Calibrating Rev Counter - Can Anyone Lend A Generator ?

[rfs1957]

The ressourceful and co-operative Chris that is behind Spiyda

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

thinks it may be possible to use the cam-end generator as a pulse-source to drive one of his widgets, which then drives the original rev-counter with an accurate voltage that it generates, rather than relying on the often problematic voltage from the generator.

He first floated that possibility 7 years ago in a separate post after a discussion with David, but says he's not aware if it has ever been tried.

Can anyone afford to do without a generator for a few days, or has a working spare, that he could confide in Chris for testing ?

I would send mine but UK-France postage is fraught so best avoided.

The advantage of this would be that instead of running extra wires from the negative coil or electronic ignition module, the original generator wires can be used to feed the widget and the installation becomes simpler and neater.

PM me and we'll confer ?

[chrisfell]

Somewhere in the deeper past on this forum (or Jag-Lovers) someone made a PCB for just this purpose. I think several were made to order. It was a while ago.

[mgcjag]

Here you go viewtopic.php?t=292&highlight=tachometer

[rfs1957]

Generator loan sorted, by kind Forum user Series1 Stu.

Will report back once Chris has had a look at it.

R

[Series1 Stu]

I just hope it does actually work properly. I'm sure Rob Beere will have made sure of that when he built the engine.

Regards

[rfs1957]

It's taken me a while to report back on this, and it's not good news.

In words of one syllable, if only to help me understand what I'm writing, this is why.

The Spyda widget interprets the frequency of the signal coming from the cam-generator, which is "inescapably" accurate since it's just a wiggle-per-rev that corresponds with the cam-rotation-speed, rather than the intensity (voltage) of that signal - which can be unpredictable.

Knowing the frequency, the Spyda is then in turn meant then to feed the rev-counter with a voltage that in an accurate reflection of the cam-rotation-speed, that the rev-counter can then translate into reliable needle-movement.

However, despite much tinkering, I was unable to get the Spyda to give a steady feed to the original Smith rev-counter.

Furthermore, and this I had not grasped, it still leaves the problem of playing with the weights and balances inside the rev-counter to try and get the reading to equate with the actual engine rpm.

Which for me meant trying to run the engine, read the rpm off the Megajolt software, and play with the intricate mechanism of the rev-counter's guts, all at the same time.

If anyone else wants to play with this, they can have the Spyda box of tricks for free.

I'm sure it's very good at taking the cam-generator signal and turning it into a feed to drive a modern rev-counter (no weights/balances on the needle etc) but you can do that from the coil anyway, and it's not apparently what someone like me needs, or can cope with.



It may be because I Is Thick.

[Heuer]

No experience of the Spyda unit but you can download a software frequency generator to help calibrate things. More info here: viewtopic.php?f=9&t=292

Download software to laptop: https://www.dropbox.com/s/6xsagrp2h40r7 ... e.zip?dl=1

I assume there is a calibration potentiometer on the Spyda unit? If so you run the above software from your laptop to the the Spyda whilst it is connected to the tach. Adjust the frequency of the software to a data point and then adjust the Spyda accordingly. In effect the software becomes/mimics the E-Type cam driven generator so no need to run the car and can all be done on the bench. Again I say I have not done this myself other than with the Dave Rawle board which is no longer available. Plans are in the thread so you could make a copy, get an electronics company to make one or see if Marek can assist.

[ralphr1780]

Rory, IIRC you have the EDIS+Megajolt set-up?
Positively then with your laptop hooked you can read the exact RPM and compare with your rev counter reading...

[rfs1957]

Hi Ralph yes absolutely, that’s what I’ve been doing.

There is a signal generating file supplied by Spyder that you run on a laptop and plug into the widget, but I was unable to get a satisfactory result out of the rev counter.

No amount of electronics upstream of the rev counter can overcome the fact that at the end of the day, the original instrument itself is an approximating device stuffed full of compromises.

I’ve decided that all I really need to know is that the red-line is accurate, or rather where the needle is pointing when the engine gets to a true 5.500engine rpm

[JerryL770]

Rory,

As you've seen on my post re: Converting my tachometer, Speedograph solved my problem beautifully.

With the Spyida RVI/RVC converter in, calibration on my desktop using a 100/200/300Hz signal from the PC was possible but adjusting the weights I think, unless one has lots of experience at it, was just about impossible to get right. I had it near enough on the bench, but in the car, the needle was all over the place. Now it is rock steady. Speedograph did a great conversion and calibration.

So, I think the RVC type essentially uses a frequency signal from the coil. Can you not get the same from your ignition system?