Original tach showing too high rpm w. MS3X

[SwedCharger-67]

Hi all,
I've built a stroker engine for my 1967 Dodge Charger, running on E85 with full sequential contol. All works well.

Since I want to have the original tach fully operational, I bought an MSD 8920 interface adpter, which is fed from the MS3X tach output and outputting the signal to the (originally ignition coil triggered) tach. Running a bench test the 5krpm on MS3X read 5krpm on the tach...all was well so far...

But, to my surprise, installing all in the car the original tach now reads approx 1000 rpm too high over the whole scale!

Is there a way in software or hardware to modify the MS3X tach output to set the old tach to show correct readings?

Martin

[CRSTune]

In "Basic/Load Settings," there is a "Tacho Output" menu. If you switch it from "Fixed" to "Variable," you can set a percentage adjustment value for the tach signal.

If you're saying the value is a constant 1000rpm high over the entire band, you might just need to adjust the needle. Was it accurate with the original setup?

[SwedCharger-67]

Thanks a lot for the reply.

I calibrated the original tach with a signal generator a few years ago so it was pretty accurate at the time. It was however not possible to set it perfectly...it wasn't entirely linear but showed an S-curve around the correct values.

Setting the duty cycle in software seems perfect, I just want the tach to show reasonably correct readings.

Thanks again, I'll have a look at it tomorrow!
Martin

[SwedCharger-67]

Yep, setting the duty to 63% and all works beautifully!
Thanks,
Martin