Proving tach accuracy...ever wonder??

mrvette

mrvette

Phantom of the Opera
Joined
Mar 24, 2008
Messages
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Location
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Ever wonder if you tach is accurate? and so by comparo with anther tach means tear it out of your buddies car for a test, PIA.....so how do you know???

I realize this means you need some test gear, another PIA for those not electronic tech oriented.....

but here is what I did.....having a Oscilloscope, and a digital audio oscillator....

figger engine at idle...600 rpm that's 10 trips up/dn for the piston.....

or 5 sparks/second....jeebus that's fast for a mechanical thingy....
but slo as hell for electronics.....

so 5 sparks/cyl X 8 cyl for a V8, obviously that is 40 sparks/second.....at the coil.....audio rep rate.....

so if engine was running 10 x as fast 6000 rpm the sparks are at 400/second, about the freq of the dial tone on a old land line phone.....

Most audio oscillators will put out a sine wave, looks similar to a power line curve....smooth transisitons + and - above and below the Zero base line....

but the spark coil output is anything but that, it is switched, and switching a coil of any type always gives that inductive kickback, which for us is useful for making sparks at the plugs.....it is a hell of a spike with very little after affects (called ringing) which are controlled

SO, being a geek, I have a certain transistor used for driving color TV deflection yokes (that coil of wire around the pix tube, for you old timers) so I used one of them driven by the audio oscillator, and it's output connected to the tach output of the HEI, engine still, key ON to supply 12 volts to the HEI.......

read the tach for accuracy...do the simple math and track that tach....

my old factory tach is dead nutz in at 1500 rpm, at 3000 it ready 300 high, and same 300 high at 4500 and even 6000 rpm.....which tells me it's not electronic but something in the mechanical balance of the pointer/readout/spring.....

naturally I wrote it all down on the gearing calculator printout I referred to in another thread, and so reminded me to do this post/thread....

hope it helps ....


:smash:
 
I checked mine with a multimeter and a digital multifunction timing light. The digital reading is a bit flakey at idle, but pretty near the same as the mechanical tack at rpm's above 1000.
 
You went about it the hard way Gene, I compared mine with my digital dial back light,and they were within a 100RPM of each other,the car tach showed 100RPM higher than the light. Close enough for me :thumbs:
 
kwplot34 said:
You went about it the hard way Gene, I compared mine with my digital dial back light,and they were within a 100RPM of each other,the car tach showed 100RPM higher than the light. Close enough for me :thumbs:
Click to expand...

You assume your secondary tach is accurate....just as much chance it's not as the first.....I know the digital oscillator is accurate as when I line synch the scope sweep and then feed the oscillator signal into it, the signal parade is totally still with very little drift....very slow...

same at 600 hz....very slow drift...just a phase shift really....

I don't trust el cheepo import tach anymore....dunno about your gear...

:stirpot:
 
mrvette said:
kwplot34 said:
Accurate enough for me :harhar:
Click to expand...

You own a micrometer, don't you???

jeesch.....

:lol:
Click to expand...

YIP!

In order for you to change my mind,you are going to have to do more than one test with the scope on more than one Tach,and then you are going to have to do the same test with my dial back and compare the results.And then we will see if there is a reason for me to change my mind.
But the bottom line is,this is not a race car and i do not need to know down to Gnats hair what my RPM is,if its off by 100 RPM so be it.
 
mrvette said:
kwplot34 said:
Accurate enough for me :harhar:
Click to expand...

You own a micrometer, don't you???

jeesch.....

:lol:
Click to expand...

Yessir....but I don't have an Oscilloscope and a digital audio oscillator or a clue about what you are talking about in your post. But I do admire that you have the ability to think these things up:friends:

BTW...I'm still waiting for you to start your elec. car project. Wasn't there some lottery winner who was bored hanging here the other day...JoeDirt...hit him up for some backing scratch$$$$$
 
kwplot34 said:
mrvette said:
kwplot34 said:
Accurate enough for me :harhar:
Click to expand...

You own a micrometer, don't you???

jeesch.....

:lol:
Click to expand...

YIP!

In order for you to change my mind,you are going to have to do more than one test with the scope on more than one Tach,and then you are going to have to do the same test with my dial back and compare the results.And then we will see if there is a reason for me to change my mind.
But the bottom line is,this is not a race car and i do not need to know down to Gnats hair what my RPM is,if its off by 100 RPM so be it.
Click to expand...

I hear you, but what prompted the test was the fact that I have NO real way to prove my speedometer was correct as it did say X speed for my tire size and gearset and o/drive ratio....the two instruments did line up....

just that nagging feeling that when I"m cruising at 90 and getting pushed by trucks/suv/18 wheelers etc....let alone other cars, and I"m not FLYING by other vehicles, I get a bit to wonder if there is a double mess up there somewhere......so I did my test my way and know for sure damn well proven that the tach is accurate as stated, tested several times....all through the band....

so after this was done, Like I said before, wife proved it by knowing the mile markers/roads /distance....and she did a quick mental math having done X miles in 20 minits time.....yup....90.....so some gas crisis, I guess....

:quote::bump:

but I have it all written down in the tech file, getting thicker....some day my son Tom will have some notes to fix it by....

or sell it...
 
If you are having problems with the speed/o and think it may be off,put a GPS in the car,those things are damm accurate,surely you know somebody that has one hanging on their windsheild you could borrow and do some runs at diffrent speeds and get some numbers.
 
kwplot34 said:
If you are having problems with the speed/o and think it may be off,put a GPS in the car,those things are damm accurate,surely you know somebody that has one hanging on their windsheild you could borrow and do some runs at diffrent speeds and get some numbers.
Click to expand...

I had considered that, but they only accurate to 80mph, so I"m told....

I was more interested in speeds above 120 to know things were in line....

and so I definately proved I have been at 150 mph with the top and windows down.....

freeking FAST do it sometime....top and windows down....not the same as a bike even as I doubt a rider could hang on at that speed without some wicked fairing/windshield, and a good helmet....


I don't ride bikes anymore.....so really dunno more than the obvious comment above....

:fishing:
 
Thats not true. they are as accurate at 1mph as they are at however fast you can go,depending on if the unit you are using will go that high in MPH.
You are using the same sattlites that the military uses,now untill recently there use to be a diffrence in the signal the military received and the general public,but that user switch was turned off and everybody gets the same accuracy,now in the time of war or some other catastrophy the gov't can flip that switch again and the general public signal will have inaccuracy again,but since so many people rely on the signal it is turned off,so if you have a GPS that will read 200mph it will be accurate.
If you borrow one make sure it is fairly new within the last 5 years or so as the newer ones can lock on to more sattelites and have 3D,accurate down too a Gnats hair. There are 24 sattelites up there in the sky only 12 of them are in use the others are for back up,with a clear line of sight to the sky you should be able to get 8 of them,they are not all in the same orbit at the same time so you cannot get all of them at the same time.
 
Gene,
I remember Duke94 did some kind of easy test. Took a 12 v battery charger, like foir a cell phone, and it put out 120HZ by changing some diodes. Tach should read 600RPM?? (That make sense)?
 
big2bird said:
Gene,
I remember Duke94 did some kind of easy test. Took a 12 v battery charger, like foir a cell phone, and it put out 120HZ by changing some diodes. Tach should read 600RPM?? (That make sense)?
Click to expand...

Nope 40 hz is 600 rpm.....so it would be 1800 rpm if you can get the tach to react....my stock '75-7 factory tach did not react with the oscillator direct....so I figgered it took more of that spike to activate it through all the filters and crap...there are input filter network in the tach also...not just under the hood....so anyway, it was proven out my way....

the GPS units the civilians have I WAS TOLD are limited to 80 mph for the readout in the unit itself.....old wives tale or not, I went with my own test kludge for ZERO cost, and info off the net.....


which all and all a hour's time and it was over with....

spent more time getting the gear set up than doing the test....

:shocking::pprrtt::trumpet:
 
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