964 Head Temp Sensor -- Adding a Gauge

[North Coast Cab]

Jack,
If you're really unsure about the effects of the aero package get 6 ring thermocouples and run them through e selector switch.

John

[RoninLB]

Originally posted by Jack Olsen

I'm going to make it a permanent install

you'll love it.

[Jack Olsen]

Is 400 degrees the 'shut it down, let it cool off' temperature (like 250 for the oil), or should it be lower than that?

[Joeaksa]

We run the airplanes at 350-375 all day long with no problems. One issue is that the gauge does not normally react real fast, and aircraft engines are operated at a steady speed, where car engines vary their speed quite often.

Would love to see what Bruce Anderson or someone who has lots of dyno time chime in here and see what their limits are on the CHT.

The aircraft engine makers generally recommend 400 or below.

http://www.lycoming.textron.com/main.jsp?bodyPage=/support/publications/keyReprints/operation/leaningEngines.html

http://members.eaa.org/home/homebuilders/building/engines/Engine%20Cooling%20Tips.html

JoeA

[350HP930]

I dunno what the porsche experts recommend for their engines, but aircraft engines are rated for operating CHT of <400 F and red line is 500F.

[RoninLB]

it takes a few seconds for the gauge to react. The slow ga reading kinda balances the heat reading from a hotter spark plug during acceleration so you won't be seeing CHT spiking as with a digital reading. The plug is cooled thru the threads and the new CHT gasket. An acceleration from 3k-5k will need a few sec at 5k for an accurate reading. An analog CHT reading won't show the spiking, good. Detonation causes a higher CHT, not sure of the increase temps but a 25degF increase is a noticable increase from your baseline.

I would test the CHT reaction to different circumstances. Once you establish a baseline you'll be able to adjust your engine load. Simply stated I know what temp I need to pull away with a cold engine without bucking, when the CHT is ok for higher acceleration, when higher CHT begins to affect oil temp and to what amount, when to shift from 5th to 4th during mountian drives, when you're using the best cool down lap technique, etc.

afaik 350degF should be max for a street&track engine. Shut down isn't necessary when you're CHT is high, just reduce the engine load.

Cyl's 2&5 are a happy medium for a ballpark indicator. CHT's will vary among the cylinders. Some cyl's are more likely to be on the hotter side for a longer time than others. The hottest cylinder floats among all cyl's to some extent and 2&5 seem to be consistant "averages".. meaning the differential between the coldest and hottest cyl CHT may be 20-30degF and the differential between 2or5 and the hottest is usually 1/2 of that differential. 5th gear, high rpm is where the envelope lies.

I don't think dyno circumstances is relevant besides knowing when the engine blows up. Conditions of street vs track vs dyno are extremely different. Every dyno guy I asked about CHT's just gave me a shrug and said "we don't pay much attention to CHT."

CHT is great engine operation monitoring imo.

my prejudical rant was learned on a street engine w/CIS & carbs using extreme CHT monitoring.

[RoninLB]

CHT is great monitoring where EGT is great diagnosis imo.

[Jim Smolka]

Not having anymore information to start the project, this is a starting point to use the piggy back the a head temp gauge of the DME's CHT signal.

1. Measure the voltage that goes to the CHT from DME If 12 VDC Simple, if not see 2b.
2. Find out the input resistance of the desired gauge
2a. If it has high resistance (above 50,000 Ohms), simple
2b. If it has low resistance (below 50,000 Ohms), an Op Amp System is needed. (Simple 741 Op Amp will work)
3. Graph the resistance vs temperature characteristics of the CHT, check to see how linear it is
4. Perform bench test to see how VDO gauge works with CHT. Compare test data to calibrate gauge
5. Parallel VDO gauge with to the input on the DME.

Hope this helps.

(Received this information from Rocky, our Horse)