Cylinder Head Temp Gauge, Step By Step Instructions Options

[bd1308]

QUOTE (Headrage @ Jun 28 2005, 07:59 PM)

QUOTE (bd1308 @ Jun 28 2005, 06:55 PM) you cant use the center console wiring to wire your CHT guage.... Why?

it's all tied into the main harness...so its like unusable without hacking through it all. I would think that it would be better just to run the new wiring.

[Mark Henry]

A lot of the VDO senders came from the factory with the wiring backwards on the gauge...so if yours doesn't work swap the wires.

I found that you must cut a grove beside most sparkplug holes to clear the sender ring even with the bend in it.

Always use it under a plug...won't read true anywhere else.

[SLITS]

Allan....as I remember, the first part of the wire from the ring under the spark plug is a thermosister wire (don't cut it)and terminates in a spade terminal after 12" or so? It then becomes normal wire to the guage.

Feasibly, if you wanted to lose the oil temp guage, I would think you could use the green? wire that ties into the main engine compartment harness (one used for the oil temp sensor..exits the harness near the battery) and plug into the commensurate spade terminal normally used for the oil temp guage in the center console.

Of course, I could be smokin' some of that "Death Star Gold" too!

The one that I had (haven't reinstalled it yet) had the ring placed under the CHT (engine was carbed though).

[type47]

use the VDO wiring harness that goes with the gauge. i think the length of the harness is important so don't cut the wires to make it shorter (it's something like 15' long). i suspect for accurate gauge reading, you have to have the full length of the harness to drop the right voltage for accurate measurement.

[lapuwali]

There are two wires and you must run both of them from the sender to the gauge. Most other types of senders measure the difference in voltage between +12 and ground through the sender (a thermistor, which varies resistance based on temperature) and +12 and ground through a fixed resistance. So, they only need one wire from the gauge to the sender (which then goes to ground through the variable resistor).

The CHT gauges, however, are thermocouples, which generate a voltage themselves based on the difference in temperature between the two ends of two different kinds of wire. One end is soldered together and crimped to the ring terminal at the spark plug. The other ends are in the white plug. The generated voltage is very weak (millivolts), and only works if you measure it across the two wires.

The exact length of the wires given with the harness isn't critical. It shouldn't be WAY shorter (like 1" instead of 15'), nor should it be a lot longer, but it can be half or double the given length w/o throwing off the measurement by enough to matter.

[Britain Smith]

How come the stock CHT sender is 1 wire? Does this reduce it effectiveness?

How hot is too hot for our type-4 motors?

-Britain

[tat2dphreak]

QUOTE (Britain Smith @ Jun 29 2005, 03:24 PM)

How come the stock CHT sender is 1 wire?  Does this reduce it effectiveness? How hot is too hot for our type-4 motors? -Britain

stock CHT isn't for a gauge... it's more like a switch, hot or cold... and is only used for the FI... can't be used for a gauge either, and where it's located, it wouldn't be very accurate...

I say over 350 is getting BAD

[Joe Ricard]

This thread was going well until the last few posts.

I think 300 is too hot????? Yea right maybe for oil temp.

I personally believe the length of the wires counts as it adds resistance to the circuit. To each his own however.

Stock cylinder head temp sender provides input to the FI brain. works the same as oil temp gauge some degree of resistance/potential to chasis ground.

So what is too hot? Just me again... I slow down when it gets past 350. But that is usually around 95 to 100 MPH during mid day in southtown.

Yup some gauges wired backwards. no reading on gauge at all. I marked Rob Watsons gauge with the color next to the spade on the gauge.

Getting the two wires through the Gooey duck looking thing is a real pain. But it does make for a nice clean appearance. Put it on your to do list while engine is out.

[tat2dphreak]

QUOTE (Joe Ricard @ Jun 29 2005, 04:04 PM)

I think 300 is too hot????? Yea right maybe for oil temp.

DOH! you're right, I had 2 numbers in my head, an shook out the wrong one... I edited my post to correct...

[Britain Smith]

So for those with aftermarket EFI experience, which type would you use? I am running Electromotive and was planning on using the stock sender...would the spark plug sender work better or does it not really matter?

-Britain

[tat2dphreak]

QUOTE (Britain Smith @ Jun 29 2005, 05:58 PM)

So for those with aftermarket EFI experience, which type would you use? I am running Electromotive and was planning on using the stock sender...would the spark plug sender work better or does it not really matter? -Britain

use one under the plug, sender matching the gauge...

[Britain Smith]

Not sure what you mean, I don't have a gauge?

-Britain

[lapuwali]

The stock sender is a thermistor (varying resistance with temp), so it only needs one wire. It also pretty much maxes out at about 220dF, which is a function of its location in the head (well away from the plug), and the basic nature of thermistors, which don't do really high temps all that well.

Using it as a warm-up parameter for an ECU is a very good idea, and that's why they used it. No need for a thermocouple amp to get a reasonable signal out of it, and for warm-up use, you don't really need full scale temp. You need "cold", "warming up", and "operating temp". By the time it gets near operating temp, any enrichment added for smooth running during warm-up is down to zero, so readings past that point aren't useful.

Using a thermocouple under the plug CHT, in fact, causes problems with a typical EFI ECU. One, it's the wrong kind of signal, so it needs some electronics to convert it into the range the ECU can cope with. Second, these kinds of CHTs actually warm up too fast. From dead cold on a 50dF morning, a VDO CHT will reach the normal operating temp range within 30 seconds, yet the engine itself won't run smoothly w/o some warm-up enrichment for 3-4 minutes. Oil temp is also not a good measure, since it warms up too slowly (on my 912, oil temp doesn't reach the operating range for 10-15 minutes after a cold start), so the warm-up enrichment would go on way too long. The fast idle control on my '69 Type 3 (w/ D-Jet) uses oil temp to set the idle speed, and it runs at fast idle for a good long time past the engine reaching a reasonable operating temp. On a cool day with no freeway driving, it may NEVER reach a temp high enough to shut off the fast idle.

Another possible temp sensor for aftermarket EFI I've seen used successfully it to take an air temp sensor (as used on D-Jet, but you can use anything, including a commonly available GM sensor), which is a thermistor, and mount it to the head so it hangs below the engine, getting cooling air off the cylinders blown onto it. This appears to strike a good balance between heating up too slowly and heating up too quickly.

[toecutter]

I have been planning to buy a CHT Gauge for a while. Bus Depot offers a "Cockpit Series" VDO Guage with sending unit for $59.99. They say that it will work with Buses 1960-1991 which, I assume, would be good for a Type 4. This is about $50 cheaper that I have seen anywhere else. Is there any reason this kit would not work?

[SLITS]

I'm glad I went and looked at mine....it is two wires from the "ring" thru a white plastic connector. Back of guage is labeled + and ground. Until I see a wiring diagram, it looks like the red wire goes to the + (which I assume you also tie into a 12V source and the black wire goes to ground.

Oh well...shoulda paid attention when I took it outta the parts car.

[tat2dphreak]

QUOTE (toecutter @ Jun 29 2005, 07:04 PM)

I have been planning to buy a CHT Gauge for a while. Bus Depot offers a "Cockpit Series" VDO Guage with sending unit for $59.99. They say that it will work with Buses 1960-1991 which, I assume, would be good for a Type 4. This is about $50 cheaper that I have seen anywhere else. Is there any reason this kit would not work?

same one CB sells for 62... should work great and look almost stock

[Jake Raby]

The function of the stock CHT boss in the head is NOT where you want to measure CHT with a gauge. Inhave tested this dozens of times and have always seen the same effects. The stock location was used to tell the ECU "Hot" or "NOT" so the engine would not go full rich on start up when its only shut off for a couple of minutes.

The stock boss takes LONGER to WARM up because its away from combustion and also in the cooling fins of the head- by the same token when the engine is turned off those fins soak heat from the chambers and KEEP IT HOT LONGER- so you don't go full rich after being parked for 20 minutes and flood the engine...

In my testing I have seen the stock CHT location be as much as TWO HUNDRED degrees lower temp than the sender under the plug- where it belongs. It seems that the stock sensor will never get above 240 degrees, even when the lead under the plug gets to 400F! By the same token the sender under the plug will cool down to 150F in 5 minutes while the stock location is still at 240.

This is the rule guys:
CHT senders for gauges GO UNDER THE SPARK PLUG- No where else! AND I don't care if you think its a big inconvenience to work with when you pull the plugs out or not! I have a racer that melted a head because he refused to listen, was hard headed and has since changed his mind.

The stock sender location is used ONLY FOR EFI sensors- PERIOD! If you hook it to a gauge you may as well throuw the gauge out the window because you are wasting your time and possibly baking the engine and thinking you are just fine! Another racer kept having valve issues with the engine from his "Other" builder and told me his heads never got over "300F" I asked where he measured it and of course he said "Stock location"... I scolded the hell out of him, got him on track and he found that his gauge was reading 450F all the time! We found the source of his heat and the valve issues ceased.

As for temps... Here are my standards ALL MEASURED UNDER THE PLUG!

275-350 perfect for flat line driving

350-375 OK for climbing the occasional hill or for engines that are driven in 5th gear BELOW 3500 RPM- DO NOT DO THAT IN A TEENER!(people wonder why 2.0 heads crack)
375-400 Very warm- don't sustain these temps for more than a few seconds.
400+ You are aging the heads, startsing cracks and waiting for a dropped seat followed by the opening of your wallet.

NO 914 engine should run higher head temps than 375 climbing a hill, if yours does- You have a tuning/configuration issue that needs special attention ASAP!

With all that being said, my 3 Liter 235 HP engine runs 275-300F and will hit 325 climbing a hill....(DTM cooled)

The 2316cc Hybrid engine (180HP) in the teener runs a SOLID 275 and has NEVER went over 300 in the past 8300 miles that I have been driving it.... (DTM cooled)

So, with that being said I'll say that most stock 2 liter engines with stock cooling run hotter than a well tuned and configured performance engine- The average I have observed in my car was 350F all the time with a bone stock 2.0. A Performance engine doesn't run hotter- It just HAS to be correctly tuned.

BTW- Volume 5 of the new handbook series is "101 tips to keep your Type IV engine running cooler" and it will go over all these things in detail along with preventive measures, theory and tons of plots from the datalogger to illustrate the subject at hand... Patience, patience....

[tat2dphreak]

what do you mean by this Jake?

QUOTE

350-375 OK for climbing the occasional hill or for engines that are driven in 5th gear BELOW 3500 RPM- DO NOT DO THAT IN A TEENER!(people wonder why 2.0 heads crack)

do you mean keep the revs above 3500 in 5th? (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif)

sorry, it's early and you lost me on that line. I sould think it would be opposite, keep the revs below 3500 in 5th

[Joe Ricard]

QUOTE (tat2dphreak @ Jun 30 2005, 05:37 AM)

what do you mean by this Jake? QUOTE 350-375 OK for climbing the occasional hill or for engines that are driven in 5th gear BELOW 3500 RPM- DO NOT DO THAT IN A TEENER!(people wonder why 2.0 heads crack) do you mean keep the revs above 3500 in 5th? (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif) sorry, it's early and you lost me on that line. I sould think it would be opposite, keep the revs below 3500 in 5th

I prefer 4000 in 5th.
Keeps ma hair on fiar. 4200 in 5th is great 4400 and you are going to jail if they catch ya. (IMG:http://www.914world.com/bbs2/html/emoticons/mueba.gif)

[Britain Smith]

Jake...do you sell the CHT gauge and sender in your type-4-store?

If not, does the one from CB performance come with the 15' extension wire?

-Britain