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Engine RPM Guage for 6V- 55 Hudson Wasp


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I would like to hook up an RPM gauge to my stock 6V 55 Wasp 202ci hydromatic for tuning and to check the RPM at various driving speeds to see if there is any slipping going on. I found a bunch of old engine analyzers on ebay for under $50.00 shipped. They are untested for the most part. Those would be good for tuning, they have a points condition and dwell mode also, but I would like to buy something that I know is correct. I have a timing light that I use on a 12V motorcycle battery. What are you guys using for tuning?

Edited by supercub (see edit history)
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  • supercub changed the title to Engine RPM Guage for 6V- 55 Hudson Wasp
2 minutes ago, padgett said:

Looking at the manual, nothing is said about 6v vs 12v systems. May not matter (has an internal 9v battery) or...

It shows a clip on sensor and the instructions say to clip it to a spark plug wire.  Meter is powered by an internal batter so no connection to the car’s battery.

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That is for the tach. For dwell, "Connect the black test lead to a grounded bare metal part of the vehicle’s frame or the negative pole of the battery, and the red test lead to the connecting pole for the low voltage of the distributor or the negative terminal “-” of the firing coil"

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Somehow I blew past that.  Still it’s probably the delta v change on the points side that triggers the measurements.  20% restocking fee if it doesn’t work so your out $10 if you return it.

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On a 12 volt system doesn't the coil run at 6 volts when the ignition switch is in the run position and the voltage to the coil is going through a ballast resistor or a resistor wire?

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a) Less 20% coupon.

b) 12v coils are usually designed to run at 9vdc, 6v for 4.5v. So it can fire when the starter is straining the battery. Then for running the ballast resistor/resistance wire cut is down.

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The permanent 6 volt tach that installed on my '54 Ranch Wagon was from the Westach company. (Westberg Mfg.) As I recall, the price was between 100-150 bucks, and very good quality. If you want a non-permanent tach for purely diagnostic purposes, I can't help you.

 

https://www.westach.com/6-volt-tachometers

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That tester should do the job if it is compatible with the positive ground for some tests. Since the pickup is inductive and it runs on it's own power source, the 6Vcar's voltage shouldn't be an issue.

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This subject is a minefield. The answer is "it depends". You'll just have to try something and see if it works. I still have not had ANY digital instrument, dwell, tach, timing light or multimeter work on my 36 Pontiac. They lock up. Even a harbor freight photo tachometer locked up, and it had no electrical connection to the car. I have a pretty good idea how fast the engine idles but I can't prove it.

 

The harbor freight device TerryB posted looks quite useful as a tachometer on *IF* it doesn't lock up. Also I have a brand new digital dialback timing light here that has a tachometer in it. It is a model that @Matt Harwood has witnessed working on 6 volt cars. I have not tried it yet. I hope it will work.

 

If you are going to get an vintage dwell meter, a two wire model is way more likely to work than one that hooks to both negative and positive on the car's battery.  That goes double for positive ground cars. Using a separate 12v battery on a 3 wire model probably wont work, and might burn up the meter. The separate 12v battery trick does work for clip on timing lights though.

 

All  a dwell meter does is show how long the points are closed versus how long they are open. A 4 cylinder car fires every 90 degrees of distributor rotation. A 6 cylinder car fires every 60 degrees of distributor rotation. An 8 cylinder car fires every 45 degrees of distributor rotation.

 

Using a 6 cylinder engine as an example, if the points were always closed, that would be 60 degrees of dwell. If the points were always open, that would be 0 degrees of dwell. Therefore if you hook your two wire dwell meter up with the ignition on, it should read 60 when the points are closed and 0 when they are open. Some dwell meters have a "set" knob to adjust the end of the scale to either 0 or 60. Those might have a better chance of dealing with different battery voltages. If the needle swings the wrong way due to a positive ground electrical system, try reversing the leads.

 

The example above assumes an old analog two wire dwell meter. Never try digital test gear on positive ground systems unless the instruction manual specifically says it is OK to do and tells you how to hook it up. Getting it wrong is extremely likely to damage the equipment.

 

On 4/4/2021 at 3:45 AM, supercub said:

What are you guys using for tuning?

 

For 6 volt cars I bought an "Electro Products" dwell meter and tachometer from the 6 volt era and restored the dwell meter. The tachometer still needs work. Mine are separate instruments, but usually the two are combined. Years ago I had some Allen equipment from the 6 volt era. The Allen stuff was especially nice, and I wish I still had it. For timing I am using a cheapie Sears clip on timing light from the late 70s, and a separate 12 volt battery. I also have a neon timing light from the 6 volt era, but those are dim and difficult to use. Neon timing lights are powered by the spark and do not care at all about the car's electrical system.

 

 

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Back in the day, Allen and Sun were considered equivalent.

 

Are passive (two wire) and transistor (three wire) tachs. All GM hood tachs (Pontiac, Buick, and even F*rd) were two wire but adding a transistor circuit board for a Vega (three wire) made them A Bunch (tm) more accurate.

 

 

tlb800.jpg

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To answer your question: What do I use?  The two items on the table top are approaching 50 years old, and still working well.  The three against the wall are flea market finds; all work well and I paid $20 or less each.  The only thing wrong with any of them is that the insulation on the leads is deteriorating.  I think the old tachs work on impulses, and work equally well with 6 or 12 volt, but read out differently for 4, 6, or 8 cyl.  I suspect some of the newer ones would be insensitive to voltage as well, but if lighted, then have a 12 bulb within.  

B4662EE1-BDD1-48D7-B4C8-4AD703ECFA3A.jpeg

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