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TM cylinder head temps....

R

Reddog

Guest
So I have my CHT installed and the sensor is on teh #3 spark plug. Yesterday, while running up a grade (probably less than 6 percent, it is a county road in the summer), I was seeing CHT's in the 450 range, possibly higher. The road had been groomed and while the surface was softish, the running was pretty easy. 3rd gear at about 3000 RPM. On the way back down, temps were more around 350.
I just installed Petronix ignition (points replacement) and rebuilt/re-jetted the carb. The rig ran great otherwise. Started easily, ran well, no hiccups.
I have all the tin in place on top, but nothing below, both heater valves are wired open, generator belt is tight, no rags or squirrel carcasses in the fan housing.
Any input? Jetted too lean? Timing too advanced? Missing lower tin?
Temps seem high to me, but the rig is running better than ever...:unsure:
 
R

Reddog

Guest
On my to-do list. The machine is not here today, but that is one of the first things on the list.
 

Snowtrac Nome

member formerly known as dds
GOLD Site Supporter
I would want to look at the plugs.running that range you are on the ragged edge of too hot. I like to see mine between 350 and 425 usually if I sart running hot it's because I have my foot all the way into it trying to push it up a hill too fast.I think I would start by cutting the timing back a couple of degrees,you should only be running at about 6 degrees
 
R

Reddog

Guest
I have been looking more into the CHT situation I have. One thought is I shortened the harness that came with the gauge, which according to what I have read, should make no difference. I found this on ratwell.com, written by Richard Atwell:
"Cylinder Head Temperature:

Another popular gauge is the Cylinder Head Temperature (CHT) gauge.

This gauge measures the head temperature. Some folks think that reading the oil temperature is enough to determine the engine temperature but it's not. Head temperatures are related to the work that the engine has to perform and will increase more quickly than the oil temperature. As the engine pushes the bus through the air at faster speeds, all of the frictional forces including the friction of the rotating engine parts resist that forward motion. Head temps increase further when you climb hills as try to maintain your speed.

The aircooled engine requires a steady flow of air for cooling and a working thermostat system. When the thermostat breaks, there is a failsafe mechanism to ensure the engine doesn't overheat. However, it's simply a spring and if not checked to make sure it's working you may end up with an overheated engine anyway. If you are missing the flaps all together, although the heads will be cooled, the oil cooler will not be so you must at least have the flaps in place!

The #1 problem with Type 4 engine is a sunken valve just like the Type 1 engine. This is due to overheating the heads because the driver was unaware of the problem and over several hundred or thousand miles the head slowly failed and the final result was lost compression on one or more cylinders or worse, a broken piston because the head of the valve fell off.

The CHT senders consist of a K-type thermocouple that is placed under the spark plug. The thermocouple itself is a pair of two dissimilar metals in the form of wires that produce a mV signal when heated. The junction near the spark plug is called the hot junction and the other end is called the cold junction. The ring under the spark plug is a simple ring terminal and the metal that it is made from doesn't affect the thermocouple according to Seebeck's law of thermocouples: it merely exists to transfer the heat from the plug to the thermocouple.

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.

Dave Bolen recently benched the VDO gauge and sender against an expensive calibrated probe. The results were even worse than the results the math predicts under a controlled environment

Dave writes:

I finally got out my new CHT gauge and sender over the weekend and did some comparisons on the temperatures using the oven and the freezer.

Here is what I found out. First of all, the sender was "mis-wired", red went to the negative terminal on the gauge rather than the positive one.

The major thing that I noticed was that the gauge reads about 20 degrees difference (higher) when tilted upward at an angle like it might be mounted in a dash(rather than horizontal).

Placing the gauge in the freezer for 15 minutes while the sender was held at a steady temp raised the temp shown on the gauge by about 20 degrees.

The senders were placed between two pieces of sheet metal in the oven I set a pyrex dish on top of the sheet metal pieces in order make sure nothing moved. Gauge temps based on "house" ambient temperature(70 degrees).

All temps are in fahrenheit, and numbers that aren't "on the mark" on the CHT gauge are approximate.

The meter I used was a fairly expensive METEX with a type k thermocouple probe. The meter probe in boiling water reads about 210 at 1100 feet above sea level.....which is pretty much exact. The probe for the digital meter was ordered with the meter when I bought it.

Metex VDO Error
400F 460F 60F
358F 405F 47F
300F 340F 40F
280F 300F 20F
250F 260F 10F
235F 250F 15F
201F 210F 19F
188F 200F 12F
So, if you graph these, what becomes apparent is that the difference between "real" temps and CHT shown temps becomes wider and wider, until at a true 400 degrees, the CHT gauge is off 60 degrees.

Fluke temperature

In other words, this gauge will not help you prevent a meltdown because it will force you to drive at such a low speed you'll never overheat the engine anyway. This may not seem like a problem but consider how you will keep up with highway traffic speeds? With variations in outside and engine temperature the VDO gauge is 100% unreliable."

So since the outside temp was 16 degrees, was my gauge reading higher than the head temp acutally was??
 

Snowtrac Nome

member formerly known as dds
GOLD Site Supporter
I have compared mine to a laser thermometer and I have found it too be accurate with in a couple of degrees I think that is pretty good. I also shortened my harness too.
 
R

Reddog

Guest
Got the rig in the shop today. Plugs look lean, so I went from my 115 main back to my 122.5 main
P1050905.JPG


P1050903.JPG


Same plugs, 1&4, just different lighting and exposure.

Also found it has a belt that is really too wide for the pulleys. Installing a correct belt also.
 

BigAl

Gone But Not Forgotten
SUPER Site Supporter
Is this a gauge I should be running on my rebuilt VW engine on my Kristi KT3 ? I would cry if I messed something up .......:sad:
 

Snowtrac Nome

member formerly known as dds
GOLD Site Supporter
Is this a gauge I should be running on my rebuilt VW engine on my Kristi KT3 ? I would cry if I messed something up .......:sad:
yes Allen I would put one on yours too this is the one goodie vw should have put on their motors. I wouldn't worry too much about calibration the cheep vdo gauge I have is plenty accurate where this comes in handy is say you are pulling through deep or heavy snow at full throttle but only making about 3 k you watch the gauge, it will start to spike quick when you see it start to climb .down shift a gear and like magic you will see it drop right back down. when running these things under load they are worth their weight in gold.
 

BigAl

Gone But Not Forgotten
SUPER Site Supporter
yes Allen I would put one on yours too this is the one goodie vw should have put on their motors. I wouldn't worry too much about calibration the cheep vdo gauge I have is plenty accurate where this comes in handy is say you are pulling through deep or heavy snow at full throttle but only making about 3 k you watch the gauge, it will start to spike quick when you see it start to climb .down shift a gear and like magic you will see it drop right back down. when running these things under load they are worth their weight in gold.

Any special machine work to do on the heads or do they mount up easily ?
 
R

Reddog

Guest
Just an ongoing update. Today I had the TM on the trailer idling, and the CHT leveled out at an indicated 340....seems high for idle.
I got the IR thermometer and from underneath, pointing at the head itself as high toward the plugs as possible, I was getting temps in the 270 range on the RH side and 220 on the LH side.
Any thoughts.....
 

Dakota Kid

New member
I am more familiar with corvair engines than vw but when you take off the lower engin shrowding and don't have the thermostates in or working your airflow will not be as designed and that could be a part of your problem. too far advance will also cause heating as will a loose fan belt. My guess would be the missing shrouding and the non-working or missing themostates Hope this helps its tough to diagnos but if it was a corvair that is where I would start
 

Snowtrac Nome

member formerly known as dds
GOLD Site Supporter
sounds like a cal issue I will check mine out mine normally stays around 300 to 350 degrees
 
R

Reddog

Guest
Just a little more info. The engine runs absolutely great. Starts with zero effort, takes gas and revs out as good as any I have been around. Running on 91 octane non-ethanol fuel.
 

Snowtrac Nome

member formerly known as dds
GOLD Site Supporter
I can't check mine it appears my kids found my laser thermometer which equates to I cant find mine now. I will check again tomorrow, but from what you have told me it sounds like a calibration issue it shouldn't be that far off .
 
R

Reddog

Guest
One other note. When I pulled the plugs to check the carb jetting, there was some carbon/exhaust traces on the #3 plug base as if some exhaust was blowing past the plug seal. While I felt the plug was tight, the CHT thermocouple ring makes installing and tightening this plug a real bear. The ring has an extreme 90 degree bend on it which has made the opening somewhat oval and the crimped connection makes getting the socket on the plug almost impossible. Would exhaust gas blowing past the plug and then past the thermocouple ring give inaccurate readings? (Am I reading exhaust gas temps, not cylinder head temps??)
Although #3 is generally accepted as the best place to put the theromocouple, it's no good if I can't get the plug tight and it won't seal, both bad situations...
 

mtmogs

New member
I agree with your observation that the sensor ring is possibly preventing a tight seal of plug #3. In addition to allowing exhaust blow by, the more sinister effect could be allowing air to enter on the intake stroke causing a lean condition. This would raise the temoerature of the head as well at that location.

This may be causing your high CHT readings. The news might be better or worse depending on how much your CHT guage calibration is off. I have a Dakota Digital guage/thermocouple and it is temperature compensated. When cool, the CHT reads the same as air temperature measured by a digital thermometer. As a point of reference, after my engine is warm, the idle temperature measured at #3 is generally around 255-270 degrees when the air temperature is in a range of -10 to 30.

My engine is a1915cc which probably generates HP on the order of 90-100 HP. It should theoretically run hotter than a smaller-bore engine for a number of reasons. I originally had the oil cooler inside the old-style (pre-doghouse) shroud. I was concerned enough about the effect of this on overheating #3 that I installed the CHT gauge when I installed the engine. I thought the engine might be running hot (over 400) on long uphill grades, and suspected that the oil cooler was responsible. I removed the oil cooler from the shroud and switched to an external cooler with a thermostatic fan. What effect did it have? As far as I could tell, it didn't have much effect at all on CHT readings, maybe they were 10 degrees colder at most, but arguably the difference was essentially undetectable. The oil temps were, however, noticeably cooler.

So what have I learned about CHT's? I guess I learned not to obsess over them. Like Don says, use the information to identify a trend.if you see your temps steadily above 400, lay off the gas and downshift to spin the fan faster.
 
R

Reddog

Guest
Thanks for the input Mtnmog. If the temps were closer to what I would want to see, I would chalk it all up to calibration, but they are pretty far off the mark. I just want to make sure I don't have a real temp issue.
I'm going to pull the #3 plug again and possibly change to NGK plugs, supposedly they are a little taller and easier to use with the temp sensor.
Does anyone else have a rig that they are running without the lower cylinder tin in place? There is some discussion that without that in place, the air passed over the fins too fast and higher temps will occur...of course I doubt that most of the folks contributing to bug and bus threads are running at well below freezing temps exclusively....
 

Snowtrac Nome

member formerly known as dds
GOLD Site Supporter
are you missing the little deflectors between the cylinders if so over heating can occur. as for the thermostats needing to be hooked up as long as the doors are in place in the fan tower you will get proper cooling the thermostat was an epa thing to help the engines warm up faster. at a hundred degrees off I believe there may be a calibration issue or a leak at the ring as you describe. I have been running mine with the sealing ring installed on the plug even though the instructions say to remove the ring also my thermo couple comes out of plug facing the rear of the motor that is where I found the most room for it to make the bend .
 

redsqwrl

Bronze Member
GOLD Site Supporter
Doug I have been running this Snow master for two seasons, It is generally on flat terrain, in firm snow. summer or winter I rarely seen CHT outside of 300-350 pushing it or putzing that gauge rarely moved beyond the previous owners marks. (dash is marked with 500 max) never gets anywhere above 350 working or cruising.

The oil temps moved around a bit as well and were always very low.

Yesterday I observed something you might want to consider.

I drove 4-5 miles out onto a frozen lake during a white out/ blizzard to retrive a fishing shack. running unloaded high gear 3.5K the carb frosted up pretty bad, so I fed warm air to the carb (there is a knob for that) while idleing and freeing up the shack. when I left to head to shore I shut off the warming air, and noticed the CHT gauge climbing, ( I slowed and down shifted and made sure I was not pulling a huge amount of snow in front of the shack)

temps continued to climb.

I stopped and looked at the frosted intake and found the engine compartment packed full of wind blown snow and what not. I am convinced I frosted up the cooliing fan...

I shut it off the snow/frost melted off the fan quickly, and I got back under way with normal low temps. I regret not having a scraper as the impending amount of moisture that came in the cabin caused the window to ice heavy on the inside....

I am headed out again to retreive another soon I will pay attention to the conditions.

are you ingesting snow in the cooling fan?
$.02
 
R

Reddog

Guest
Great observation! I'll look at that next time I'm out. Thanks!
 
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