Tach accuracy question...

[d240zx2]

'70 Z.

My tach is seating at 200-300 reading when the engine is turned off. It gradually seats to zero after about 30 seconds and/or when I tap the dash. Is this a matter of a failing tach or is there dirt in the way somewhere? Coil connection is tight.

Would you assume or speculate that I should use the added 200 - 300 rpm reading when determining actual red-line?

Thanks for your help!

Regards,

[d240zx2]

Any ideas yet?

Thanks!

[jmortensen]

You could buy a cheapo aftermarket tach and check the stock tach against it. My impression of the stock tachs is not favorable.

[moritz55]

Mine actually reads ~400 RPM above Zero when off. Not sure why, I thought it may be since my tach was originaly in-operable becuase the prior owner cut the Green/White wire that once was connected to the Ballast resistor when he installed a new coil. Anyway I just rewired the harness in the front and got the Original tach to work yesterday. Have been driving adding >100miles in 2 days and I noticed the Tach rests at ~400 RPM.

So - to calibrate the Tach I had a Digital Timing Light with Digital Spark Advance. I used it to validate my Tach at various speeds.

Here's what I got:

Correct Reading Old Tach Reading

Off - Zero 400 RPM

1000 RPM 1100 RPM

2000 RPM 2100 - 2200 RPM

3000 RPM 3100 - 3200 RPM

I felt that it was good enough since my car actual RPM was ~100-200 RPM lower than what the tach was reading.

I bought the Digital Timing light with RPM gauge for assruing my timing was set at the exact RPM. So it came in handy once I git the stock tach to work.

Hope this helps and good luck, Mark.

[hls30.com]

Sounds like an aftermarket ignition was added and either the tach wiring was not modified for it, or it was not replaced with a later tach. There is a tech article around here from Bambikiller240Z on this...

Will

[EScanlon]

Or simply that the needle has been relocated on it's spindle to read 400 rpm at 0.

Moritz:

Did you compare idle readings? What about more data points to see if your discrepancies are constant or isolated to certain ranges. Will your digital tach be accurate below a certain RPM? That might put doubt on the balance of it's readings as well as the original tach's.

But to both Moritz and the d240zx2:

I wonder why you both require such a tight tach reading. Or are you shooting to redline it? If so, you might want to consider that the "redline" is just an approximation of what was considered too fast for the engine to be revving as oppoised to a definitive point of engine destruction.

An engine that is well maintained and in perfect working order won't have a problem with occasional bursts into the "redline", but another engine without the same care and health might blow up at a much lower RPM reading. But neither one will do well with constants and prolonged surges into the "Red".

And don't forget that it's over the total history of the engine that you have to consider it's health. A poorly abused engine that is now being cared for meticulously will operate for a longer time, but won't be as strong or healthy as one that was maintained very carefully since day one.

Just my 2¢

E

[d240zx2]

Excellent observation, EScanlon.

My primary concern is that because I run the old gal on the track occasionally and that the engine still has the original 5 CW crankshaft in it that a "slight" excursion beyond 7 grand will create a two-piece crankshaft.

In order to be as competitive as I want to be with the car, yet keeping it streetable and as original as possible (those may be diametrically opposing goals), I was wanting the tach to be dead-on accurate. But it appears that may not be possible.

Do you think that my installation of the Pertronix system (stock coil) would have affected the reading?

Thanks,

Frank

[EScanlon]

This is where "book" knowledge and "in-use" knowledge start to show the strain.

The early tach, commonly referred to as the "4-wire" tach (due to it's connector) gets it's signal directly from the current going to the coil (Black/White and Green/White wires) from the Ignition Switch. Later style tachs went to a one wire sensor and not a loop, which reduced the number of wires to the tach's mechanism by one, hence the reference as "3-wire" tachs.

While I've read that people feel the later style tach (3-wire) is more accurate (definition still to be determined), I've not read of anyone being able to convert a 4-wire wiring harness to use a 3-wire tach. The reason for this is that the 4-wire Loop is a requirement of the ignition system in order to provide power to the coil. Cut that wire and you won't get any current to the coil at all.

Now as far as your question goes, there have been numerous people that have swapped out the points disributor for a ZX dizzy in order to gain the E12-80 module (and loose the points), and have also swapped the coil in order to allow eliminating the Resistor from the circuit. What this may imply is that the change in the coil might not have had any effect on the tach...but actual experience may differ. That's the problem in saying for a fact that it would or wouldn't affect the tach.

But since your intent is function and not originality, why not check into some of the conversions to more modern tach's? That way you'll gain the accuracy you want, and bypass the old technology.

FWIW

E

[moritz55]

Back to EScalan's Question:

- Yes I did run the range for comparison and stopped at 3500 RPM since my neighbors were a bit upset with the loud engine noise. The net was that the stock tach always measured 100-200 RPM higher than actual. I'm sure my error in reading the tach was greater than the true measurement error as I peered into the car from the window. I did drop my idle down as low as i could go ~750 RPM on the digital tach so the engine still ran smooth using my digital timing light tach, and the RPM gauge in the car actually measured around 50-75 higher. At the low idle it waivered a bit but wasn't terribly off by much given it rests Off at 400 RPM.

From my perspective I'm not terribly concerned about the exactness of the Stock Tach to true engine RPM, only it's reasonable accuracy. I am a more a spirited driver and do not race competitively. I've never Red lined the car since rebuilding the engine so for me this works fine and keeps my car as close to Stock on the inside as possible.

When I added my Mallory Unilte distributor and Promaster Coil/MAL 700 Ballast I used this link to assure my 240z 4wire Tach could be integrated without major calibration/accuracy issues. Hope this helps some:

240z Tach Wire Hookup: http://www.zhome.com/ZCMnL/tech/MalloryDist.htm

Many thanks all,