Cylinder Head Temperature Gauge

[ranger372]

Hello All,
What is considered "critical" Cylinder head temperature? I've read that the VW guys begin to panic at 425. What say the Corvair people?
Car is a '65 Corsa Turbo. I am going to be purchasing all new aftermarket gauges and want to make certain the scale on the Cyl. Head Temp. goes high enough.
There are many manufactures of gauges. I've done a considerable amount of research and have only found Speed Hut to have ALL of the required gauges for the Corsa and all the same style. My issue with the Speed Hut line is they are pricey. Is there other manufactures that I should be looking at?
I am certainly looking for quality.

All assistance is appreciated, Thank You!
Steve D.

[66vairguy]

Hello All,
What is considered "critical" Cylinder head temperature? I've read that the VW guys begin to panic at 425. What say the Corvair people?
Car is a '65 Corsa Turbo. I am going to be purchasing all new aftermarket gauges and want to make certain the scale on the Cyl. Head Temp. goes high enough.
There are many manufactures of gauges. I've done a considerable amount of research and have only found Speed Hut to have ALL of the required gauges for the Corsa and all the same style. My issue with the Speed Hut line is they are pricey. Is there other manufactures that I should be looking at?
I am certainly looking for quality.

All assistance is appreciated, Thank You!
Steve D.

This topic has been looked at for many years and without getting into a small book of data I'll summarize as follows:

1) Most modern cylinder head temperature gauges use a thermal couple. If the gauge DOES NOT HAVE COLD JUNCTION COMPENSATION circuitry it won't be very accurate and was a problem on many of the older VW cylinder head gauges.

2) LOCATION, LOCATION -- GM did an extensive SAE paper on what PART OF THE HEAD gets to different temperatures, or a sensor near the spark plug will read different than a sensor near the exhaust port. The Corvair temperature gauge sender location used a gauge center temperature of 400F and a maximum of 600F. When new anything over 450F was of concern and you DID NOT want to operate at 500F or above except for short periods.

3) The Corsa engines use a thermistor as a sender to the gauge that is now OBSOLETE. A thermistor chances resistance depending on temperture to drive the gauge. With age the thermsitor response changes. What to do --- most just see were the gauge is during normal driving and if the needle starts heading to a higher temperture then it's time to be concerned. WARNING you will find a lot of graphs done by folks measuring passive resistance vs. temperature. Per the GM documents you have to take measurements with it conducting amperage from the gauge, called "self heating effect". So the amateur data is well meaning, but not accurate

4) Clark's sells an "electronic" replacement to drive the Corsa gauge. I have no idea how it works, but many are happy with it - BUT BEWARE the electronics package is in the cable and must be kept away from extreme heat. The "bulge" in the cable must NOT be inside the shroud near the hot air off the head or exhaust manifold.

BTW - The Corsa gauge is "usually" fairly accurate, or from one to the next they indicate the same - usually! A lot more, but I don't want to get too technical.

[Deadwolf]

4) Clark's sells an "electronic" replacement to drive the Corsa gauge. I have no idea how it works, but many are happy with it - BUT BEWARE the electronics package is in the cable and must be kept away from extreme heat. The "bulge" in the cable must NOT be inside the shroud near the hot air off the head or exhaust manifold.

I actually have this currently installed and so far with limited run time it seems to jump all over the place. Once I get running again I will let the car run for 10-15 minutes to see if the gauge settles down and comes into range. If that doesn't work I will repeat the install instructions. If that doesn't work I will call Clarks for tech support. Actually the guy that sells them through clarks also sells them on his own, I came across his page once and now can't seem to find it again.

If I can't get it to work I might have to go with the more expensive Corvair Ranch option.

[66vairguy]

4) Clark's sells an "electronic" replacement to drive the Corsa gauge. I have no idea how it works, but many are happy with it - BUT BEWARE the electronics package is in the cable and must be kept away from extreme heat. The "bulge" in the cable must NOT be inside the shroud near the hot air off the head or exhaust manifold.

I actually have this currently installed and so far with limited run time it seems to jump all over the place. Once I get running again I will let the car run for 10-15 minutes to see if the gauge settles down and comes into range. If that doesn't work I will repeat the install instructions. If that doesn't work I will call Clarks for tech support. Actually the guy that sells them through clarks also sells them on his own, I came across his page once and now can't seem to find it again.

If I can't get it to work I might have to go with the more expensive Corvair Ranch option.

DW -- The fellow (and his company) worked with the Corvair Ranch to develop the same unit Clark's sells - at least at first. Not sure if the Corvair ranch has changed to another unit - hmmmm. Anyway ----- keep in mind the EM Spyder temperature gauge was not as accurate as the LM gauge. I know from experience they work similar, but the LM construction was changed to better respond to the non-linear output of the thermistor sender. However the EM gauge works O.K., no need to worry about it.

You are not the first to report this problem on forums. Do be aware that a poor ground (either engine or at gauge) will cause problems. I have often had to dismantle and clean the gauge contacts on LM units. The EM units tended to have the balance resistors soldered on the back and are subject to damage!!! If the gauge, connections, and grounds are good then you may have a defective sender. As I stated previously, they are subject to heat damage if the cable is not installed correctly. Once thermal damage occurs the vendor has told others the unit must be replaced - that's what I've seen on the CCF. Check connections and work with the vendor.

[Corvair-EFI]

Since I had to create a special thermistor for the EFI, I figured why not make a gauge set for it.

Since I got sick with Covid-19 in early Jan, I got way behind on EFI and everything else, so the gauge took a back seat. My first prototype has the wrong number (425 instead of 475), but seems workable. I hope to have a few ready by summer. The target price is $89 with 3/8" thread thermistor.

If anyone is interested, let me know and I will continue development. Just like Electronic Speed Control... sometimes you have a good idea that no one wants.



Ted Brown

[66vairguy]

Since I had to create a special thermistor for the EFI, I figured why not make a gauge set for it.

Since I got sick with Covid-19 in early Jan, I got way behind on EFI and everything else, so the gauge took a back seat. My first prototype has the wrong number (425 instead of 475), but seems workable. I hope to have a few ready by summer. The target price is $89 with 3/8" thread thermistor.

If anyone is interested, let me know and I will continue development. Just like Electronic Speed Control... sometimes you have a good idea that no one wants.



Ted Brown

Ted - Why are you using a thermistor? When I did research I found NONE of the affordable thermistors were rated for Corvair head temperatures (and remain reliable).

Did you mean to say your sender is a thermocouple?

[Corvair-EFI]

Ted - Why are you using a thermistor? When I did research I found NONE of the affordable thermistors were rated for Corvair head temperatures (and remain reliable).

Did you mean to say your sender is a thermocouple?

No. I use a thermistor rated to 572f (300c), so it all about the quality of thermistors, and have been using these for a few years. They cant be used on the Corvair dash gauges due to low wattage rating will induce self-heating. So far no failures except due to my poor soldering skills at the connector.



Thanks

Ted

[joelsplace]

Will they be available with fine or coarse threads?

[66vairguy]

Ted - Why are you using a thermistor? When I did research I found NONE of the affordable thermistors were rated for Corvair head temperatures (and remain reliable).

Did you mean to say your sender is a thermocouple?

No. I use a thermistor rated to 572f (300c), so it all about the quality of thermistors, and have been using these for a few years. They cant be used on the Corvair dash gauges due to low wattage rating will induce self-heating. So far no failures except due to my poor soldering skills at the connector.



Thanks

Ted

O.K. - Yes I'm aware of the thermistor 'self heating effect" and why all the charts folks make for the stock Corvair thermistor are incorrect since measurements were done by passively heating the thermister and using a low power ohm meter to take measurements. This process does NOT reflect the true resistance to drive the gauge since the thermsitor is NOT conducting amperage to drive the gauge. I did find a China company that would make the correct "die" size to handle the Corvair gauge amperage, but the minimum order was 10,000 pieces!!! So that was the end of that idea - LOL.

[joelsplace]

If they were .25 each...

[Corvair-EFI]

Will they be available with fine or coarse threads?

Both... but right now I am concentrating on an add-on gauge for the 95/110 engine, so Coarse right now. Can create both, but again don't want people to think that these will work on Corsa dash.

I will keep the forum informed or you can email me individually if interested. If it has to be sold through Clarks to get people to purchase, it will push the price up %25.

Ted

[joelsplace]

Are the thermocouples the only ones that need temperature compensation? Thermistors don't?

[66vairguy]

Are the thermocouples the only ones that need temperature compensation? Thermistors don't?

Correct, but it's more complicated than that. In a labratory environment with stable receiver junction temperatures you don't need temperature compensation. Temperature compensation circuits can reduce accuracy, but in an environment were the receiver temperature is not controlled, a temperature compensation circuit is required.

Thermistors are a passive device - so they are simple and inexpensive IF they are designed to be stable in the working environment. Self heating effects must be minimized. Resistance vs. temperature can change with age and thermal cycles, especially for higher temperature use unless you use a more expensive material. For the Corvair heads a good rule of thumb is to use a material that will tolerate up to 800F repeatedly without changing characteristics. That said many items now have little margin do to the need to reduce costs.

Simply put thermocouples are now more accurate and cost effective vs. thermistors in many applications, especially higher temperature applications.

[Corvair-EFI]

Sounds like a thumbs down here. No problem, I already got some VW interest...and again, I have to build thermistors for the EFI where I sell any gauge sets or not. Just thought it might be a good way to get the gauges to the people who constantly complain about and the price.

If thermocouples were more accurate and cost-effective vs. thermistors, we would be using them. If you buy cheap Chinese thermistors, that's the results you get. BTW, we are running an analog gauge to give the approximate temp of a gas-fired engine... not the fuel temp of the space shuttle.

Thanks for your input.

Ted

[wbabst]

I have installed Cylinder Head Temperature Gauges on my Corvairs. I have used a VDO gauge and Dakota Digital. The VDO gauge is not available any more, as far as I know, and is not temperature compensated. Really unless your climate swings in extreme differences in temperature you won't notice it on the gauge. The Dakota Digital is Temperature Compensated and a bit more costly but a nice gauge. You can also get CHT gauges from Aircraft Spruce. All these gauges read from a sensor under the Sparkplug. CHT gauges for air cooled engines run up to around 600 F usually, I CHT gauge for liquid cooled engine won't go high enough for your air cooled engine.

[joelsplace]

No thumbs down here. I'll take 4.