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Rpm - tach readings vs multimeter vs Ear

PostPosted: Sun May 20, 2018 2:35 pm
by richardcox_lotus
Fettling today, wanted to check the timing, but I can?t see the tach from outside the car so hooked up my multimeter. The differences were surprising:

Tach in car showed 6-700 revs
Multimeter showed 1500-1700 revs
My ear thought about 1100 revs.

I?ve always suspected the tach in the car to be a little low but not that much out. Has anyone else had strange readings from a multimeter ?

Nb car is stock, with points.

Regards
Richard

Re: Rpm - tach readings vs multimeter vs Ear

PostPosted: Sun May 20, 2018 10:10 pm
by billwill
Sorry, but what kind of multimeter can count pulses ???

You would need a frequency meter with an adjustable input circuit as the pulses are not simple square shaped.

Re: Rpm - tach readings vs multimeter vs Ear

PostPosted: Mon May 21, 2018 1:07 am
by Chrispy
I've seen several multi's over the years with a RPM readout.

Re: Rpm - tach readings vs multimeter vs Ear

PostPosted: Mon May 21, 2018 5:59 am
by richardcox_lotus
Bill it?s a Gunson -G4187; now discontinued.

Re: Rpm - tach readings vs multimeter vs Ear

PostPosted: Mon May 21, 2018 12:38 pm
by pauljones
Hz x 60 = rpm

Ie 10hz reading is 600 rpm.

Different on wasted spark systems because you pick up the 2nd spark on the exhaust cycle. So for me i half to half the answer to correct it.

Re: Rpm - tach readings vs multimeter vs Ear

PostPosted: Mon May 21, 2018 2:04 pm
by bob_rich
Hi Richard

that looks an interesting multimeter. I could not down load the book anywhere but as is only has probes I assume ( for +ve earth) that it is placed on the coil CB terminal +ve probe and earth -ve probe so with a standard coil ignition system that would be 2 pulses per rev for a 4 cylinder engine.

Just guessing here but could you check it on say an "old fashioned" battery charger ? this would give a rectified AC pulses at 100pps and 20V or so peak which would be 3000rpm as a guide that the meter is working ok.

but without the book can't be certain if it would work !

hope this helps

best of luck

Bob

Re: Rpm - tach readings vs multimeter vs Ear

PostPosted: Mon May 21, 2018 2:37 pm
by ericbushby
My Fluke 87 has a frequency range on it, but I have not tried it.
I use a Tektronix THS720A digital oscilloscope with the low pass filter selected. It seems to work fine.
100 cycles/sec on the coil circuit is 3000 RPM.
Not usual garage kit I know, but I have it left over from retirement so I use that.
Eric in Burnley
1967 S3SE DHC