cht gauge accuracy

Seems to be some contention over the accuracy of cylinder head temperature gauges. This centers around the wiring itself emanating from a very hot place going to and through a much cooler place. Results, at 350 degrees about 30 to 40 degrees inaccuracy. Any comments from you wrenches? Thanks in advance. Also, best brand to purchase.

As most thermocouples develop their "potential" only in the area where the dissimilar wires form a connection, I would believe temp differences along the wiring/cable path would have no effect on the gauge reading......I used thermocouples, along with process controls in several industrial applications demanding +/- 1 deg.f. as normal operating spec.....

Bob,
This was talked about not too long ago.
Here is one that is temperature compensating:

http://www.aircraftspruce.com/catalog/inpages/CHT_gauges.php

Check out this thread:

https://www.speedsterowners.com/forum/readmsg.asp?t=18883

Always remember that the "wire" between the thermocouple pickup point (usually plug numbr 3) and the temperature gauge itself on the dashboard, is NOT wire in the conventional sense. It is "thermocouple extension cable". The gauge itself is calilbrated to the exact length of the extension cable supplied. NEVER cut this wire short, even if you have excess.

why not use a thermistor or RTD?

I am buying one right now for my Megasquirt system and this is a thermistor:
http://www.aircooled.net/new-bin/viewproductdetail.php?keyword2=FSI0002&cartid=0526201089551534

Not sure what this is yet and am waiting for an answer:
https://secure.cbperformance.com/catalog.asp?productid=333

Megasquirt cannot read a thermistor

CB advises that theirs is a thermistor.
1/8 NPT so one would need to drill and tap the head

You guys are too good. Terry, thanks for the site. Will try to call today.
Bob

Bob, I just received my Micro1000 cht gauge from Aircraftspruce. Not the easiest gauge in the world to read, but I wanted an accurate gauge-function won over form.

The key is purchasing an instrument that is temperature compensated.. the only units that have this are SPA, Dakota Digital and a few others that can be bought under different brands from aircraft spruce.

I use SPA for my datalogging equipment, nothing else I have seen has the sampling rate or response time to temperature changes. The worst is VDO.

I made a cht gauge dashboard extension to get it in exactly the perfect position for my eye-line without totally screwing up the dashboard look. The face and the light match the other instruments. I made the radii of the extension match those of the rest of the dash and concealed all screws etc, so it looks about right. I made a mould for a RHD car, so if anyone wants one, shout.

Nice fab.

Nice job on that addition. Looks good!

Good interesting topic.

First, my cht gauge saved my engine coming home after Morro Bay last April going over the Tehachapi Pass. We were climbing up the pass
and at 75 MPG and 3,500 feet my CHT went from 310 degrees to 475. I absolutely freaked when I saw that. As I was [pulling over to the side I said to Alice "there's something in that fan" and sure as hell it was---a piece of friggin' paper towel. The car was running good and not loosing power---just rolling up that mountain strong as hell.

If I didn't have a CHT Gauge I wiuld have fried that engine for certain. No question about it.

Right now I have an issue with the circle sender around the #3 plug
--there is some binding when turning out the sparkplug. I believe the wire and ring sended is involved anyway I plan to move the sender to around a head bolt nearest #3. This will make it easier to change the plug too, which for me is a challenge. Westach says that I will see a lower temprature by 25 to 50 degrees but it will still read the trend which is all I need. Really just green, orange and red lines on my meter would suffice to show me the trend.

So if I get a 50 degree lower reading, instead of seeing 325 degrees going down the road, I'll be looking at 275--that's my new "normal" and I know it's really 325. When it rises from 275 to 425 I'll know I have the same amount of increase as I had going up the Tehachapi
Pass that day.

The point of all this (if there is one) is that I believe the trend is as valuable as the actual temp anyway--that the precise number of degrees at #3 isn't that critical.

Whatever--the CHT saved my engine and quite a few dollars too!

Anyone see flaws in this thinking?

I had the same issue as Jack when installing the sender under #3 spark plug. It's a PITA, but I wanted it there to get a rapid response to any elevated temperatures. IMHO the rate of response, if under a cylinder head bolt, will be a bit slower. I wanted the optimum response time, so I keep mine under #3 plug.

When ambient temperature is often over 100 and the freeway speeds are often over 80 (both conditions can exist here) I get a little paranoid and watch that gauge way more than I want to.

As for gauge location (authenticity not being an issue for me), I just tucked the CHT and the oil pressure gauge under the edge of the tonneau cover, where they would still be visible when driving.

Dave--did you ever experience a spark plug in #3 not wanting to unscrew? Mine gets damned hard at just one turn out---maybe just 2/3 of a turn. I'll have it looked at but when it got hard to turn I just tightened her right back dowm. I feel like the big wire going beside the plug plus mayve the 13mm ring binding against the 13 1/2 mm plug are binding if it wasn't filed out to slip onto the plug.

My plugs are hidden inside the tins---there just a 16mm hole to put the plug through by feel. I did the other 3 ok but #3 was a PITA for a newbie mechanic like me.

What was yours like?

Jack, those under-the-spark-plug senders can get caught or gall the spark plug threads a little. Kind of a necessary evil. IMHO, you will still see your trend if you move it, and it may respond a little slower and read a little lower. So what! Drive it around and pay attention to it under normal conditions, and you'll still know when it is ABnormal conditions, which is all you really need anyway.

My temp sensor is screwed in a boss on the head, in the number 4 head, which runs hottest on a 911 fan conversion. The only time it gets high is when the car is stuck in traffic, recirculating the same hot air. Today at 58 F ambient, in traffic for 5 minutes, the temp went from 220 to 270, because I was sitting.

Danny--that's exactly what I was thinking. My plugs are gdifficult to change aanyway because tou can't see where they go. I'm getting better at it but with that ring around the plugs it will be a fight for me every time.

Thanks for your advice. I'm glad to know that you already have what I'm thinkong about--that is, to put the sender somewhere else than the plug. Like you, I can accept a little slower change and just need the trend.


Good to hear from you!

Bob, I presume you'll have got your CHT sensor sorted by now. For what it's worth, I think most Temperature Sensors are realistically only a general indication and not accurate. That isn't really a problem as what you're looking for on them is something that's out of the ordinary from you'd normally see. That is gross over-heating of the cylinder. I look at mine in correlation with the oil-temp sensor.

You're right about how you can soon stuff the heli-coil of the spark-plug thread. I put a bit of Copper-Eze High Temperature Lubricant on mine before inserting - and try to hold the sensor ring whilst tightening. Fiddly job.........

Regards, Mike

Mike --how loose is your ring around the sparkplug? I measured a new ring's opening at 13mm and the plug at 13.5 mm. Obviously the rings should be filed out to allow a loose enough fit so it doesn't bind.

Rings come in other sizes too but I have the 13mm.

Getting the plugs without that big wire and ring in my Type IV is
tough enough but with, I just don't know if I'm up to that.

How do some others accomplish that?

Rite here Jack:
http://www.germansupply.com/xcart/customer/home.php?gspage=customer/main/tech/cht/cht_install_2.tpl

Did Jake install your CHT ?

Personally i've put off installing mine until i figure a way to snap it onto the "nut" of the spark plug , rather than under the plug. Some kind of spring-steel/copper gescheft that can simply be attached/detached and moved aside when checking the plugs. The steel portion of the plug should easily carry the same temp as the head. A small FG sock over the whole thing to keep the cooling air out.
Any sledgehammar machinists out there wanna take a crack at this...

Daniel should not be the 2nd cylinder the hottest running in a 911 conversion?

#4 is under the fan and against the axial flow of the fan, #1 gets the most air, #2 the next, then #3, and finally #4.

I hear a lot of discussion on what gauge to use, but is there anyway to make the engines run cooler? I took mine for a drive in the foothills today and noticed that it was running toward the higher end of the gauge. Not sure how hot that was, as it is the normal set of gauges that comes with a Vintage Speedster. Anyway, any ideas on how to cool the engines down. If it is running this hot now, I can imagine what it will be doing next summer. PS...it was 68 degrees today.

Mechanical heat temp test:
If the valves are a' rattl'n the motor is run'n hot.

I'm going to adjust my valves today! Hopefully that should help. Thanks for the info!

If a valve adjust doesn't cool it enough, then an external oil cooler is in order. There's an article over on speedstershop.com about how to install one of those, and it's been mentioned on here a few times, too.

WHY would anyone bother with CHT if all you need is OIL temp gauge ???

Kool oil does not necessarily mean cool head.
(Its a frying pan AND a fire up deah)


The Jakester has commented on this B4 .

Gordon, how would an external oil cooler bring down cylinder head temperatures?

Ron:

It won't, but I was directly answering a question he had asked:

"I took mine for a drive in the foothills today and noticed that it was running toward the higher end of the gauge. Not sure how hot that was, as it is the normal set of gauges that comes with a Vintage Speedster."

Last I new, that means oil temp, not CHT. If he wants to bring his oil temp down, an external oil cooler is the way to go (assuming he already has all of the cooling tins and shields in place).

Still, to avoid a lot of work and $$$$ unnecessarily, the FIRST thing I would do is get (or borrow) a $50 dip stick thermometer from www.mainelycustombydesign.com and find out how hot it really is - maybe it's just a generous gauge.....

gn

During a resent conversation with Henry the Mainely dip stick thermometer came up (I have one). To my surprise Henry said that they weren't that accurate. I'll have to do some testing with mine.

Henry's right, in a way.

There is a nut on the bottom of the gauge scale, and sometimes they loosen up and allow the gauge scale to rotate, thereby skewing the read-out.

All's ya gotta do is loosen the scale nut a bit, put the thermometer it in a small pot or measuring cup with three inches or so of water (make sure it's a Pyrex measuring cup) and bring the water up to boiling. Note the reading and rotate the scale to read 212 F to dial it in.

Gordon; how do you dial a VDO CHT? is there a screw or something?

http://www.etbinstruments.com/Gauge_Range_Overview.htm