CHT probe placement

[Ron,K]

I'm getting ready to install a CHT gauge. Which cylinder runs hotter on a 911. I'm quessing 2 or 5, since they are in the middle of the bank. Anyone know for sure?

[Grady Clay]

Ron,

Yes, those are the best choices.

What is your sensor and how is it attached?
What is your data acquisition method?
What is your use for the information?

Best,
Grady

[Ron,K]

Grady,

I'm using Westach guages that I found cheap on Ebay. Data acquisition will be with an LM1. Primary use simply is to monitor temps on the cylinder heads as I fine tune the MFI.

[Grady Clay]

Ron,

Cool, that looks like a good set-up. I have compared the under-sparkplug thermistors to planted-in-the-head thermocouples and the latter is far better. Your set-up will work OK.

Somehow I think your “’69T” needs some further explanation for all with MFI.

If it is OK with you (hijack), I would propose a data acquisition preference.
1) Head temperatures at the intake and exhaust on all six heads.
2) Exhaust gas temperatures for each cylinder.
3) Intake air temperature, humidity, and pressure.
4) Exhaust gas CO and O2 measurements.
5) Fuel flow.
6) RPM
7) Ignition timing.
8) Throttle position.
9) Acceleration.

XX) What more?

There is a lot to be gained with good data acquisition. These days it is widely available and relatively inexpensive.

Best,
Grady

[Rot 911]

Do a search. I know Roninlb has a similar setup and has posted on it before.

[cgarr]

I have this (CHT) on my 912 and it works great, I am thinking of adding an EGT too, (poor mans oxygen sensor) they respond much quicker to any mixture changes. One other thing I did when I had the engine out was to put a 5/8 alum tube thru the center tunnel of the car up to the front that way if I want to add future engine gauges the wire run is there.

Craig

[73911guy]

I've been told that #6 is the hottest since it is next to the engine mounted oil cooler and doesn't get good air flow. The comment was that on a turbo motor, #6 will be the first to go on any abnormal situation. Any turbo guys heard the same thing?

Jim

[Ron,K]

Grady,

Motor is a hybrid. 72 T with 86mm 9.5 pistons, Elgin Mod S cam, stock throttle bodies and stacks for the time being, and MFI pump rebuit to S spec.

Having just moved to Phoenix, I was experiencing very high oil temps during the summer months. I want to monitor cylinder head temps as well as oil temp during the hotter months to hopefully prevent engine damage due to excessive heat. I already have a 28 tube fender well cooler installed but I will install an additional cooler if needed to maintain reasonable temps. As a side benefit, I'll be monitoring O2 and CHT temps as I adjust mixture settings.

Where would you mount the planted in the head thermocouples?

[bell]

i have a dual cht guage too......but ron has the cats meow.
mine is on cylinders 2 & 5, using a spark plug gasket sensor (replace the washer on the spark plug)
fyi.....you absolutly HAVE to notch a spark plug socket or you will tear/twist the wires all to hell when you put the plug back in.
it's a great little tool

[Randy Webb]

or maybe its #3 -- that's the one PAG put their CHT in.

[Grady Clay]

Ron,

On well instrumented engines, I had two thermocouples on each head.

I take some Type-K thermocouple wire and thread it through a 1/8” x ˝” steel roll pin. After the wire is through, strip and form the thermocouple junction with a reducing (rich) flame on an oxy-acetylene torch. I then ball up some of the wires at the junction so the wires can’t pull back out of the pin but not larger than the OD of the pin. I drill a 1/8” x Ľ” hole where I want the thermocouples. I made a driver tool from some steel rod to fit the pin and a side slot for the wires.
This technique lets you grab the pin with Vice Grips for removal.

Here is the source for the wire, connectors and much more. Omega wrote the book on the subject.
http://www.omega.com/ppt/pptsc.asp?ref=EXGG_KX_WIRE&Nav=temh07


The placement is somewhat dependent on the type head. Many heads have a flat just outboard of the exhaust port where a drilling would be for air injection. That is a great place.
Most heads have a rib/reinforcement on the piston side of the fins at the intake runner near where the casting date is on some. That is a good location; just make sure there isn’t any interference to the cylinder. When you assemble the heads on the engine, make sure you don’t get a wire between the head and cylinder.

I run the exhaust wires up alongside the cylinders and bundle with the intake wires and finally out the fan shroud near the breather plate. It is best to notch the fan shroud so it is independent of all the wires and connectors. I like the very small Type-K connectors and bundle them on the cross member.

The cost of data acquisition systems has become very reasonable.
Data logging can be as complete or inexpensive as you want – your budget will determine that.

Some places to look:
http://www.veracitydata.com/
http://www.corsa-inst.com/
http://www.piresearch.com/
http://www.advantagemotorsports.com/
http://www.competitiondata.com/
http://www.truechoice.com/menu.asp
http://www.maxqdata.com/

For a skilled tech, this is basic easy stuff. An off-the-shelf system is as inexpensive as you want and is very functional. For complex and custom systems, there are a large number of (probably unemployed or under employed) experts. Many will advise for little or no compensation just to keep up to date with the technology.


For a simple solution you can use a spring clip on a fin near the exhaust.


For a solution to the engine temp problem, look here:
Fan drive ratio:
Engine Fan Drive Ratio


Best,
Grady

[Randy Webb]

Grady - since you have all 6 heads instrumented... do you find one is consistently hotter than the others? If so, which one?

[}{arlequin]

I'm still meaning to do the dual cht setup so thanks for bringing this up.

[Grady Clay]

Randy,

I wish I still had all my original data, lost in ’87. I am relying on my memory from 15-35 years ago.


In general, at WOT and high RPM, the head temperatures (both top & bottom) were remarkably equal among cylinders. There was usually a slight difference left-to-right. I think Porsche slightly over-did the ducting on the back of the alternator. On some race engines I reduced the fins by about 5 mm and better equalized the temperatures L-R.

At idle some engines are all over the map. This is only partially air flow. Usually it is the slight differences in idle mixture. Most carbureted engines are terrible until you adjust seeing exhaust gas temperature and mixture for each cylinder. CIS is best, MFI is pretty good when properly adjusted but can very dramatically L-R when not correct.

With any differences L-R, cam timing should be the thing to confirm equal. With special engines, differences in compression ratio can make a big difference in temperature.


For most on this Forum I think the 1.82:1 fan system is an engine saver and the horsepower spent is significantly exceeded by the power gained. Same with the “Rubbermaid Solution.”

I would really like to use the current instrumentation that is available. The price and size makes it usable both on an engine & chassis dyno and on the road & track.

There is a lot yet to be learned.

Best,
Grady
gradyclay@hotmail.com

[Randy Webb]

Maybe Ron will will get into this and do the tests for us all.....


The ability to cheaply read thermocouples is from the miniaturization of the "cold junction" in the mid to late 1970's - linear op amps also as the signal needs a lot of amplification. A tcpl works like a batter in reverse - the electricity generated between two dissimilar metals varies with temperature (tho it is a very low voltage - some field dataloggers read in nV).

The upshot -- for anyone planning to use them -- is that you have to run a pair of leads of tcpl wire all the way - you can't splice in ordinary wire and still get accurate results.

OK, end reminiscence...

[RoninLB]

I had the exact same CHT model that bell uses today. The 2&5 cylinders were used for 2 reasons. [This is only my weekend wrench reasons so don't quote me in the pro circuit.] The 2&5 were centered for adjusting carbs. The fact that 1&3 or 4&6 may have been cooler or warmer wasn't the issue. 2&5 have been confirmed to be more stable in operating temps. The difference between 1&3 or 4&6 could be be 20deg F. The 2&5 are usually within 10degF of each side cylinder. These temps is for highway driving between 70-90 in 5th. At WOT and lower gearing variations get wider and a bit scattered. Such as 3 going from the hotest EGT to the coldest within a few seconds. Using 2&5 for a dual CHT would be my suggestion for a clearer pic of what's happening. As Grady mentioned some fuel systems are more consistant than others. CIS is very stable. My carbs Were scattered until I went neurotic on adjusting them. You really can't fine tune using the CHT. You can fine tune with EGT imo. So the CHT info is to track future oil temps and prevent the high oil temps from happening.

What was interesting about the gauge I used is how sensitive it is to other elec wiring. When it was mounted in the console the left bank was significantly cooler than the right. If I removed the gauge and held it away from the console the temps would equalize. OK what's causing this. I immediately thought that the gauge placement angle, foreward/back, was an issue. I then checked with a small level across the top. So now I start figuring that the gauge responds to small voltage variation and the near wiring in the console were throwing off an induction field. But why only the left gauge. I never figured it out. I tried moving wires nearby and straightning out the bundle somewhat to prevent the coil induction effect. Nothing cleared up the variation. I've mentioned it before but figured I'd waste your time mentioning it again. I'm not sure if my reasoning is correct because some have argued with me that DC current doesn't have induction. My answer to that is that we have pulsed DC in our cars. So my bottom line it that electro magnetic particles will affect the CHT gauge. My next method of answering the dilemma was a sheet of lead around the gauge..

The gauge I used was placed on the lower console. It is below a Volt gauge. The CHT ga doesn't have to be in your line of sight imo. You kinda know when you may be heating up your engine and will pay attention then. It's an immediate heads up to what will happen to your oil temps in 5-10 minutes. The idea is to control you oil temps. Cooling hot oil temps when driving is a long boring process imo.

ps: when warming up a cold engine the CHT gauge is wonderful. Not only as an oil temp cold assist but when to start a cold carb engine rolling. In my act a head temp of 175degF will prevent that cold bogging when driving away.. again all this is only imo.

[Randy Webb]

OK what's causing this.

- Probably not RF so lead shiled will not work. The cold jct. is in the gauge and is inadequately thermally compensated. That's the most likely explanation.

[RoninLB]

Originally posted by Randy Webb
OK what's causing this.

- Probably not RF so lead shiled will not work. The cold jct. is in the gauge and is inadequately thermally compensated. That's the most likely explanation.

but if the gauge is pulled away from the console it equals each other.

[Ron,K]

Randy,

What is tcpl wire. I'm probably going to have to extend the leads and was planning on using 18 ga. wire. If tcpl wire is necesary, where can I source it.

[RoninLB]

Originally posted by Randy Webb


- Probably not RF so lead shiled will not work.

so lead is only effective at high powered particles and does not affect low energy particles.. right

then maybe a magnet placed somewhere to neturalize my
"maybe" induction? Overpaid Slacker just got a trick 4ga combo for the clock and it's already all hooked up except the mounting.. so this issue is something I'll have to pay attention to.