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HEI & Factory Tachs....A new "discovery"...!

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by Greg72, Mar 21, 2002.

  1. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    HEI & Factory Tachs....A new \"discovery\"...!

    For those of you who have been on this list for a while, you already know we've discussed how the GM HEI wreaks havoc with the factory tach and makes it indicate an RPM that is too high..... I have always suspected this was the cause of my high readings in my '72, and today I had a chance to prove it to myself.

    I installed the MSD 6AL box today (actually, I installed it yesterday and finished the wiring today /forums/images/icons/smile.gif) and hooked the "signal wire" from the MSD box back into the factory tach. After a short test drive, it became obvious that even though I'd competely removed the HEI module from the distributor....the tach was STILL reading too high!!! /forums/images/icons/crazy.gif

    So then I REALLY started questioning whether maybe the tach was right, and my engine was just revving higher than I wanted.....forunately, the package from Summit that contained the MSD ignition ALSO contained a new Autometer Pro-Comp Tachometer!! I installed it in a VERY temporary way (Duct taped it to the dashpad!) and wired it up.....but I decided it would be cool to split the signal wire at the MSD box, so that BOTH tachs would be operating at the same time.

    The result was that they both APPEARED to read correctly at idle and at very low RPMs, but as the engine revved higher.....the factory tach began climbing extremely fast. Here are some examples of readings

    Factory Tach vs. Autometer (RPMs)

    1000 .....714
    2000 .....1428
    2500 .....1785
    3000 .....2142
    3500 .....2500
    4000 .....2857


    My "old" crusing speed of 65MPH used to always read at around 3500RPM......it was nice to find out that the TRUE engine speed is actually only 2500RPM.

    The bottom line, is that there must be more to the "Inaccurate Factory Tach" phenomenon than just HEI ignition upgrades. I don't know what else it could be, but I wanted to let people know that you probably shouldn't trust your factory tach even AFTER an MSD Ignition upgrade unless you have a way to verify it's accuracy with a second gauge.

    FWIW
     
  2. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Very interesting. Thanks for the info dude. /forums/images/icons/smile.gif
     
  3. NoSmog73

    NoSmog73 1/2 ton status

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Hey greg thanks for the info..
     
  4. 69K5

    69K5 1/2 ton status

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Greg'
    How hard was that to put in? I've been thinking about upgrading from points. is it worth the trouble?

    Nathan
     
  5. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Nathan,

    I don't know how much EXTRA work the points add to a normal HEI conversion....but it was pretty easy to do. About 8 wires total....

    I took pics of the steps, so I will probably post them as a "How To" instruction in my Members Gallery area.

    I also put a post in the "Garage" forum on my impressions of the MSD box.....you can read more about it there too..... /forums/images/icons/cool.gif
     
  6. Steve_Chin

    Steve_Chin 1/2 ton status

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    The ignitor module's trigger signal (whether it's for an MSD or and HEI) has a very sharp, more or less quare shaped leading edge to it. That is in contrast to a more sinusoidal waveform put out by a points ignition. The rate of change of voltage is much higher, causing an erroneous reading on the tachometer. On the plus side, it gives the coil more saturation time to get off a "hotter" spark.

    Anyhow, a filter circuit is necessary to rectify the problem with many older OEM and aftermarket tachometers. Some newer vehicles actually have this problem as well. My Fiero has a filter attached to the coil to rectify the problem...
     
  7. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Dr. Chin,

    I was hoping to "bait" you into responding with this post!!! /forums/images/icons/laugh.gif

    I figured that something about the MSD was still not right....you're explanation makes sense to me. If I really wanted to keep the factory 5000RPM tach, I'd probably be asking you for a schematic of the circuit to "soften" the edge transitions of that tach signal.....but since I have my new 8000RPM Autometer tach anyway......I suppose it's kind of a wasted excercise......

    Thanks for responding though...... it's always good to get some "validation" from the Master! /forums/images/icons/smile.gif
     
  8. BLAZEN70

    BLAZEN70 Registered Member

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Thanks for the education guys. Those are two things i eventually will upgrade to. Any ideas or opinions on which brand of ignition is better for performance?
     
  9. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Robert,

    The change is very subtle....I wouldn't look to an MSD (or any other brand of ignition) to provide dramatic improvements in performance.

    I guess the best way to think of this swap is that it removes a possible "weak link" in the whole system. You'll know that you're getting a nice FAT spark and consistent combustion with an aftermarket ignition....so if you have "issues" it's probably NOT related to the ignition system. /forums/images/icons/smile.gif
     
  10. BobK

    BobK 1/2 ton status

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Steve,post that filter circuit if you can.I know I'm interested in it.
     
  11. Chris Demartini

    Chris Demartini 1/2 ton status

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    <font color=blue>A year or so ago, I made my factory tach read correctly with HEI. Just take the dash out and adjust the little screw in the back of the tach with a small flathead screwdriver until it reads acurately. I used a timing light to tell when I had the tach adjusted properly
     
  12. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Chris,

    It seems like any "screwdriver adjustment" is only going to allow you to correct the tach for a specific RPM..... Steve can probably comment more on this, but it seems that the error is somewhat non-linear, so it needs something a bit more sophisticated to be accurate at ALL rpms...

    Pure speculation on my part. /forums/images/icons/smile.gif
     
  13. Chris Demartini

    Chris Demartini 1/2 ton status

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    <font color=blue>When it's not 30 degrees out, I'll put the timing light on it and check again. When I did this a while ago, it was acurate at higher RPM's.
     
  14. Sledge

    Sledge Registered Member

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Could the points' resistor wire provide enough parasitic capacitance or inductance to shape the signal?

    W few years ago, I friend and I did a fuel-only EFI conversion on an BL A-series engine using the stock points ignition for RPM pickup. When we were dinking around, we had the switched-side of the coil primary on the scope and it was nice and sharp. Scope was nothing fancy, just a 100MHZ bench scope, but I was very surprised how sharp the rise and fall times were. I don't know how the probe would have loaded it compared to the GM resistor wire.
     
  15. Sledge

    Sledge Registered Member

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    OK, so I'm slow tonight...since the resistor wire is on the constant hot side of the coil (which I'm assuming it is, mine's had an HEI since I bought it) then any influence from the wire is moot compared to the coil windings.
     
  16. Steve_Chin

    Steve_Chin 1/2 ton status

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Bob, the "filter" required is actually available from either M&amp;H Electrical Fabricators or MAD (at least I was told that one of them had it). I don't have a schematic, since the earlier GM cars I have/had came equipped with tachometers that functioned just fine with an electronic ignition and the Blazer has a 7K RPM '78 Camaro tachometer in it (i.e. I haven't needed it). I know of a few guys who have cobbled together their own filters. I'll see if I can't find a way to get in touch with them and pick their brains on this.

    FWIW the stock tachometer is useless anyhow, since it has a large error (even with points) past about 3K RPM. The later tachs aren't a lot better, but since the scale is larger, the error takes longer to show itself.
     
  17. 69K5

    69K5 1/2 ton status

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    steve
    do you need chevy schematics? I have the chevrolet shop manual for car and the truck manual for 1974.

    nathan
     
  18. BobK

    BobK 1/2 ton status

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    Thanks Steve.
     
  19. Supergas

    Supergas 1/2 ton status

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    I am going to update my instruments &amp; would like to know the model numbers of the Autometer or VDO units that you used in yours.. Steve did you use a small tach tha fit in the original location?? Or what ...

    Thanks
     
  20. Steve_Chin

    Steve_Chin 1/2 ton status

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    Re: HEI & Factory Tachs....A new \"discovery\"...!

    I had to modify the Camaro tachometer a little. The face of the tachometer is a little too large to fit in the gauge panel, but the radial location of the numeration and hash marks works out great. All I needed to do to the tach was to cut away part of the face so that it'd fit in the cluster and the modify the cluster to accept the tach. I hacked up the circuit boards off of the Camaro and the Blazer (I had a few of the one from the Blazer/pickup) and then jumped all the circuits to work.
     

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