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Old 05-01-2005, 01:03 PM
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Elec vs Mech gauges

Im going to be ordering some new gauges for the new dash insert for my 56 that im expecting in the mail. Whats the difference between Mech and Elec gauges. Actually, the real question is, which should i get? Should i get certain gauges Mech and others Elec or what. Any info would help, thanks

Joseph

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Old 05-01-2005, 03:32 PM
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In a nutshell electric are easier to install, a sending unit in the engine and a wire to the gauge. Mechanical require a tube from the engine to the gauge which can be hard to route cleanly and if the tube is bent or cut you can have oil where you don't want it or a gauge that dosen't work, in the case of a water temp gauge you have to throw it away and buy a new one.

Personally I prefer the mechanical because of the 270 degree sweep that the gauge face has, much easier to read at a glance compaired to the typical 90 degree sweep of most electric gauges.

Some say that mechanical are more accurate, some say they are the same...
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Old 05-01-2005, 04:26 PM
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Mechanical Vs Electrical

Doc here,

The Major differences Between Mechanical and Electrical Gauges is The premise on which they operate.

The Basics..

MECHANICAL:

Relay on physical pressure, or bourden tube type senders (non electrical.)

The oil pressure for instance works off a tube hooked to the block in an oil gallery tap and actually pumps oil up the tube and into the gauge.

The Temperature sender works from a stainless steel tube, that is liquid filled. The Block end is a bourden tube, that reacts with / from heat to make the liquid rise in the tube/gauge as the temperature rises and falls.

Tachometer, is driven by a Speedometer type cable that hooks into a tap and mechanically into the block via a pickup unit and samples the engine speed.

The drawbacks to the mechanical units, are lines and cables break and fray, sometimes spilling fluids inside the car, They usually require some degree of plumbing, and tend to be generally larger than electric units.

ELECTRICAL:

The basics:

Electric Gauges relay on electric senders, usually one wire is needed, and on most vehicles, is already in place. The Senders have bourden tubes and pressure senders built right into the sender at the engine block, and mechanically move a resistor or rheostat internal to the sender to vary the amount of resistance to ground to the gauge.

The Electric gauge itself has, has a resistance value,(the meter movement)and is strapped across a "Whetstone" bridge or 3 or 4 balanced value resistors +/- 1% to make the movements on the gauge. If all 3 values or 4 values are equal, (in theory) The gauge reads at or near zero...If you vary the resistance at the sender it imposes a load on the bridge unbalancing it, the electric gauge then moves in an attempt to balance the load across the bridge. As the resistance goes up and or down at the sender, the gauge follows suit.

Usually Electric Gauges only require 3 or 4 wires for operation, Power, ground,Sender and dimmer...in the case of a 4 wire 3 can be hooked up locally at the dash, the sender being the only long external run, and as said, is usually already in place from the stock harness.

The Tachometer works of the high energy impulse from the Distributor, (points or Mag pickup opening and closing as the rotor turns) It sends this impulse up the wire to the gauge, where there is a circuit board with a Digital, or linear integrated circuit. The pulse goes through a "Buffer" to lower the current impact on the Chip, and shape the pulse into a quasi square wave..then from there, it goes to the chip which is a Divide by "N" or divide by 4, 6,or 8 (in older versions) and through to a low output driver, this driver supplies the electrical gauge signal to drive the meter...resulting in a visual reference to RPM's...

They usually Require 4 wires, power, ground, dimmer, and distributer. Simple to hook up.

Electrical gauges are much easier to hook up and run compared to mechanical, they don't require plumbing runs other than installing the sender, Have nothing to leak inside the car or elsewhere, and (In my opinion only) Are much more reliable.

If you have already ordered the Electric Gauges for your car, Be sure you get the proper senders for them (some include them, some don't) The points to look for are Total resistance to the gauge (I.E. 0 to 30 ohms, 0 to 90 ohms etc..) and The physical thread size of the sender..(will it fit in your block?)

The rest is simple, just follow the directions with your set..

Hope it helps!

Doc
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Old 05-01-2005, 04:54 PM
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Wow, thats a lot of information right there Doc. Just learned a few things thanks to you. My guess was that Elec. gauges were better, but something i have noticed is that when you go to order a set of gauges, theres Mech and Elec gauges included. Certains ones are Elec, certain ones are Mech. Duhh..When i decide on a set of gauges i like ill post up some info about them or the website where i found them and maybe you could help me out and let me know if i could use them or not. One thing im not really understanding is who whole sender thing, how do i know if itll fit in my block or not. Im a amatuer when it comes to all of this, just starting to get my hands in on it so im a lil slow.
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Old 05-01-2005, 05:20 PM
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Gauges

Doc here,

Sure No problem....

The Sender Thread sizes are pretty much industry standard, If you order them for, Say, like a GM 350 , that sender will fit most SBC's... and If not, FLUSH mount adapters can be obtained at any good hardware store for a few bucks in the brass fittings section...

Note I say FLUSH mount fittings...These are like threaded rings that adapt between the sender and block , but don't recess the sender outside of the block.....

Not so awfully important in the oil sender, but for water, it can cause air gaps or bubbles That settle in the fittings....and a sender can't read air...It will cause at best, excursions on the meter (full swings up and down) or worst case scenario, no reading at all...Plus moving water is hotter then still water..so if it isn't in the moving water stream, it will read inaccurately...

Always use Teflon tape, or Teflon sealer When installing (ANY TYPE) senders (white for water, Yellow for oil) and be sure to leak test before you "Call it all Good"....

Doc
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