CHT Sensor - Alternate Locations

[Floating VW]

I want to start by saying that I am aware there are several other threads that have also dealt with this subject, such as these here:

https://www.thesamba.com/vw/forum/viewtopic.php?t=...mp;start=0
https://www.thesamba.com/vw/forum/viewtopic.php?t=129036&highlight=cht+sensor+location
https://www.thesamba.com/vw/forum/viewtopic.php?t=745116&highlight=cht+sensor+location
https://www.thesamba.com/vw/forum/viewtopic.php?t=424332&highlight=cht+sensor+location

But I feel these threads were all rather lacking in useful data. In fact, the main reason why I wanted to start this one is that almost all of the others were basically three or four pages of people telling the OP to stop being a dumbass and just put the damn sensor ring under the #3 plug.

Of course, we all recognize the benefits of using the #3 plug as a location for the CHT sensor, and if you want to measure your CHT's this way, nobody is gonna ding you for that. So, please let's not discuss the #3 plug any further, here. There is no need.

Instead, I would like to hear from those of you who went a different direction; how did you do it and what were your results, both good and bad.

I'll start with my own experience:

I decided not to go with the #3 plug for a couple of reasons. It makes getting the plug in and out tricky; the plug doesn't sit as deep as it should in the chamber, unless you remove the gasket; and the sensor ring tends to get chewed up over time. But most of all, I don't really care how hot my spark plug is. I did start off with a 14mm ring sensor (in case I ever changed my mind about putting it on the plug), but I had to reshape it to make it fit in the location I chose:

To prevent the sensor from sliding around, I cut out and soldered a piece of copper sheet to the inner part of the "ring", and drilled a hole through the center for a bolt to go through:

If the most important thing to know is the temperature of the valve seats, and the second most important thing to know is the temperature of the hottest spot in the chamber (i.e. the spark plug), then it seems to me the best location for a CHT sensor is right square in the middle of these. On my cylinder heads, there is a little plateau directly beneath the plug and in between the intake and exhaust ports. This is where I drilled a 5mm deep hole and tapped it for an M4 bolt (The hole is not exactly in the center because the shape of the cylinder head makes it very difficult to get a drill in there, but this is not a bad thing. Also, I was a little concerned at first about weakening the material here and possibly causing a crack to form, but the metal is about 25mm thick in this area, and even though you can't see it in this picture, the edges of the hole were later cleaned up and chamfered, so we'll see what happens. I think I'll be fine):

The idea of using JB weld didn't appeal to me, so to clamp the sensor to the head, I fabricated a steel plate and added three layers of gasket material (not visible in this photo) to the back, to insulate it from the sensor. The idea is to not only keep the sensor snug up against the cylinder head, but also to prevent any air flow from cooling the back side of the sensor.

And here is the CHT sensor bolted to the cylinder head. If you notice, the hot junction of the leads is closer to the exhaust valve seat and the spark plug than to the intake valve seat, which is a good thing, in my opinion:

The lead goes through the rear of the cylinder tin, through a small rubber grommet, which makes for a nice, clean installation:

I chose the Micro 1000 dual CHT gauge, which is temperature compensated, to accurately monitor both sides of the engine at the same time (more specifically, cylinders 1 and 3). I calibrated the gauge by dipping the thermocouples in a bath of hot oil, and comparing the reading on the gauge with a digital thermometer. Below 200 degrees F, the gauge is not very accurate, but is spot on between 210 and 400 degrees F, and that suits me just fine. You might also notice that the gauge is in a round, aluminum bezel, instead of a square, black plastic one. That took quite a bit of delicate fabricating on my part, but I wanted it to match my Autometer gauges a little better.

Results?

Let me start with a little caveat here. My engine runs cold, it always has. It was designed and built to do so, and even more so after installing the ram-air cooling ducts and the venturi ring in the fan shroud. So if the following temperatures seem a bit on the low side to you, well, they are on the low side, but that fits in perfectly with what I consider normal for my engine. With that in mind:

Temperature changes on the gauge occur instantaneously (which is something very important to consider when selecting a location for the sensor). In the hot summer months, average highway cruising CHT's are a cool 240 to 260 degrees F. At a sustained full roar in fourth gear on level roads, the heads level off at around 365 degrees F. In the winter months, however, cruising CHT's barely get over 210 degrees F, and this concerns me. To corroborate this, the oil barely gets warm enough to register on the gauge (which starts at 140 degrees F), and that concerns me even more.

Something interesting I noticed is that, once the engine is up to operating temps and the cooling flaps are open, both sides of the engine run almost dead even in temperature. However, at start up, the right bank heats up faster than the left, but only until the flaps begin to open, and then the left bank heats up faster. Eventually, they both level off somewhere a little above, or a little below 250 degrees F.

About the only negative thing I have to say about the gauge (apart from the fact that it comes with a square bezel), is that the resolution is not very high and the face is set very deep in the case. The low resolution means it's very difficult to differentiate between, say, 263 degrees and 269 degrees, so using the gauge as an additional tool for tuning the engine is not really an option. And the deep-set face causes a considerable amount of parallax unless viewed from directly straight on.

So that's what I got. Hopefully this will provide someone with some good information (even if only what NOT to do), and if there are any other like-minded people out there, please feel free to post your results.
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"It's time you started treating people as individuals, rather than mathematically predictable members of an aggregate set, regardless of how well that works."

[texastomeh]

VERY timely post - at least for me!!

I am to the point in my build where I need to decide/do the CHT sensor installation.

I really don't want to do the "ring under the #3 spark plug" and am considering this method as well as other options presented in the referenced threads.

I wish that I could contribute a recommendation, but the PO of the engine that I am replacing had the ring under #3, and I swore; "NEVER AGAIN"!!

I know that it has been discussed ad nauseam, but am anxious to see if there are any "new" opinions!

Tom
_________________
A new engine build won't solve ALL of life's problems - only those THAT REALLY MATTER!!!

GETTING old was GREAT - BEING old SUCKS!!

Too bad that the guys that know how to solve all of the WORLD's problems are too busy working on old Volkswagens!!

[Chip]

I like this. I had an install with 4 under plug sensors. Totally sucked to work on. I personally care less about the actual number, and more about side to side and cylinder to cylinder consistency. It's cool to see those results from your situation. I do however understand your ultra cool running concerns. Might be worth blocking some of the fresh air intake on the rear of the car to assist with that, think diesel truck during the winter, in places that actually get winter. I'm not a thermostat guy (just move on) so I don't know if there are different temp thermostat options out there. Might be nice to find a higher temp unit that opened later, to help that sucker get up to temp and stay there.

[clonebug]

I have a higher temperature thermostat and it didn’t give me any noticeable temp changes. It didn’t hurt anything either so it’s been on there for 5-6 years at least.
There are some thicker spots on the head that can be drilled and a thermistor glued in there. I helped install one in the Skyway Flyer last winter and he uses it to monitor head temps for starting his cooling fans.
It’s all controlled by his ECU but you can easily hook it to a cylinder temp or EGT sensor and monitor that way.
_________________

[ottobevis]

I'm going to be using four O-ring thermocouplers with a TC4 plus but am locating them on the lower head studs (the ones closest to the pulley and flywheel). The thermocouplers will basically be on the outside of the chamber, near the bottom of the chamber).

I have 044 heads and there is a small space between the lower fins of the head where the stud passes through. I have springs that will keep the thermocoupler pushed against the chamber of the head. The disadvantage of this method is that it is not the hottest part of the head. It should still show variances between the cylinders (which will help me adjust the cooling of a 911 style shroud). The advantage is the location is not in the direct path of my fans airflow (which may reduce the temperature of the thermocoupler).

[raygreenwood]

Re-stating the obvious......The main reason to put the TC on cylinder number 3 is that with "many" acvw engine configurations....#3 tends to run hotter. I know for sure that type 4 engines are a little lopsided due to combined shroud output imbalance and the tight eucting on the 3/4 side that causes static pressure stack up turbulence back by #3.

I also understand that where the sparkplug actually is tends to be just ahout the hottest part of the head....usually. All of that said.....I think that actually under the spark plug as a replacement for the gasket is pretty poor in numerous ways.

Here..... check out this mod. I cant remember if I posted the pictorial on this one yet. Scroll down. It puts the thermocouple withjjn about 1.0 to 1.5mm of where it would be when the ring terminal is replacing the plug gasket.....its in tight contact with the head.....but its not actually replacing the plug gasket or in contact with it. It allows you to change or gap plugs without disturbing the TC.


https://www.thesamba.com/vw/forum/viewtopic.php?t=756796&highlight=cht

https://www.thesamba.com/vw/forum/viewtopic.php?t=752545&highlight=

Ray

[jim martin]

if i remember i did this years ago on my old nitrous street strip motor .
i deformed the CHT connection under the plug after a while .engine was out for clutch work and i modified and mounted the connection directly to the head .its been a long time but i remember this .
temperature readings were like 75-100 deg less and took way longer to react .
but had my info from before !

since in this case you have a 2 sensor input i would log the same #cylinder
under the plug and mounted on the head at the same time .
then you will have real information
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[FreeBug]

One of the reasons to use the n°3 plug, is that it's a sort of "standard" used throughout the VW world, and so you can compare apples to apples, when discussing different engines/combos.

I totally agree it's a pain, I epoxied mine into place, it works so far, doesn't come out.

Nice work!

[Floating VW]

[Floating VW]

[Floating VW]

[raygreenwood]

For those who are wondering.....and I will show the pictures and measurements when I get around to posting the whole pictorial "how to"......if there is a difference in temperature readings between the sensor being used UNDER the spark plug in place of the spark plug gasket (like most CHT thermocouples are installed)....or as I have done it in this mod with the ring terminal AROUND the spark plug and not interfering with the gasket..........No......there is not.

At least not from what i was able to measure many years ago when I first did this mod and drove on it for quite a few miles......and not as I have currently tested this one. There is also not really an expectation that there would be a difference.

In a normal copper TC rimg terminal.....the weld bead for the TC is typically on the inner end of the barrel crimp about 0.060-ish from the gasket. Usually the weld bead is slightly exposed and you can see it. Sometimes on crappier crimp rings ...the weld bead is buried somewhere in the middle of the barrel crimp on the copper ring.

This is poor form....because if they crimp the barrel too hard it can crack the bead or crack one of the wires. Over a period of time....temperature cycles and vibration....you start getting screwy readings or it quits altogether.

So suffice to say that my terminal puts the TC bead (hot junction).....probably with about 0.50-ish from the same place a normal VDO type ring does.

The part that I have not tested with this particular ring.....is on a running engine that has qn operating spark plug ....adding ITS heat to the ring terminal.

That factor can be more heat than you think.

To this point.....testing this.....I have made two terminals. The one you see in these pictures.....NOT UNDER the spark plug......and one that is a knock off of a VDO that goes UNDER the spark plug.

Both TCs were identical and came from a set of 5 I bought. Both were connected to TC mini plugs. Both were tested in the same hot water bath at the same time and plugged into a true RMS volt meter. Both were within ~0.5°F to each other.

Then they were tested in free air the same way in a small oven. They were both within ~1.75° to each other in the 400° range. Thats pretty good.

I put them both on my test mule 1.7L head.....one on #3 and one on #4. Set the head in the oven and let it come up to 350° F......to saturate. Both TC's read within 1.5° and about 2.5° to each other ......so there is some small difference. Small.

This is going off memory. When I finally post this up I will post pics of the TCs and voltmeter readings.

I suspect.....actually I expect......that with an operating sparkplug......you will see about 40-50° higher. Pure speculation....but eventually we will see.

Ray

[veltror]

i would try this instead

https://kaefertuer.com/en/produkt-kategorie/en/

[gorbur]

Quite timely for me as well as I'm rethinking my CHT.
Originally, I settled on a single sensor plug #3 install. With a new engine more of a early issue indicator. However, after the break-in I found I was having issues getting the temperature up to normal and as you found, imbalances. I flipped to two NTC thermistors mounted on the forward inner head studs on each bank. There is just enough thread on the head stud to use a ring connector and a jamb nut.
Not sure if ill leave it that way after the heat up issue is resolved. This last week it hit -39C so not really a realistic temperature to do any testing.
Very interesting post and hope to hear how you and others progress
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73 - Ghia Cabriolet, currently under resto
76 - Type1, 1776, full restomod

[raygreenwood]

[texastomeh]

Guys,

I don't mean to sidetrack the thread, but have a question re the CHT gauge.

I have a new SPEEDHUT REVOLUTION CHT gauge that I ordered: "customized" (color dial, pointer, font).

IF and WHEN we get this SENSOR location/installation thing settled , I would like to be able to monitor all four cylinders by using a selector switch.

I am just not sure how sensitive the gauge/probes are to "Selector Switch Resistivity".

The four probes will be OE identical "K Type Thermocouple Temp Senders - 9 Ft."
It's a kit car, so the length is fine.

Can anyone that may have "Been there - Done that" suggest a four-position selector switch?

I have found one made by WESTACH for aircraft, but it is prewired.

Thanx in advance for any help.

Tom
_________________
A new engine build won't solve ALL of life's problems - only those THAT REALLY MATTER!!!

GETTING old was GREAT - BEING old SUCKS!!

Too bad that the guys that know how to solve all of the WORLD's problems are too busy working on old Volkswagens!!

[raygreenwood]

[texastomeh]

Ray,

I REALLY appreciate the response - ESPECIALLY the schematics!!

The SPEEDHUT literature included with the gauge describes the sensor as:

[raygreenwood]

[Floating VW]