Tach experts; conversion of factory tachs and use of a aftermarket tach

[Billccm]

So I spent some time this weekend researching my Road Runner's MSD Street Fire set up and the factory cluster tach (which is inoperable currently).
The MSD Street Fire literature describes 'two types of tachs; current sense and voltage sense'.
From my 2.2l Mopar days of repairing many a tach driver module for the club members, I realize that the 'modern' tachs of today are voltage sense. A pulse train from the 'trigger signal' is integrated into a voltage and the tach is simply a voltmeter. Understood.
So, is it a safe assumption my cluster tach in the Road Runner is a current sense? I assume the tach is pulsed to GND by the old points at the coil and the current is integrated, and converted to a voltage by a capacitor?

Having said all this, I have a few questions:

1) The 'conversion' of the old tachs in the cluster is to a voltage sense type?
2) Are the aftermarket low cost tachs available today all voltage sense types?
3) Anyone out there with a MSD Street Fire and MSD Billet Distributor using a aftermarket tach, or MSD converter?

Thanks for any help with this as I want to understand the method of operation. I may just design and breadboard an integrator using an LM358 and see what I get with a current source.

 

[Billccm]

http://rt-eng.com/rte/index.php/RTE_67-74_Tach_kit

Did some research on tachometers. So a current sensing tach is simply an ammeter in series with the coil positive source. Not very accurate and used in British cars.

Mopar used a voltage integrator tach.

Going to contact rt engineering and get more info on their solution.

 

[Wietse]

How i understood the story on the OEM tachometer (Ralley dash) is that it works on the voltage pulses caused by the triggering of the ignition to the individual spark plugs.
The tachometer converts the pulses to a reading, the old style ignitions pulses were not of the "clean" type compared to modern type ignitions, like the MSD systems.
The "clean" pulses from the MSD requires to be converted again back to the "dirty" pulses for the tachometer to work properly, which requires a tach adapter as MSD provides.
Depending on what type/model ignition you are using there are 2 different types, both are described on the website.
Best bet maybe is to contact MSD and ask them as they can give you a more clear answer on what is required for your application.

https://www.msdperformance.com/products/accessories/tach_adapters/
I am running a RTR distributor and a Blaster SS coil, i am using the 8920 and the tachometer works well.

 

[Billccm]

Tach converted installed and working. The guys at rt-eng.com are great. Great and really fast email support. They helped me get the movement working and their solution has calibration jumpers. Took less than an hour to convert and calibration on the bench.
I also bought their ivr4 and will install that tomorrow.
It's so nice to have the in dash tach working.
Let me know if you need help.

 

[Billccm]

Thanks, glad your happy with the tachboard.

On the pertronix, a probable maybe?
From the my experience they (like MSD) are designed for a performance spark, but a clean signal for the tach
isn't on the pertronix radar for some models.
A waveform (pertronix) of one cylinder fire that the tachboard filter had trouble with.

White line (my) sketch of a normal signal ignition pulse.
Note: pertronix makes many models, the above is simply my experience with one model long ago.
Kind regards,
Tom

 

[Billccm]

TACH Scope pic from rt-eng.com