Tachometer pulses per Rev??

[magic930]

Anybody know how many "pulses per revolution" an 87 Porsche Tachometer is expecting?

[ChrisBennet]

I don't know if this will apply to a 930. This is from a Carrera ('88?) measured at the back of the tach:

100 Hz at 2000 RPM


200 Hz at 4000 RPM


My cat is better at math than I am (despite doing lots of 'nip when she was younger) so I asked her. She gave me that "Are you an idiot?" look and then told me:

"100Hz equals 6000 pulses per minute at 2000 RPM or 3 pulses per revolution."

Hey, I could have figured that out...

Hope that helps,
Chris

[Tim Polzin]

Originally posted by ChrisBennet
"100Hz equals 6000 pulses per minute at 2000 RPM or 3 pulses per revolution."

Hey, I could have figured that out...

Of course, one pulse per point contact opening on an old style distributer. How ya doin Chris?

Tim

[ChrisBennet]

Originally posted by Tim Polzin
Of course, one pulse per point contact opening on an old style distributer. How ya doin Chris?

Tim

Hi Tim,
So my cat was right? Good kitty!

My circumstances are a bit difficult but I'm irrationally exuberant (as usual). Did my first motor drop of the year yesterday. Maybe I can beat my record this year (8 in one year)?

-Chris

[magic930]

Thanks Chris this was very helpful. On my latest software upgrade for my DTA EFI system the "tach pulse per revolution" has been added as a user input from 1-6.

[Tim Polzin]

Originally posted by ChrisBennet
So my cat was right? Good kitty!

Your lucky, I've got a Siamese who is still big into the nip.

Tim

[magic930]

Chris,

I am curious about the images you posted; are they from a regular computer...? if so which software, etc.

David

[ChrisBennet]

Originally posted by magic930
Chris,

I am curious about the images you posted; are they from a regular computer...? if so which software, etc.

David

I have a couple of handheld Fluke oscilloscopes. I took those shots last summer with my Fluke 192 and used some Fluke software to save them to my computer. I was tinkering with the idea of making an electronic tach telltale.
-Chris

[kjb]

Originally posted by magic930
I am curious about the images you posted; are they from a regular computer...? if so which software, etc.

That looks like a screenshot from a simple oscilloscope.

/ J

edit: oops, chris beat me to it. I should have refreshed the thread before replying.

[Tippy]

So the tach receives the signal on Carreras from the speed sensor, not the reference sensor?

[Bill Verburg]

Originally Posted by magic930

Anybody know how many "pulses per revolution" an 87 Porsche Tachometer is expecting?

w/ a stock ignition w/o a wasted spark

3

[Tippy]

So the tach receives the signal on Carreras from the speed sensor, not the reference sensor?

Anyone?

[Bill Verburg]

Originally Posted by Tippy

Anyone?

no the tach signal comes from the Motronic DME unit

the DME receives a reference signal from the flywheel sensor, the Motronic brain then calculates when to fire the ignition based on that signal and a couple of other inputs.

[Tippy]

Just figured out from an older post by Lorenfb.

The reference sensor is only for startup, the speed is for the DME which then translates firing impulses which ultimately feeds the tach.