Just tap the ring out a hair and install it as designed. I used a sparkplug thread chaser to make half a thread in the ring and have no issues.
Can someone please explain why a temperature compensated gauge is needed?
If the themocouple is reading the head Temp, why isn't that accurate?
Why isn't the temperature taken good enough?
Cole Thompson- posted:Can someone please explain why a temperature compensated gauge is needed?
If the themocouple is reading the head Temp, why isn't that accurate?
Why isn't the temperature taken good enough?
A non temp compensated cht gauge can read as much as 40' low in the summer heat. It will read correctly at 70'F ambient and as the ambient temp goes up it reads correspondingly lower. As the ambient temp drops below 70'F the cht gauge will read higher.
A VDO cylinder head temperature gauge is non temp compensated.
Is VDO making CHT gauges again? Last year I tried to get one and they weren't available yet.
But why? Why isn't the temperature taken accurate?
if I put a thermometer in a cup of 110 degree hot water, I expect it to read 110 if the outside temp is 32 or it it's 95.
Maybe something to do with the 8' or 10' of wire the signal has to travel through?
I'm not a scientist like some on here, but here are my thoughts:
It has nothing to do with the length of wire transmitting the signal. A non-temperature compensated CHT gauge is giving the correct head temperature at any/all ambient temperatures. However, what we really want to know is the head temperature without influence of ambient air temps.
At 110 degrees F, CHT may be 450 degrees F, while only 390 degrees F at 50 degrees F ambient air temp. Both readings are correct, but they ignore the affect of ambient air temps.
A temp-compensated gauge has an inline thermocouple (or something similar) that adjusts the CHT shown by taking ambient air temp into consideration, thus giving the user a true differential head temp, the data we are looking for. I am guessing that, near the other end of the wire, i.e., the gauge end, there is a temp gauge that measures ambient air temp under the dash or somewhere NOT in the engine compartment, and uses that data to determine CHT unaffected by ambient air temp.
That means that at 3200 rpm in fourth gear on a 6% grade, CHT will be higher on a hot day without a temp-compensated gauge. I think the real usefulness of any gauge is watching the dial/needle/indicator/readout/whatever under various conditions, and adjust load accordingly to maintain lubricant temperature for a long-lasting engine.
Jim, you are just about right, according to the innerwebs.
"The voltage produced in the thermocouple is based on the temperature of the hot junction near the ring terminal relative to the temperature of the cold junction at the end of the connector that leads to the CHT gauge. The gauge and cold junction are usually calibrated around 72F. What matters is not what length of gauge of wire is used between the thermocouple and the gauge or which cylinder the ring terminal should be under (although #3 is ideal) or where the thermocouple should be located (although under the spark plug is the only correct location for VWs) but what temperature the engine compartment is at.
"Because the thermocouple is calibrated for 72F, the readings will only be accurate at that temperature in the engine compartment. If it's colder, the CHT temperature will appear higher giving a false impression that the engine has fully warmed up. If the temperature is higher, say 100F or more, then the reading will be lower by 30F (the difference between the engine compartment temp and the calibration temp). This is unfortunate because the very time when we want to know that the engine is too hot, it is actually low by 30-40F because of the outside temperature.
"The reason this happens is that the VDO CHT wiring harness contains a 1 ft. length of thermocouple wire and the rest of it is regular wire so the cold junction will be in the engine compartment. If you manage to run the thermocouple portion of the wiring through the engine tin and expose it outside of the bus you have similar temperature calibration problems because of the airflow and exhaust heat, etc. Even if VDO ran thermocouple wire all the way to the dash, then the cabin temperature at the cold junction would still affect the the readings (hot day, low reading). You shouldn't have to install air conditioning to have accurate readings!
"This is the nature of thermocouples and in this form they are basically unsuitable for automotive applications. The cure is use a system that has a cold-junction compensating thermistor. What this does is measure the temperature at the cold junction so the gauge can adjust the reading. Very simple but for some reason you can't buy this type of system from the low budget gauge vendors."
I'm installing gauge so I know the differential between the cylinders. It is good to know about the different readings based on cold junction temperature OR if it's compensated.
The MAX6675 chip I'm using(x 4) is temperature compensated.
Regarding CHT and oil temp:
I'd often heard that CHT could move independently of oil temperature, in some high-load circumstances, such as driving in the mountains. But in 15 years of driving agressively and monitoring both sets of data, I never observed this phenomenon in the wild. Oil and head temps always rose in sync-- out on the plains, in the Rockies, in the desert, and in the Sierra Nevada range.
... then I went to the Smokys last year, and was hammering on the Blue Ridge Parkway. The grade was just subtle enough that I was able to drive in 4th gear, with my foot pretty deep in the accelerator and the tach under 3000 RPM. The engine wasn't "lugging" in the classic sense of actually slowing down with the pedal matted, but it was under a LOT of load. Nonetheless, I was able to accelerate smartly and run pretty much as fast as I wanted to go-- until I looked at the CHT gauges. With the oil temp under 200*, I hit 400* CHT on all 4 cylinders. Since the choice I had was to drive slower, or downshift-- I chose the latter, and instead of turning just under 3000 RPM, I was spinning over 4000 all the time and up to 4500 on acceleration. This was enough to spin the fan enough to cool the heads almost immediately to under 350*.
It was at that point I was really pining for a 5 speed for the first time-- not for the speed, but to drop the engine into a nice 3500 RPM range. If I'd have had a Subaru, it would've been no big thing, but I was fan-dependent.
Cool oil temps and high head temps are a thing. Not every day, but it only takes once to drop a valve seat. I'd never have known had I not been monitoring. Knowing what is going on is a good thing.
@DANNYP We may have a similar setup.
I went for the Octo MAX31855 : https://www.ebay.com/itm/Octo-...?hash=item4632b8eb3e
and some pressure sensors for Fuel and Oil : https://www.ebay.com/itm/1-8NP...p2057872.m2749.l2649
All that on a raspberry PI 7inch touchscreen that can flip hidden under the dash when not required. Also added GPS Nav using the NavIT package and music library.
Code and bill of material will be shared on this site when I am done tweaking it. I still have to run the wires for the Oil Pressure sensor and road test it to make sure all the goodies play nice together.
jc
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Jim Kelly posted:I'm not a scientist like some on here, but here are my thoughts:
It has nothing to do with the length of wire transmitting the signal. A non-temperature compensated CHT gauge is giving the correct head temperature at any/all ambient temperatures. However, what we really want to know is the head temperature without influence of ambient air temps.
At 110 degrees F, CHT may be 450 degrees F, while only 390 degrees F at 50 degrees F ambient air temp. Both readings are correct, but they ignore the affect of ambient air temps.
A temp-compensated gauge has an inline thermocouple (or something similar) that adjusts the CHT shown by taking ambient air temp into consideration, thus giving the user a true differential head temp, the data we are looking for. I am guessing that, near the other end of the wire, i.e., the gauge end, there is a temp gauge that measures ambient air temp under the dash or somewhere NOT in the engine compartment, and uses that data to determine CHT unaffected by ambient air temp.
That means that at 3200 rpm in fourth gear on a 6% grade, CHT will be higher on a hot day without a temp-compensated gauge. I think the real usefulness of any gauge is watching the dial/needle/indicator/readout/whatever under various conditions, and adjust load accordingly to maintain lubricant temperature for a long-lasting engine.
Stan Galat posted:Regarding CHT and oil temp:
I'd often heard that CHT could move independently of oil temperature, in some high-load circumstances, such as driving in the mountains. But in 15 years of driving agressively and monitoring both sets of data, I never observed this phenomenon in the wild. Oil and head temps always rose in sync-- out on the plains, in the Rockies, in the desert, and in the Sierra Nevada range.
... then I went to the Smokys last year, and was hammering on the Blue Ridge Parkway. The grade was just subtle enough that I was able to drive in 4th gear, with my foot pretty deep in the accelerator and the tach under 3000 RPM. The engine wasn't "lugging" in the classic sense of actually slowing down with the pedal matted, but it was under a LOT of load. Nonetheless, I was able to accelerate smartly and run pretty much as fast as I wanted to go-- until I looked at the CHT gauges. With the oil temp under 200*, I hit 400* CHT on all 4 cylinders. Since the choice I had was to drive slower, or downshift-- I chose the latter, and instead of turning just under 3000 RPM, I was spinning over 4000 all the time and up to 4500 on acceleration. This was enough to spin the fan enough to cool the heads almost immediately to under 350*.
It was at that point I was really pining for a 5 speed for the first time-- not for the speed, but to drop the engine into a nice 3500 RPM range. If I'd have had a Subaru, it would've been no big thing, but I was fan-dependent.
Cool oil temps and high head temps are a thing. Not every day, but it only takes once to drop a valve seat. I'd never have known had I not been monitoring. Knowing what is going on is a good thing.
edsnova posted:Jim, you are just about right, according to the innerwebs.
"The voltage produced in the thermocouple is based on the temperature of the hot junction near the ring terminal relative to the temperature of the cold junction at the end of the connector that leads to the CHT gauge. The gauge and cold junction are usually calibrated around 72F. What matters is not what length of gauge of wire is used between the thermocouple and the gauge or which cylinder the ring terminal should be under (although #3 is ideal) or where the thermocouple should be located (although under the spark plug is the only correct location for VWs) but what temperature the engine compartment is at.
"Because the thermocouple is calibrated for 72F, the readings will only be accurate at that temperature in the engine compartment. If it's colder, the CHT temperature will appear higher giving a false impression that the engine has fully warmed up. If the temperature is higher, say 100F or more, then the reading will be lower by 30F (the difference between the engine compartment temp and the calibration temp). This is unfortunate because the very time when we want to know that the engine is too hot, it is actually low by 30-40F because of the outside temperature.
"The reason this happens is that the VDO CHT wiring harness contains a 1 ft. length of thermocouple wire and the rest of it is regular wire so the cold junction will be in the engine compartment. If you manage to run the thermocouple portion of the wiring through the engine tin and expose it outside of the bus you have similar temperature calibration problems because of the airflow and exhaust heat, etc. Even if VDO ran thermocouple wire all the way to the dash, then the cabin temperature at the cold junction would still affect the the readings (hot day, low reading). You shouldn't have to install air conditioning to have accurate readings!
"This is the nature of thermocouples and in this form they are basically unsuitable for automotive applications. The cure is use a system that has a cold-junction compensating thermistor. What this does is measure the temperature at the cold junction so the gauge can adjust the reading. Very simple but for some reason you can't buy this type of system from the low budget gauge vendors."
Cole Thompson- posted:But why? Why isn't the temperature taken accurate?
if I put a thermometer in a cup of 110 degree hot water, I expect it to read 110 if the outside temp is 32 or it it's 95.
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Ed's explanation is good - but maybe too good.
The crux of it is that a thermometer measures temperature, but a thermocouple measures temperature difference.
So, if what you're comparing the head temperature to (in this case, the air in the engine compartment) is itself changing over time, the indicated CHT will change as the engine compartment warms up.
There. Fifty words or less.
Stan Galat posted:
... then I went to the Smokys last year, and was hammering on the Blue Ridge Parkway. The grade was just subtle enough that I was able to drive in 4th gear, with my foot pretty deep in the accelerator and the tach under 3000 RPM. The engine wasn't "lugging" in the classic sense of actually slowing down with the pedal matted, but it was under a LOT of load...
...Since the choice I had was to drive slower, or downshift-- I chose the latter, and instead of turning just under 3000 RPM, I was spinning over 4000 all the time and up to 4500 on acceleration...
...It was at that point I was really pining for a 5 speed for the first time-- not for the speed, but to drop the engine into a nice 3500 RPM range...
Bingo.
And you've probably got bags more torque than my modest mill.
Around here, there are short, steep hills pretty much wherever I want to drive. And some long, steep ones, too. And little old ladies in silver Priuses who somehow know the perfect speed to go to trap you just under your shift point.
With those five magic cogs, you always have the right gear, you're never lugging, and you can go anywhere at 3500 rpm.
And all the world's a sunny day.
Does that mean we're talking 5 speed gear ratios again? Oh boy- one of my favorite subjects!
ALB posted:Does that mean we're talking 5 speed gear ratios again? Oh boy- one of my favorite subjects!
No. Just... No.
TRP posted:ALB posted:Does that mean we're talking 5 speed gear ratios again? Oh boy- one of my favorite subjects!
No. Just... No.
And you're a poopie pants!
No, Al, the term is "Poopie HEAD."
Re CHT monitoring gear, Raspberry Pi, etc.: Unlike with industrial/mechanical stuff, it really can pay off to make your own electronical stuff.
https://www.ebay.com/i/232636542392?chn=ps
I look forward to the tutorials on these.
Lane Anderson posted:No, Al, the term is "Poopie HEAD."
That's very true, Lane, thank you for pointing that out. It was very late and I just wanted to go to bed.
And btw-
YOU'RE ALL POOPIE HEADS!!!!!!
Spoken Yoda has. I feel much better now...
Some good laughs here, and truths as well.
Krusty, the thermocouple chips I got on bangood.com were very cheap. Yours I believe is the next evolution. Keep us posted. I forced myself to learn C++ for Arduino, as they are dirt cheap. I got nano versions for less than $3. I picked my platform with my wallet rather than go Raspberry Pi.
edsnova posted:Re CHT monitoring gear, Raspberry Pi, etc.: Unlike with industrial/mechanical stuff, it really can pay off to make your own electronical stuff.
https://www.ebay.com/i/232636542392?chn=ps
I look forward to the tutorials on these.
Ed, I paid $6 a chip with thermocouple. I see the chips for $2.50 by themselves, then you can purchase thermocouples elsewhere like I did. Don't fall in the rabbit hole! The code is REALLY unforgiving and frustrating. I bought all these parts years ago, it took me 2 years of messing with it from time to time to get it to work.
These are cool too:
Clearly above my paygrade!
And on the flip side of all this engine temp stuff...
My '95 VS, 1835cc, 1:25 rockers, dual 40mm Kadrons, external oil cooler mounted in the badlands area on the bulkhead above my 3:88 4-speed trans. Odometer showing 121,000Km (75,000 miles) at the time of our trip cruising through Colorado. 
On our way to Colorado, spent the night in Moab Utah and took a sunset cruise through Arches National Park. 
Met up with Dusty Johnson (Pleasant Valley Saddleworks) in Loveland, CO.
Cruising with Dusty and a few of his fellow PCA friends through the Rockies. My nebulous temp gauge rarely went above 3/4 mark, so actually had no idea what any of my engine temps were....
...didn't matter though, just mindlessly enjoyed the ride and the scenery. Gratuitous photo op in the Rockies. 
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When did you come through? I'm in Laporte, a few minutes away from Loveland.
Cole Thompson- posted:When did you come through? I'm in Laporte, a few minutes away from Loveland.
Cole, this was about 10 years ago. We came up through Las Vegas NV, then Moab and Aspen CO. Swung by Air Force Academy in Colorado Springs, then north to Loveland and Fort Collins. Epic trip! @Cole Thompson-
MusbJim posted:Cole Thompson- posted:When did you come through? I'm in Laporte, a few minutes away from Loveland.
Cole, this was about 10 years ago. We came up through Las Vegas NV, then Moab and Aspen CO. Swung by Air Force Academy in Colorado Springs, then north to Loveland and Fort Collins. Epic trip!
The silver car is the time machine car, as in, he had that one a while back. The modern car is the black one.
I think there is quite a large heat difference between a low-compression 1835 and my 2165 hi-compression motor. I'm running 170hp, what's Jim, about 80-90 on a good day? Which is basically half, so that's my concern with temp monitoring.