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Test your Tach (you may be surprised)
#1446102
06/01/13 09:14 PM
06/01/13 09:14 PM
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Joined: Sep 2004
Posts: 2,091
oberlin, Ohio
Rapid340
OP
top fuel
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OP
top fuel
Joined: Sep 2004
Posts: 2,091
oberlin, Ohio
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I tested a Mallory and Autometer tach and found them both to be off by what I consider to be quite a bit (more than 5%) at different points from 1K to 7.5K RPM.
Since most of us dont have a frequency generator, I sketched up a simple schematic (parts that can be purchased from digikey for under $20) that can be used instead. All you really need is an accurate frequency meter. Many digital volt meters are accurate to within .5% in this range (you can do a rough check by measuring the line frequency of your house AC, it should be 60Hz).
Build the circuit powering it with the battery of your car and connect the tach green wire to the output (LM555 pin 3 thru a 100 ohm resistor). The tach wire should go to this circuit only. You will need to disconnect it from the coil or ignition box to do this measurement.
Vary the 25 turn pot until your tach reads the desired RPM and then record the frequency (Hz) that produces that tach reading.
Actual RPM = DVM frequency (Hz) * 15
This allows you to check the upper RPMs without the engine running (The key or power switch will need to be on so the tach is powered up). Dont use this circuit with a voltage that exceeds 16V. A battery charger for a 16V system would exceed 16V.
Example: A 400Hz signal should produce a 6000 RPM Tach reading on the nose. One of my tachs read 5700 RPM with a 400Hz input. The Autometer Tach was way off around idle RPM range.
Note: Use a little 9V battery instead of your car battery in case you make a mistake. (or use a fused car battery power source)
Last edited by Rapid340; 06/02/13 09:06 AM.
1971 Factory Appearing Duster 340 11.000 @ 122 mph
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Re: Test your Tach (you may be surprised)
[Re: Rapid340]
#1446109
06/02/13 12:07 AM
06/02/13 12:07 AM
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Joined: Aug 2003
Posts: 43,172
Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,172
Bend,OR USA
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Quote:
The reason I started investigating this in the first place is because I wanted to get an idea what the converter slip was. When I went through the calculations, based on my tach reading and trap speed, I was coming up with zero slip. We all know that is not possible with a conventional torque converter.
I saw this very thing years ago, I built two Identicle NHRA legal 426 Street hemi motors, one would rev to 7000 RPM in 1st,2nd and 3rd gear, that car ahd a earlt Sun tach, the other one was a stck car and it would stop pulling at 6000 RPM according to the very expensive auto Meter race tach    I ended up putting Jones Motorola mecahnical cable driven tach in it and when the Autometer showed 6000 RPM in third gear the mechanical was reading 7000 RPM I saw it once more with a cheap Auto Pro or what ever the cheaper line of Auto Meter was in the mid 1970s, it was on a stickk shift 340 car with a big solid roller cam, it would quit pullling around 6800 to 6900 RPM, he put a Stewart warner mechanical tach drive tach in it, the car had 5.57 gears in it awith a 3.09 1st gear tranny, he look down in third gear just before shifting to see what both tachs read and the electric read 7000 RPM and the mechanical read 8200 RPM,  It spun a rod bearing shortly after that while idling in nuetral after a run  Check your tachs, all of you  
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Test your Tach (you may be surprised)
[Re: Cab_Burge]
#1446110
06/02/13 12:43 AM
06/02/13 12:43 AM
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Joined: Feb 2011
Posts: 4,302
Nebraska
72Swinger
master
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master
Joined: Feb 2011
Posts: 4,302
Nebraska
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I know my tach doesnt match my rev limiter. Im guessing its off about 100-150 rpm. I do wonder where my idle REALLY is.
Mopar to the bone!!!
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Re: Test your Tach (you may be surprised)
[Re: 72Swinger]
#1446111
06/02/13 07:06 AM
06/02/13 07:06 AM
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Joined: Jan 2003
Posts: 2,917
A shed in England
Tig
master
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master
Joined: Jan 2003
Posts: 2,917
A shed in England
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I'm pretty sure if anyone has a digital type ign system you can monitor and/or download engine rpm. I'd guess this to be more accurate than the tach. Mine is out -200 rpm at the bottom of the range and and -210 at the top a fairly linear error.
My indicated 1100rpm idle is really a rowdy 1300.
If anyone has access to a waveform generator remember to switch to square wave 12v o/p and common connected the -ve to the psu.
'74 Challenger..9.46 @ 145.9 1/4, 6.001 @ 118 1/8 so far. 4023lb !!! # N/A, Marsh performance 655ci, Indy Maxx, T/R, Indy 600-13 X's, Street legal, pump gas, full interior, Cal-Tracs, mufflers, 3:73's and real 10.5 radials.
9.51 @ 142.4 1/4, 6.003 @ 114 1/8 with our old mule KB, 572-13, 580 wedge.
RHD '68 Barracuda Fastback 323ci street/strip. Best ET 13.88 @ 99.03
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Re: Test your Tach (you may be surprised)
[Re: Tig]
#1446112
06/02/13 10:52 AM
06/02/13 10:52 AM
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Joined: Jan 2006
Posts: 419
NY
F1Scamp
mopar
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mopar
Joined: Jan 2006
Posts: 419
NY
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Sorry to hijack Jeff. 11.000? When the hell did you run that? Congrats man.
You going to Martin?
Work In Progress- 71' Duster F.A.S.T.- 10.36@130 Smallblock Record Holder.
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Re: Test your Tach (you may be surprised)
[Re: Rapid340]
#1446113
06/02/13 11:30 AM
06/02/13 11:30 AM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
Master
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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When doing this test will the tach jump up to the RPM when you have it hooked up to the tester.... also do you have any other values for different rpm's as in 1500 rpm and maybe 4000 rpm to see if its linear thanks 
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Re: Test your Tach (you may be surprised)
[Re: nss guy]
#1446115
06/02/13 12:44 PM
06/02/13 12:44 PM
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Joined: May 2003
Posts: 6,562
Downtown Roebuck Ont
Twostick
Still wishing...
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Still wishing...
Joined: May 2003
Posts: 6,562
Downtown Roebuck Ont
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Quote:
I check the electronic tachs against my cable drive tach
That would work assuming the mechanical tach is known to be accurate ie been calibrated.
Kevin
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Re: Test your Tach (you may be surprised)
[Re: RodStRace]
#1446117
06/02/13 01:25 PM
06/02/13 01:25 PM
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Joined: Apr 2006
Posts: 3,675
Akron, Ohio U.S.A.
roadhazard
master
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master
Joined: Apr 2006
Posts: 3,675
Akron, Ohio U.S.A.
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Quote:
Great info!
I know fuel pressure gauges are all over too.
Why are we paying so much for these guages when their accuracy is questionable 
So if you find your tach is lets just say 300 RPM off..... what can you do about it? Is there a internal adjustment you can make
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Re: Test your Tach (you may be surprised)
[Re: roadhazard]
#1446118
06/02/13 01:37 PM
06/02/13 01:37 PM
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Joined: Aug 2003
Posts: 43,172
Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,172
Bend,OR USA
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Quote:
Quote:
Great info!
I know fuel pressure gauges are all over too.
Why are we paying so much for these guages when their accuracy is questionable
So if you find your tach is lets just say 300 RPM off..... what can you do about it? Is there a internal adjustment you can make
I think the main message is to not take the instrument readings as the absolute truth, some gauges are not ACCURATE I've seen a bunch of supposedily high quality aftermarket mechanical temp. gauges be off 40 degrees, reading 140 F when the actual water temps are right at 100 F according to a certified claibrated thermometer All instruments are a measuring device to provide us information, accurate information  When in doubt, check it out I learned on that first electric tach to move the lever when the motor quit pulling,no matter what the tach read, not to keep it in gear with the right fooot down waiting for the tach to read 7000 RPM
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Test your Tach (you may be surprised)
[Re: roadhazard]
#1446119
06/02/13 02:32 PM
06/02/13 02:32 PM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
Master
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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Quote:
Quote:
Great info!
I know fuel pressure gauges are all over too.
Why are we paying so much for these guages when their accuracy is questionable
So if you find your tach is lets just say 300 RPM off..... what can you do about it? Is there a internal adjustment you can make
Accuracy sure is nice ... specially if you are trying
to figure your conv slippage rate... when I was working
and setting up test cars we had to test and calibrate
all the gauges and transducers before we put them
on the car and sign off on the calibration... we
had the calibration equipment right in the lab so
it was pretty easy... before that we had to take
them to another lab just to get calibrated and that
could take a week or better depending on where we
were in line of other customers
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I wonder how many cars will burn down from people like me trying this..?? 
