylinder engine.

[OldGoat]

 

I wonder why that makes a difference?

 

Richard

I was going to make a facetious comment on the basis of "rev-counters don't need petrol or diesel - they run on 'leccy."

 

Boom Boom

 

but thought better of it

Of course a petrol engine version is probably 0-10,000 rpm, but a proper boat engine one is 0-1500 rpm (haha)

Now who's replied to point this out (before me...)

[Paul C]

My best guess is that the 'petrol only' ones are fed by the contact breaker or electronic ignition. Even then many are calibrated for a four cylinder engine.

 

This

 

I was going to make a facetious comment on the basis of "rev-counters don't need petrol or diesel - they run on 'leccy."

 

Boom Boom

 

but thought better of it

Of course a petrol engine version is probably 0-10,000 rpm, but a proper boat engine one is 0-1500 rpm (haha)

Now who's replied to point this out (before me...)

 

Not this

[Tony Brooks]

The petrol engine ones often have a loop of cable that goes into one of the coil 12V cables (can't remember which one) and passes through some kind of inductor/hall effect thing on the back of the gauge and I doubt the W terminal feed would drive it within the parameters required. Goodness knows what they do now we have high energy, one coil per plug, programmed electronic ignition. I rather suspect a modern petrol car will use the W feed but the revcounter will be calibrated for a longer (higher number) scale.

 

I suspect it is even possible that some my be directly driven form the engine ECU.

[RLWP]

The last bit is most likely. The ECU has to know the engine speed to do its job, providing a signals for a tacho display would be easy

 

Richard

[ditchcrawler]

All the petrol cars I owned that used rev counters didn't have ECU's I don't think they had been invented. One of the rev counters I had was mechanically driven off the back of the dynamo, others picked up a signal from the low voltage side of the ignition between the coil and the points.

[Paul C]

The petrol engine ones often have a loop of cable that goes into one of the coil 12V cables (can't remember which one) and passes through some kind of inductor/hall effect thing on the back of the gauge and I doubt the W terminal feed would drive it within the parameters required. Goodness knows what they do now we have high energy, one coil per plug, programmed electronic ignition. I rather suspect a modern petrol car will use the W feed but the revcounter will be calibrated for a longer (higher number) scale.

 

I suspect it is even possible that some my be directly driven form the engine ECU.

 

The dashboard of a modern car communicates with the engine ECU, and probably other ECUs such as the transmission ECU (on an auto), body ECU, etc or the processing power for eg some of the functions may be included in the dashboard. In amongst the other data will be a revs/min signal from the ECU, which is derived from the crank sensor(it will also have a cam sensor to determine which cylinder is due to fire, on the 4 stroke cycle but the cam sensor won't affect the rev counter).

PS older tractors often have a mechanical drive cable similar to a speedo cable from the engine to the dashboard. Modern tractors have an engine ECU.

[BGA]

 

 

 

Where's the edit?

 

Richard

Maybe its taking its timing from the sending computer, if that computer is set to an earlier time it might appear to have been sent before the post originated.

[cuthound]

Maybe its taking its timing from the sending computer, if that computer is set to an earlier time it might appear to have been sent before the post originated.

Yes, it is almost certainly getting its time from the sending computer. In the early days of email I once reset the time and date on my computer to send an e-mail to my then boss. It apparently arrived a fortnight before I actually sent it I then convinced him that he must have overlooked it in his inbox.

 

Nowadays I think you need access to both computers and to change the date/time on both for it to work.

[by'eck]

Bottom line, whilst petrol engined tacho's usually derive speed sensing directly from engine (ignition system), diesel ones rely are triggered by pulses from one phase of alternator output. Since this only relates to engine speed via alt drive pulley ratio, some calibration has to be performed to compensate.

 

One dedicated to a particular engine model with standard charge system would be pre-calibrated, otherwise they are available to read engine revs based on selection of pre-set alternator drive ratio's 1.5:1, 2:1, 2.5:1 etc., or are infinitely variable and require calibration to a known engine rpm.

Edited December 6, 2014 by by'eck

[Alan Saunders]

Bottom line, whilst petrol engined tacho's usually derive speed sensing directly from engine (ignition system), diesel ones rely are triggered by pulses from one phase of alternator output. Since this only relates to engine speed via alt drive pulley ratio, some calibration has to be performed to compensate.

 

One dedicated to a particular engine model with standard charge system would be pre-calibrated, otherwise they are available to read engine revs based on selection of pre-set alternator drive ratio's 1.5:1, 2:1, 2.5:1 etc., or are infinitely variable and require calibration to a known engine rpm.

Or, as a previous owner of my boat did, re-paint the dial of the tachometer. Mine is a little crudely done but Letraset transfers could make a neater DIY job or a clockmaker could do a professional re-enamel after you have calibrated it.

 

It is nice if the maximum rpm on the dial matches the maximum rpm for the engine. Even better if all dials read 'normal' at 12 o'clock. e.g. My Vetus M3.10 (800-3K rpm) cruises at 1200 - 1600 rpm and my Gardner 3LW (400-1,750 rpm) at 750-1,000 rpm on shallow canals and rivers. I guess in deep water a tacho. is a good guide to optimal cruiing rpm. Reference those WW1/WW2 films where the captain calls for 200rpm or Scotty replies "She can no' tak' it Cap'n".

 

Calibrating the Gardner tacho. (a dc or ac motor mechanically driven from the end of the dynamo) is easy; at tickover I just count the 'beats' for 40 seconds, =rpm.

 

An engine tacho. is not essential but it can be a useful guide, e.g. when I filled up with dodgy diesel I soon saw that the engine was idling at less than the previous, consistent 800 rpm before it began to stall at minimum throttle.

 

HTH, Alan

[by'eck]

Yes I had a new dial made for my Smiths mechanical tacho to exactly suit my JP2. The drive ratio was such that I had to have an up-converter gearbox added within the cable, so it made sense to increase the gearbox ratio to give 1200 rpm FSD from the previous 2000 rpm. See here

 

Most instrument companies have specialist software and printers to design and produce custom dial faces nowadays. I also had one made for my roof mounted oil pressure gauge at no extra charge.

Edited December 7, 2014 by by'eck