Hi all, can some kind soul enlighten me on ignition / tach signals.

If I have a 4 cylinder tach connected to the neg of the coil with no conversion logic in series, would I expect the reading to be half or double of what the actual RPM is?

Web search shows conflicting info. Some say half, some say double.

Cheers
Rob

Post research it would display half the actual. expects 2 ppr and all only gets 1 ppr.

Sorted.

R

lazylobster said:

Post research it would display half the actual. expects 2 ppr and all only gets 1 ppr.

Sorted.

R

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Sorry late to thread. You are correct it will under-read. You could try a motorcycle tacho. One for an American thumper, not a Japanese screamer

Robert G8RPI.

What make of Tacho have you got fitted? Smiths make an electronic tacho that can be adjusted for 2 + 4 + 6 + 8 cylinder engines. It is what I have got fitted in my cluster. It is altered by what sequence a little row of switch es internally are altered to. And, as a bonus, it only reads to 7,000rpm.

I bought a brand new Harley one from a guy who said he had it for a 500 engine but never fitted it. When I hooked it up it read double. A knowledgable mate told me you could modify the electronics. So I carefully prized it open than contacted my mate to ask him what to do. He said " I don't know, I had just heard it was possible"
Aaaargghhh !

Toshi 975 said:

I bought a brand new Harley one from a guy who said he had it for a 500 engine but never fitted it. When I hooked it up it read double. A knowledgable mate told me you could modify the electronics. So I carefully prized it open than contacted my mate to ask him what to do. He said " I don't know, I had just heard it was possible"
Aaaargghhh !

Click to expand...

I had my Veglia Borletti one modified for the two cylinder operation.
I'll have to dig out his details.
I ended up with eight period Veglia gauges
Revs, petrol, speedo, oil pressure, oil temp (super rare most are water temp) amps volts and clock

Thanks for all the answers.


I have a digital tach that supports 4, 6 or 8 cylinder operation. I have an electronic tach convertor in line, so this doubles the signal to feed the 4 cylinder setting. To me, at tickover, of around 800RPM, it all sounds right. But I've posted pictures on the gram at 100kmh and the tach showing approx 3200rpm and I've had a few comments that that is on the low side. However the max torque of the 594cc is 3400 and max power about 4800. I also have a sync gearbox, so expect the final drive ratio plays a part as well.


So I've purchased a cheap LCD 4 cylinder tach to connect to the neg of the coil and see what this says. If I'm getting circa 400rpm at tick over, then the original tach I have installed must be correct.If not . . . .

R

Me again. So as you may know I had some doubts raised over the accuracy of my RPM gauge when I had posted pictures up on Instagram. In essence, at around 100kmh, RPM shows around 3200. 594cc engine connected to a later sync gearbox. The factory specs for the 594 being max power at 4800rpm and max torque at 3400.

I have a digital Tach that is configured to 4 cylinder and use a signal converter to double the signal input.

After a run today, engine hot. oil showing 77c and idle steady. I connected up an inductive tachometer to a spark plug lead. The tach is configured for 2 pulse per revolution.

Results seem conclusive. I’ll shut up about it now.

Rob