hey everyone
i pretty much run my fans anytime i'm traveling slower than 30mph. in conjunction with a fluidyne radiator and miata thermoswitch i hope i'm managing the cooling system well enough not to overheat. anyways, a water temp gauge is long overdue and i'm going to get an autometer carbon fiber series water temp gauge to match my boost gauge.
should i go for electrical or mechanical? which is easier to install? pros/cons? i didn't get too much information about this topic off a search other than the different wires i'll have to tap with each gauge.
thanks for the help guys
-kris

p.s.
i remember mahjik posted something about the temp senders from autometer gauges can be installed on the coolant filler neck without drilling. can anyone verify/elaborate?

 

The mechanical gauge will be more accurate, but the downside is that you will have to run water directly to the gauge...ie, into the cabin. If it leaks, you have a big mess on your hands. My buddy uses ONLY mechanical gauges in all of his race cars, but those things are stripped to the bone.

I used an electric Autometer gauge and I was gonna put the sender in the filler neck plug hole, but i could not get that damn plug out! I ended up using the throttle body line for now. When I get my upgraded radiator, I'll probably move the sender to the thermostat housing.

Sonny

 

https://www.rx7club.com/showthread.p...echanical+temp

 

yeah i would go with electrical everything besides boost gauge. I prefer mechanical boost gauge since its so simple.

 

thanks sonny, sgtblue, and saburo... and anyone else who chips in

 

yea, elictrical most definitaly..my oil press gauge is mechanical and the line broke once..oil everywhere, and this was while i was on the freeway..bad excperiance...

 

Dude... for a water temp gauge you don't have to worry about the coolant running all the way to the gauge. Most (if not all) mechanical water temp gauges have a capillary tube with a sensor at the end which sends the measurements to the gauge. Besides, the reading won't be accurate if coolant were traveling from the engine to the gauge. The only problem would probably be mounting the sensor since it's a 1/2 NPT fitting (huge) on the Autometer gauges.

 

If you look at the top of the filler neck, you'll see the sensor with the wire running behind it. There is a HEX bolt on the top of the filler neck. You can remove that bolt and install the sensor there. However, you will have to cut the filler neck cap a little (trim one of the Mickey Mouse ear sides) so it will clear the sender.

 

Mahjik: Did you heat the area around the hex plug up to get that **** out? I had an 8mm L-shaped hex wrench in there plus a 12" extension and was really leaning on it. I got the feeling that I was either gonna break something or tear off 3 of my knuckles, so I opted for the throttle body lines.

Sonny

 

Agreed, you don't have to run coolant into the cabin. My guage gets its reading from a sensor T'd from the coolant hose that leads to the elbow. The only thing running through my cabin is the necessary wiring to the gauge itself.

 

Nope. I used pretty much the same technique as you described. I guess I just used a little more elbow grease.

 

Wait...to those that stated that you don't need to run coolant to the gauge...if you have a sensor in the engine bay and running a wire into the cockpit, isn't that considered an electrical gauge? I always thought gauges were "electrical" if they utilized sensors in the engine bay that ran a signal through an electrical wire into the cockpit...

Sorry...just need to clarify a bit...I'll do some research.

 

dont some temp gageues just use a metal liune that heats up when the car does and shows it on the gauges..kinda like a thermometer..or sumtin..

 

I personally perfer Electrical Gauges in about every aspect. Easier to install is a main point and not having to run what i'm monitering into the cable is also a great plus.

 

Here is how mine went together:

http://web.newsguy.com/geesaman/Installation.ppt

Dunno if it was luck, but I can open my water pump cap without touching the sensor or needing to cut the cap.

Dave

 

Mechanical temperature (oil or water) gauges use a glass bulb with an attached capillary tube, filled with inert gas. The bulb is placed in contact with the fluid whose temperature is to be measured. Snce the volume is constant, the pressure of the gas is linear with respect to temp (PV=nRT). The gauge converts the pressure in the capillary tube to an analog scale temp. If these break, the gauge won't work, but there will be no coolant/oil spurting into the cockpit.

Mechanical pressure gauges (oil, fuel, boost) have a tube that connects directly between the substance being measured and the gauge. These are the gauges that can spill oil, fuel, etc. in the cockpit if they break. There are fuel line isolators - I presume you could use them for oil, as well. They basically seal the line at the firewall, and transmit the pressure via a flexible membrane to a different fluid that goes to your gauge.

Pros of mechanical gauges: very little to go wrong, simple to install, fairly sturdy once they are installed. typically cheaper than electrical gauges.

Cons: that capillary tube is fragile - break it and you have to buy a whole new set. The bulb is big enough that it is sometimes a challenge to find an appropriate place to put it.

I have a mechanical gauge in my Spec Miata - I have a bung welded into a 3" piece of pipe that replaces 3" of upper radiator hose. Works great - no problems despite the abuse of racing.

I have an electrical gauge in my FD - didn't want to drill that big a hole in the thermostat housing. It works great, too.

 

Nice post, macdaddy!

Sonny

 

macdaddy:

Good post - Saved me from typing out the same explanation of how a mecanical temp guage works - But just to clarify - Usually the sensor bulb is brass or copper (metals conduct heat much better than glass) and the Bulb, Capilary Tube and Gauge, are filled with an inert LIQUID, NOT GAS (b/k liquids are Non-Compressible)

 

You are correct that the bulb and capillary tube can be made of metal - autometer says theirs is copper. However, it is filled with a gas - autometer says theirs are filled with ether.

http://hp.autometer.com/techtips/faq...ap_tubing.html

This is basic physics. Liquids do not appreciably alter their volume (or, in a sealed volume, their pressure) based on change in temp. They only do so at the extremes - i.e. state change to solid or gas. Gases, however, have a linear relationship between temp and pressure (if volume is held constant) - or between temp and volume (if pressure is held constant). That's the gas law: PV=nRT (pressure x volume = number of moles of gas x Avagadro's number (a constant) x temperature.

 

Ahh...nice post indeed. That clarifies my confusion!

 

MacDady - You are correct re the Temp Gauges - The Oil Pressure Gauges measure the working fluid directly (ie are filled w/ oil) the fluid is fed through the capillary to a Bourdon tube movement that is basically a spiral tube that unwinds as the pressure inside is increased.

However while Liquids do not change volume with a change in PRESSURE (being basicly uncompressable) most do EXPAND significantly with an increase in TEMPERATURE - that is why you need a catch tank for the cooling system - the coolant expands as it is heated and gets pushed out of the system as the pressure excedes the strength of the radiator cap spring - the pressure in a sealed container filled with a fluid increses very rapidly with small changes in volume because a fluid is not compressable.