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Kirkham Motorsports

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Old 07-01-2008, 05:40 AM
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Question Tach reading high ?

It seems my SW tach is reading high, this is only based on my comparision of the reading with the sound of the engine. When driving it seems to be reading around 4000 to 6000 rpms, but does not sound like it is running very high at all. I have been driving ans shifting by sound but it just seems off in comparision.

Is there a way to double check this or verify the reading. It is a standard SW tach and ford distributor, no MSD or anything fancy.

Jim
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Old 07-01-2008, 06:05 AM
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Check to make sure the switch on the tack is on the 8 cylinder position.
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Old 07-01-2008, 06:13 AM
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Is this switch on the back of the tach, ?? I do not recall seeing one, but then again I was not looking when I was wiring the gages.

Jim
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Old 07-01-2008, 06:17 AM
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Should be on the back. Do you have the instructions? If not, see if they have them on the SW web site.
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Old 07-01-2008, 06:39 AM
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If that is ok get one of the Isky Ratio Computers which is a small cardboard wheel that has a spinner on each side. One side will tell you about your tach readings, speedometer reading, speeds at what RPM & etc. The other side will give you all of the displacement figures. I have used them for years and they are very accurate. To determine if the tach is correct for example you would measure the diameter of your rear tires with them inflated and then use your rear end gear ratio and the speed your speedometer shows. Then when you move the part that turns to line these up it will show what the tach should read. They work for all of these things. I used to carry one in my car when I had it.

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Old 07-01-2008, 08:00 AM
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Of course, if you are allergic to cardboard, you could always use one of the electronic calculators on the INTERNET.

http://www.cobranet.com/rpmmph.html
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Old 07-01-2008, 08:40 AM
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I'm having a similar problem with an S&W Tach I had rebuilt. When I accelerate, the needle seems to show a progressive increase in RPM until it hits 4000. Then it jumps to 6000. What is more confusing is I also have a shift light installed that is set for 6000. I have a red conventional Brittish dashlight as the indicator mounted right next to the tach so I can see the needle and the light together. When the needle hits 6000, the light comes on.

I'm running a conventional point-type distributor withan MSD 6A. Is it possible that I'm getting some points flutter that's tricking the Tach and the shift light?
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Old 07-01-2008, 01:17 PM
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I'll take a look tonight and see if I have a switch on the back side of the tach, I think I can reach up there yet. If not I will be back with some more possible questions.

Jim
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Old 07-01-2008, 10:15 PM
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If it's proportionally off throughout the RPM band it's a problem with the tach. (switch setting/defective)

Noise would cause erratic readings through the RPM band. (noise will vary depending on loading of the ignition system)
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Old 07-02-2008, 04:50 AM
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Well, checked it last night, not sue if the switch is right on the back as I can't get to it without pulling the dash and steering column, man I hate to do that. The other thing I found out check the tach part number and instructions, which I never checked before as I never had a reason to question it. The part number on the box is for a Marine only tach and not one for both auto and marine. Not sur if there is a real difference as two how they work or just how hey are built and sealed for water ect.... I guess I need to get a hold of SW to find this out, unless someone here has some insight.

Jim
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Old 07-02-2008, 07:32 AM
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Well, a basic understanding of what may be happening might help.

The Tach reads or accepts the pulse from the distributor for each negative portion of the coil discharge. Or, in other words, when the distributor sends the grounding signal to the coil, the tach counts a pulse.

If you have a CD system with multi-strike ( more than one discharge ) firing, you can get more than one pulse per cylinder making the tach read higher than it should. CD system have an "ON" cycle signal designated for the Tach. You will need to use it.

If you have a bundle harness ( wiring that is all in a bundle and then fed to the engine ), you may be getting noise to the point that the noise itself is adding pulses to the tach reading. A quick fix for this is to run a shielded line between the tach and the tach signal. Ground the shield to the block/chassis.

If you have a double strike firing system ( two strikes per cylinder ), you will need to find out how to provide a single pulse for each. I used a conditional Flip Flop once that worked fine. I think it all depends on your system. The distributor supplier should have a fix.

Well, I hope this helps.

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