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Thread: The stock digital tach is apparently very inaccurate

  1. #1
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    Default The stock digital tach is apparently very inaccurate

    The stock digital tach is apparently very inaccurate. I recently bought an Autometer tach/shift light so I can shift my car manually at the track (it has an epoxy mod AOD). I hooked it up and set the Pulse Per Revoluation setting to 4 (what Autometer says a single coil V8 should be set to). I knew from previously using a tach/dwell meter on my car that the stock tach is around 200-300rpm off at idle. Now you can see it in the car.



    It gets worse at higher RPM. According to the stock tach the car's cruise rpm at 70mph is 3000rpm. It's actually 2300-2400rpm. The yellow shift light is on as I set the shift light to come on at 2250rpm for testing.



    It gets worse. At 4000rpm on the stock digital tach the Autometer tach is displaying 3400rpm. Above idle I observed the stock digital tach reading 500-700rpm high. The Autometer tach is being fed a signal from the ignition coil wire under the hood so it's getting a good signal. So the moral of the story is the stock digital tach is way off. Don't use it for any kind of performance driving.
    '88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, Edelbrock 70mm TB, 76mm C&L MAF, 30lb injectors, 2.5" exhaust, AOD with 2800 PI converter, 8.8 with 3.73 gears, 31 spline Traction-Lok, 31 spline Moser axles, 04 Cobra front arms, Maximum Motorsports extreme duty rear arms, subframes.
    '11 Focus, '12 Mustang 3.7, '17 Accord EX-L V6

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    I'm not really suprised, stock gauges basically only give you a general idea of where it's at. although, my speedo is dead-on with my phone's speedo app.

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    so what I have learned here is .........
    when I go and buy any new duplicate part to compare to my car, if the new thing say something different, my car must be wrong.

    are you running the stock size tire????
    wheels?

    this isn't any way to troubleshoot, you gotta get more than just one sample.
    maybe its time for a dealership calibration inspection,,

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    Well the speedometer has nothing to do with tachometer calibration. I just revved the engine to 3000rpm (what the stock tach shows when the car is traveling 70mph) and the autometer tach reads 2300rpm. I'm running stock size tires and the speedometer driven gear matches the rear end gears so it's fairly accurate. A rpm calculator shows 2400rpm at 70mph with my setup. The Autometer tach is pretty dead nuts on with the calculator, the stock tach not so much. I've run two different tach/dwell meters on my car to check base idle. Both have said 700rpm in park, just like the Autometer tach. The stock tach has always read 1000rpm at that same idle speed. Checking the stock tach against the Autometer unit the stock tach reads around 25% faster than actual engine speed. I'm pretty sure that it's just crappy calibration by Ford. Stock fox body Mustang tachometers have been know to be up to 25+% off. I'm betting our cars are probably the same.

    Also note my car is not moving during this test.
    '88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, Edelbrock 70mm TB, 76mm C&L MAF, 30lb injectors, 2.5" exhaust, AOD with 2800 PI converter, 8.8 with 3.73 gears, 31 spline Traction-Lok, 31 spline Moser axles, 04 Cobra front arms, Maximum Motorsports extreme duty rear arms, subframes.
    '11 Focus, '12 Mustang 3.7, '17 Accord EX-L V6

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    I think the inaccuracy engine rpm displayed has nothing to do with tire size or rear gears. I always figured the digital tach wasnt totally spot on, but it sure looks cool......
    Mike

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    I'm wondering if Ford didn't calibrate the digital tachometer for the V6/V8 but instead just used something that worked "close enough" for both engines. That would save them the trouble of calibrating the tachometer for each different engine. The factory tachometer in my car is between 20-25% off at different rpm. I wonder how far the tachometer is off on V6 cars with the full digital dash....
    '88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, Edelbrock 70mm TB, 76mm C&L MAF, 30lb injectors, 2.5" exhaust, AOD with 2800 PI converter, 8.8 with 3.73 gears, 31 spline Traction-Lok, 31 spline Moser axles, 04 Cobra front arms, Maximum Motorsports extreme duty rear arms, subframes.
    '11 Focus, '12 Mustang 3.7, '17 Accord EX-L V6

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    Remember what I've always said: everything concerning our era digital gauges are representative only. Nothing was accurate. Analog wasn't much better. But it was closer than digital.

    One bar on the tach represents...what? 250 rpms, maybe? But that is wrong, as proven above. It makes for an interesting light show but it is not a true picture of the actual rpms.

    The closest we have to an accurate digital gauge is the speedo. Everything else, take it with a grain of salt.

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    I had a 90 LS, which had the full digital, mine would be around 18-1900 rpm @ 65mph.
    Current: 1986 Thunderbird, 105k 3.8/C5, 2 tone Midnight Wine/Medium Taupe, wing windows, wire wheel covers.


    Former: 1985 Cougar GS 115k Oxford White/Regatta Blue, 5.0, full console, 14" 8 hole aluminum T-Bird rims, Edelbrock valve covers.

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    Quote Originally Posted by EricCoolCats View Post
    Remember what I've always said: everything concerning our era digital gauges are representative only. Nothing was accurate. Analog wasn't much better. But it was closer than digital.

    One bar on the tach represents...what? 250 rpms, maybe? But that is wrong, as proven above. It makes for an interesting light show but it is not a true picture of the actual rpms.

    The closest we have to an accurate digital gauge is the speedo. Everything else, take it with a grain of salt.
    True. Each bar represents 200rpm. So when the bars are lit up to 3000rpm it could be any engine speed between 2800-3000rpm. I wish it was slightly more accurate. Then I wouldn't have to run the aftermarket tach/shift light.
    '88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, Edelbrock 70mm TB, 76mm C&L MAF, 30lb injectors, 2.5" exhaust, AOD with 2800 PI converter, 8.8 with 3.73 gears, 31 spline Traction-Lok, 31 spline Moser axles, 04 Cobra front arms, Maximum Motorsports extreme duty rear arms, subframes.
    '11 Focus, '12 Mustang 3.7, '17 Accord EX-L V6

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    as much as I like these cars more than the MN-12, I will say this, the ONE thing that I feel was better was the layout of the electronic cluster in them, oh, and the HO 5.0 :-D the 85-88 just has a kooky layout that I've never really liked.

    Also, I forgot about the one that could be had in the Aerostar, looks a lot like the Fox one, doesn't it?
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    Current: 1986 Thunderbird, 105k 3.8/C5, 2 tone Midnight Wine/Medium Taupe, wing windows, wire wheel covers.


    Former: 1985 Cougar GS 115k Oxford White/Regatta Blue, 5.0, full console, 14" 8 hole aluminum T-Bird rims, Edelbrock valve covers.

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