CHT readings vs EGT readings as indicator of Engine efficency

[schosh]

OK here's the background.
I am lucky to work alongside a seat of the pants hands-on mechanical genius. His reusme includes FAA certified aircraft powerplant mechanic since 1970, a Degree in Aeronautical Engineering , and currently as an Engineer emeritus he manages the "skunk works" for the largest aircraft fleet out side of the military . He holds patents on a number of tools and devices and is always ready to talk and teach on a variety of subjects. He has a pretty extensive hands on understanding of aircooled engines from aviation to harleys to old vws.
So I asked him about the viablity of using cylinder head temperature as the primary indictor of engine health. His opnion was that CHT is adequate , but felt that exhaust tempature provides a better, faster feedback.

He recomended installing two EGT sensors into the exhaust flow , one for each bank of cylinders. He then mentioned that you don't really expect them to match but you work with the carb mixture settings ( assuming close to correct timing) to determine the maximum possible exhaust tempature for each tailpipe. You then lean the mixture back so that your full load exhaust readings are 50 degrees cooler then maximum. After this inital calibration /tuning he claims that any changes in Engine efficency would be imediately apparent in the exhaust tempatures and that potential heat problems would be noted significantly faster then with a CHT based gauge.

So my question for you eperienced engine builders and tuners out there is this : Do any of you have any experience with comparing CHT readings with EGT readings? Does the idea of an EGT gauge as a primary indicator of engine efficency make sense? If not why?

I look forward to hearing some informed opnions
Bill

[Dave at Pelican Parts]

From my readings, CHT is not used as an indicator of engine efficiency. It is used as an indicator of, "OH CRAP!!! My engine is melting down!"

EGT, from my talks with a couple of racers, is a good indicator. However, you have to dyno first to find what EGT your engine makes its best power at. You do all your tuning for max power, and make a note of what the EGTs are. Then you can use your EGT gauge to help you re-tune that particular engine when conditions change (e.g., air temps or humidity are different).

I am not expert or even that well informed on this subject, but the main guy I was talking to about this built and ran a 200+ HP 2.0 914 motor in his race car. (It broke every other time he ran it, but boy was it fast!!!)

--DD

[BURN-BROS]

My Dad did this in the 80s and really makes tuning carbs easy. The readings you get are real time and it works great.

[Jake Raby]

A CHT and EGT set of quad gauges can transform your car into a rolling dyno of sorts for tuning..

EGT is most definately a tool for measuring efficiency for us..

[matt_o_70]

Any sugestions for where to get parts to do this sort of thing?

Who make the best setup? and then what is the best DIY setup?

Also anyone out there done this sort of thing and then added FI.. I am thinking megasquirt etc.. as a possible winter project. So any one who has gone the DIY FI route have any suggestions on what worked well? or what did not.

[Jake Raby]

www.westach.com

[Alfred1]

I just got a new book and this is what it says

The exhaust gas temperature varies with the equivalence ratio. The exhaust gas temperature also varies continuously as the gas leaves the engine cylinder and flows through the exhaust port and the manifold and pipe, so an appropriate definition of an average exhaust gas temperature should be used in quantifying this variation. However, time-averaged thermocouple measurements from specific locations in the exhaust system can provide useful information on trends. Figure 14-10 shows examples of predictions of the enthalpy averaged exhaust gas temperature at the exhaust port exit as a function of equivalence ratio compared with time-averaged measurements. The enthalpy-averaged temperature is defined by Eq. (6.19). These are typically 50 to 100 K higher than time-averaged measurements. The exhaust temperature peaks at the stoichiometric point and decreases as the mixture is richened and leaned on either side.

The equivalence ratio is the actual F/A ratio divided by the stoichiometric F/A ratio or the inverse of lambda.

[Jake Raby]

Of course.........

[Alfred1]

Originally posted by Jake Raby
Of course.........

Be good, Jake, or you'll get me banned.

[Jake Raby]

Seems that you are pretty good at that yourself...

[Alfred1]

Do your dyno tests show that the lowest BSFC numbers are obtained when the mixture is slightly lean and when the exhaust gas temperature is near its maximum? I'm no expert but that's what the book seems to be saying at least for certain operating conditions. I know you said that you hate the stock cam because all of the heat that it produces indicates a lack of efficiency.

[schosh]

enthalpy? We don't need no stinkin' entthalpy!