I read somewhere that the senders are difference types calibrated for the connector being @ 70 degrees. But when the connector is colder (or hotter) the accuracy is off. IOW, on a cold day your heads look hotter?
Can someone explain this behavior? How should I infer the readings on the gauge?
Should we get accuracy by putting the connector in an "oven" like those used for quartz crystal oscillators? Hahaa
We have to go way back to 1822 to understand this:
With today's micro-circuits, cold junction compensation has become a huge market.
Speedy Jim
Thanks for the theory. I like theory. Found where I read it:
So that means the reading on the gauge is 50F degrees low (120-70==50)? So 325F is really 375F? I didn't know about this before a few days ago and I have yet to fully grasp how to read the meter.
................I addressed this problem by putting a sensor in the engine compartment not far from the CHT's connector. This sensor is monitored while I'm driving and I mentally figure out what the CHT 'ought' to be reading. It works for me! One thing that I am going to change is the CHT sensor location from the #3 spark plug to a head bolt. Since I'm running with an L-Jetronic system, I may even try to double it up on the boss where the FI's CHT sensor is located. In the past, I've read somewhere that there can be enough heated air passing around the spark plug on a type1 to interfere with the sensor causing higher than actual readings.
timmy
I have those threads, too. I'm pondering 2 sensors in those holes; one each side with a DPDT switch up front prior to the gauge. Maybe I should thermal wrap the connectors in the engine compartment ;)
Ok... Enough over-thinking this... Off to paint my diagonal arms :)
On Thu, 1 Jul 2004 15:33:18 -0400, "Tim Rogers" ran around screaming and yelling:
you fellows WAY overthink this stuff...really...the actual temps are not as important as consistancy....the vdo head temp guages are great for reading temps that are all relative to YOUR engine....let you see trouble...actual temps are less important than being able to notice a sudden spike in temp.. JT
No we don't!! We're perfectionists and out to perfect our cars. Our cars mean more to us than our marriages.
Okay, just kidding on that last one.
.............Ambient temperature variation can make a CHT inconsistent. On a hot day, your reading an inaccurately low temp and then it's reading too high on a cold day. Once you've factored in this variable, consistency is then based on what your engine is actually doing. Next time you need some help with this technical stuff Chris, I'm here for ya!
:-)
timmy
Never have heard what the VDO gauges are actually calibrated for, officially. My gauge is a Westach Quad CHT gauge. After cooking #2 cylinder on a T4 engine when the injector backed out slightly, I decided that the toggle switch idea wasn't gonna cut it any more. I was running a sensor on #1 & #3 with a toggle switch at the gauge. #2's stuck ring was never indicated. If something DOES go wrong, you have to be lucky enough to have selected the correct head to monitor beforehand, and as the readings are directly related to temp where the connector is and the fact that as your engine gets hotter, the engine compartment does too, thus lowering the reading on the gauge... You have to be really lucky when you're having a streak of bad luck!
My Westach gauge is calibrated for 75 degrees F. cabin temp (airplane.) It has 48" stainless covered leads so the connectors theoretically can be placed within the cabin which should be about 75 degrees. I placed my connectors on top of the tranny and added a digital thermometer sensor so I could watch the trends for the first year. As I live in the Seattle area the cold seldom gets below 50 for long and the hot seldom gets above 90 (at all!) That means that if the temp indicated on the digi thermometer is 50, the gauge(s) (four scales) are reading 25 degrees high. If the temp is 100 degrees on top of the tranny, the gauge is reading 25 degrees lower than actual temp. I almost never saw the digital thermometer outside of that swing.
The key is, in order to monitor something meaningful, you need to be able to either monitor all four chambers at the sparkplugs or at the very least, both heads at the same time. Now, if you choose to monitor both heads at a point roughly in between the two sparkplugs, any change at one of the cumbustion chambers will be indicated very subtly on the gauge. If you monitor at the base of the sparkplug, you have instantaneous reaction indicated at the gauge. What is neat about the quad gauge is that you can see the differences easily at a glance and tell immediately if a cylinder is "out of range" compared to the others. It was interesting to note that #2 & #4 ran colder than #1 & #3 until a certain point, then they all were dead even. When going back down the other side of the hill, they cooled evenly to a point, then # 2&4 went cooler still. This was after I honed #3 cylinder .001" looser than the others so it then ran dead even with #1.
If you monitor one cylinder you would have to be extremely lucky to be monitoring the one that has a problem, especially in a fuel injected engine. The FI engines have so many things that can go wrong with a single cylinder that will not necessarily affect the others, it is virtually meaningless to monitor temp that way. Much as I disagree with oil temp monitoring, I have to admit it is better than a single CHT gauge. The oil temp will rise if any component that is oiled runs hotter than usual. My complaint is that it basically tells you that you cooked something ten miles back whereas a CHT tells you that something is beginning to happen NOW.
On the other hand, the final chapter of my last T4 motor was a main bearing failure. The headtemp gauge(s) never caught my attention cuz even though they ran hot, they ALL ran hot and I had programmed myself to believe that as long as they were all close, everything was hunkey dorey! The lesson I learned was that I should have had an oil temp gauge as well. The engine was ruined and would have been ruined anyway, even if I had seen the temp rise on the oil gauge. I might have seen the interuption in oil flow at the pressure gauge, but the interuption was only brief and that engine still had high oil pressure even with the center main mangled!
I have written about the sensors and the gauges and all several times in the past. If interested, check Google for archives. Scott Fraser was working on some electronic gizmos that automatically compensated for the temp differences and for automatically switching or sequencing the readings on an LED screen. Don't know if he ever perfected it though.
-BaH
On Thu, 01 Jul 2004 18:54:21 -0700, David Gravereaux ran around screaming and yelling:
perfection is NOT obtainable....you will kill yourself trying....you are working with 60+ year old technology, perfection is a stretch at minimum
you sure?
JT
On Thu, 1 Jul 2004 22:06:28 -0400, "Tim Rogers" ran around screaming and yelling:
no help needed, but thanks...i have understood the shortcomings of the standard vdo headtemp guages for years...i ran a full set of guages even before it was "kewl"...i also run two guages....#1 and #3 for a bit of added "protection"....but ask *any* reputable retailer or engine builder and they too will tell you the guages are great for taking "relative" measurements and that the actual temp is less important than the pattern of temps.... hope this helps you out tim, i too am here for YOU... JT
On Fri, 02 Jul 2004 03:26:05 GMT, "Busahaulic" ran around screaming and yelling:
i run two separate guages one for number 1 and one for number
3....everything was wonderful the day my engine dropped an exhaust valve on number 2...... JT
All thermocouple temperature gauges have a cold junction somewhere. No other kind of gauge has this, so this must be a thermocouple temperature gauge.
It's necessary because the thermocouple concept requires that you make a loop from 2 different kinds of metal. Once you have a loop, you're forced to have 2 junctions, so the one that isn't hot is always called the cold junction. The temperature you read is really the difference between the 2 junctions.
For measurements like this the variation in "room temp" is not particularly important, but for precision measurements, the cold junction is sometimes immersed in ice water, just to assure a fixed temp of 0 deg C.
------------------------------------------------ Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA
-----------------------------------------------
Sarcasm. You either missed it or your playing me back.
Hey Norton come down here! I know that you know, but I bet you didn't know that I knew that you knew.
I'm not one for using emoticons, but I'll try to use more in the future.
On Sat, 03 Jul 2004 02:16:05 -0700, David Gravereaux ran around screaming and yelling:
the latter.... JT
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