Accurate Tach

[BDA]

Well, this may not be the correct place for this question but I thought it was close enough.

I have a stock Smiths tach in my car (TCS). I recently read that when tuning, do not trust the tach in the car but use an accurate tach. Well, the only other tach I have is a dwell-tach (the All American Dwell-Tach on sale on ebay from several people such as this guy: https://www.ebay.com/itm/164891959400?hash=item266453a068:g:WQ8AAOSwSktgtSw5). It was cheap when I bought it over forty years ago but the few times I needed to use it back then, it seemed to be pretty accurate. In fact, I remember being surprised how accurate I thought it was but I don't remember what I was comparing it against so I don't trust my memory as evidence that it's accurate. Well, I've noticed that it reads about 200 rpm less than my Smiths tach and now I'm left wondering which is accurate (presuming one of them is!).

I would note that my Smiths tach has not been altered even though I am running a Pertronix ignition. Interestingly, when I went from a light ignition (Allison IIRC), which drove it crazy over about 4000 rpm, to the Ignitor I, my tach behaved much better. Now that I have an Ignitor II, it seems to behave itself up to about 5000 rpm and then it goes crazy. But when I'm tuning, I don't get up that high so I'm not sure that should concern me.

So the question is which one should I trust? Or can I trust either of them for best accuracy? I would think that the circuitry for determining rpm from the ignition (which both the Smiths and my dwell-tach use) would be well known to "everybody" and that even a cheap dwell-tach like mine should be trusted. On the other hand, it would surely be known by Smiths too, and so I should be able to trust it also. But then they differ.

Thoughts out there?

[MRN I J]

I would think the Smiths tacho would be the innacurate one, use a decent tacho like a Stack

[jbcollier]

As patriotic as the "All-American" moniker sounds, it's a cheap and nasty tach.  Get a digital tach.  Lawnmower shops have them, cheap. They are very accurate.

[BDA]

Chris, in case you don’t know anything about my car, there is very little stock about it but using a different brand of gauges is bridge too far for me. Thanks for the suggestion.

JB, that sounds very intriguing. I guess you divide the rpm shown by four, right?

[jbcollier]

There are 4-stroke lawnmower engines so they have the ability to do both 2-stroke and 4-stroke.  If it only gave a 2-stroke readout, then you would multiply the reading by 2:

4000 sparks = 4000 rpm on a 2-stroke
4000 sparks = 8000 rpm on a 4-stroke

There are wasted spark systems that complicate matters further.

4000 sparks = 4000 rpm on a 4-stroke with a wasted spark system.

Some 2-strokes have a wasted spark system (the mind boggles).  Not very common but one of my performance 2-stroke engines had such a system.

4000 sparks = 2000 rpm on a 2-stroke with a wasted spark system.

[EuropaTC]

As patriotic as the "All-American" moniker sounds, it's a cheap and nasty tach.  Get a digital tach.  Lawnmower shops have them, cheap. They are very accurate.

By coincidence there's been a thread about tachs on Lotuselan.net and the consensus was that a cheap digital tach was a good way of calibrating/checking the OEM tach.  These were also lawnmower, motorbike, etc units and the recommended ones were very cheap, under £10 ($14 US), and apparently work very well. I've had a moderately cheap digital tach for when I'm servicing the cars, a clip on unit which I can just place in the engine bay rather than relying on the one in the car.

Brian

[surfguitar58]

Following this thread with great interest. My car came from its DDPO (deceased dreaded previous owner) with standard ignition and a new Pertronix setup in a box of parts. The tach was jumpy at high RPM from the get go, leading me to think (hope?) the DDPO had already modified the tach for electronic ignition but just hadn't gotten around to installing it before he kicked the bucket. I have since installed the Pertronix dizzy, coil and plug wires (massive reduction in misfiring and plug fouling btw) but the tach behaves exactly the same; works well at low RPM, oscillates wildly at high revs. I am dreading getting behind the dashboard, but my fuel and amp gauges also need attention, almost certainly a ground issue, which is 90% of the problems on this car. Since it doesn't prevent me from driving, I'll be shifting by ear until I work up the courage.
Tom

[BDA]

Recognizing that a bad ground should usually be the first suspicion, it sounds as though your tach and ignition don’t play well together. I was preparing to send my tach to Nisonger but you reminded me that I thought I had heard about a kit or instructions for upgrading the tach.

Can anyone confirm that and supply some details?

[2766R]

Might I suggest a conversion kit from Spyida which converts a Smiths RVI to RVC and should solve any issues causing erratic tach behavior.  I purchased and installed on on a Tach sourced from feebay.  Although I still have the original tack and ignition system, my plan is to upgrade to Pertonics ignition along with installing the converted tach (I have all the pieces, just need a window of opportunity).  The actual conversion was quite straight forward following their instructions.  The only trouble I had was during calibration.  I had to partially unplug the Tip/Ring/Sleeve connector from Spyida's patch cable with the extension cable from my PC.  It worked out to about $73 including shipping (converted from GBP) which I have to believe is quite a bit less than Nisonger would charge and for probably the same work.(?)  See the following link for info:

https://spiyda.com/tachometer-electronics.html

Gerry

[BDA]

Thanks Gerry. I think that's what I was thinking about.

Just to make things interesting (and a bit more confusing) it seems that a guy on lotuselan.net was able to achieve the same thing by changing a bit of wiring in the car (or am I misunderstanding him):

rdssdi wrote:
I do not understand the difference between wiring the +12 for the Pertronix from the + 12 coil terminal or the ignition switch. It appears the +12 coil terminal is powered by the ignition switch.

I believe the Tach power is supplied from the loom at the tach. The tach uses the "trigger" wire White / black for the signal.

Bob


Ok, right. It's a bit difficult to explain but you know how a coil works (you put 12v across it and it builds up an Electro field until you remove the neg which releases the stored charge down the HT lead). The problem comes if you connect your Electric ignition 12v supply to the coil 12v, when the coil is doing its thing the electro magnetic field causes interference to the electronic ignition module (basically the 12v feed is not clean it has interference caused by the coil's magnetic field collapsing, you can see the bad signal if you hook up an oscilloscope)

On my car what i got was the Tacho getting confused, it would shoot over to red line when the engine was idling things like that. So to fix this i ran a fresh 12v power from the Ignition as far away from the coil as possible and fitted a small gizmo Spydr made for me to clean the Tacho's 12v signal (i believe the latest kits have this gizmo integrated link to my issues lotus-electrical-f38/odd-rev-counter-jumping-t33543-15.html)
Chris

Any comments, anyone?

[jbcollier]

I have installed literally tons of Pertronix kits.  Always hook to coil+ and coil-.  No issues.  On some tachs which get their signal from a loop of the white ignition feed, you had to remove one loop, IIRC.

[BDA]

I have installed literally tons of Pertronix kits.  Always hook to coil+ and coil-.  No issues.  On some tachs which get their signal from a loop of the white ignition feed, you had to remove one loop, IIRC.

Loop of white ignition feed? I don't remember seeing anything like that in my under-dash travels. Do you remember enough to be more specific or descriptive?

[jbcollier]

Everything you didn't want to know about RVI and RVC Smiths tachs:

https://www.triumphclub.co.nz/wp-content/gallery/pdfs/Smiths.pdf

The mod we used to do.  Not all tachs require it, just a few:

http://www.mgaguru.com/mgtech/ignition/pdf/smiths_tachs.pdf

[BDA]

Thanks JB!

[surfguitar58]

Question for the group: How hard is it to wiggle the tach out of the dash and put it back without taking the whole dashboard out? Anybody done this, and if so, any tricks or tips? Looks like the 2 brackets would come off easily and enough wire length to disconnect outside the binnacle.
Tom