Choosing Gauges - Things to consider

[mccorry]

I got into a discusson on my local board about Mechanical guages after another member spouted off about how mechanical guages will spray fuel into your car cabin and cause fires.....

 

Anyhow... I thought that this might be helpful to some of you over here when considering what kind of gauges to purchase.

I want to note that this is all IMHO... so take if for what it's work and with a grain of salt.

 

Mechanical gauges work via the transmission of fluid pressure or direct temperature measurement. This means that they are connected to the item being measured by a hose, a capilary tube, a glycol filled temperature probe, a liquid filled diaphram, or via electrial wires directly from an RTD probe (inserted into a line or hose). It should be noted that the only gauges that bring actual under hood fluids/vapos into the cabin space are oil pressure gauges and Boost/vacuum gauges. It is illegal to bring fuel into the drivers compartment. Therefore, gauge manufacturers handle fuel pressure via a firewall mounted diaphram, which seperates the fuel from the cockpit / cabin. I've never seen a fuel pressure guage that has fuel piped directly to it.

 

Electrical gauges work via an underhood mounted sending unit which measures the pressure or temperature, coverts this measurement into a resistance (oHms) or a current (mA). Sending units come in many different configurations and are 95% of the time propietary (meaning that you'd have to purchase a new sending unit from the gauge manufacturer). For example... oil pressure guages can use a sending unit that is 0-100 ohms, 0-200 ohms, 0-250 ohms, etc. Plus... the plug on the sending unit may be special for the gauge wiring harness.

 

The Pro's of mechanical gauges are:

1. They are "actual" measurements, meaning that the reading isn't being converted to a mA or Ohm signal and there is no manipulation or filtering.

2. They are often cheaper than electrical gauges due to a sender not being required. In my experience, it is usually $60-$100 cheaper for a mechanical gauge.

3. They don't require any proprietary sending units so it's great for flexability. Sending units are usually somewhat bulky... which can limit your options as far as tap and mounting locations.

 

The Con's of mechanical gauges are:

1. You can develop a leak inside of the car's cabin. I've had oil seep out of a compression fitting that wasn't tight enough. These capillary tubes are small, however, so you aren't going to get spraying.

2. You can develop a leak under the hood from the capillary tubing if the compression fitting isn't tight enough. I have also seen this happen before with an oil pressure gauge. No spraying.. only some drips.

2. The capillary hoses that come with these gauges are a ROYAL PITA to work with. I am talking abou the coiled up white tubing of death. If you've worked with it, you know what I'm talking about. This stuff kink's if you look at it cross-eyed. Instead, I use Tygon tubing, which is super flexable and uneffected by underhood temperatures, chemicals (oil) or kinking.

3. They can be slightly slower to respond than electrical gauges since you are transmitting the information via tubing. This is only an issue with oil pressure, since the fluid is viscous. Boost gauges are immediate.

4. Cold weather can also make oil prssure gauges respond more slowly since the oil is more viscous.

5. Since the needles are driven by the fluid pressure, there aren't many special "features" on the guage. They pretty much tell you pressure/temperature, etc... and that's all.

 

The Pro's of Electical gauges are:

1. Quick response.

2. Installation is all wiring once the sending units are mounted in the lines.

3. Running wiring can be easier than running tubing. Plus you don't have to worry about leaking compression fittings at the hose ends.

4. Electrical gauges usually have more "bells and whistles" than mechanical gauges. You can get "opening ceremonies" with electrical gauges, along with other features.

 

The Con's of Electrical gauges are:

1. They are slightly to moderately more expensive than mechancial gauges.

2. You can develop a leak under the hood from a faulty sending unit. I've had this happen twice.

3. Sending units can and do fail (esp. more common if you go with cheaper gauges) and you might not be able to find a replacement. I've also had this happen.

4. If your sending units are proprietary, it limits your sources of getting replacement parts.

 

So.... Mechanical gauges aren't going to spray you down with fuel or oil. Scalding glycol won't be pumped into your seat bottom thereby burning your sensitive tush. Gauge manufacturers are not going to be selling units that will pump raw fuel into your face.

 

Basically the decision of electrical vs. mechanical gauges is is a choice of form vs. function vs. $$.

 

For example.....

- You can go out and buy a $550 Defi Electronic guage and be perfectly happy. It will flash, have a cool opening ceremony, move the needle, and look great. It will also be accurate. This is a quality piece... if you have the money.

- You could also buy a $100-$200 Ebay or no-name gauge and have the sending unit crap out after 2 month with no way of getting a replacement. Been there, done that. I don't think you'd be very satisfied with your purchase.

- Or you could purchase a reputable, name brand $150 Mechanical gauge (Auto-meter, etc) and install it and have it tell you accurately what your boost pressure and oil pressure are.

 

It really comes down to what you want, what you looking for and how much are you willing to spend. My priorities when I select gauges are:

1.) Accuracy. What good is a gauges if it tells you nothing.

2.) Appearance. I want a gauge that looks like it belongs. That looks like a quality piece.

3.) Reliability. I don't want to have to mess with the thing for a few years... at a minimum.

 

 

I hope this helps. Feel free to correct me if I'm wrong or you disagree.

[rao]

100% right on the function, but price is not the most accurate indicator of quality.

[RaGe]

subscribe.

[mccorry]

100% right on the function, but price is not the most accurate indicator of quality.

 

True. I like to go by reputation.

Usually if you haven't heard of them before... you're running a higher risk.