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Thread: Exhaust Gas Temps

  1. #11

    Exhaust Gas Temps

    Thanks to those that have posted info links. Very helpful

    Sean

  2. #12
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    Exhaust Gas Temps

    Hi all....apologies for not responding earlier but a) the weather's been bad and daylight hours short, b} I don't live close to where my aircraft is based and c) each time I DO fly she has to be rigged and de-rigged soooo....

    I got down to fly her last week but had no time to check the jet sizes etc. I can confirm, however, that the EGT figures were 450 on one cylinder and just about 430 on the other. I admit it was a pretty cold day but these figures seem abnormally low.....

    At the very least, next time I'm at the strip, I'll check jet sizes and if they're correct, lower the needle a notch and report back.

    Thanks everyone,

    Phil.

  3. #13
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    Exhaust Gas Temps

    Phil,
    Your EGT gauging system - is it the standard (Westach I think?? with 1. something mtr of K type dissimilar metal sensor wires)?
    Just occurred to me - on my Chaser when I bought it someone had chopped the sensor cables (to about 15") and would therefore have got very low EGT's.

    I.e. are your sensor cables the correct length?

  4. #14
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    Exhaust Gas Temps


  5. #15

    Exhaust Gas Temps

    Or this:

    http://www.rst-engr.com/rst/articles/egtcht.pdf

    Cheers,
    Paul.

  6. #16

    Exhaust Gas Temps

    Thermocouple sensor lead lengths don't matter.
    Dave

  7. #17

    Exhaust Gas Temps

    Paul PM'd me with this:
    "Dave,
    Appreciate your comment re length of egt sensor cables.
    However from Westach installation instructions "patch leads may be extended up to 22 feet but sensor cables should not be shortened"!
    Paul"

    But my comment above was public, so should the answer - but it is my theory, not proof...

    I suspect that Westach's comment about not shortening the sensor leads is to be sure they are far enough away from the hot junction to be truly at ambient temperature.
    In the complete thermocouple circuit, EVERY junction of different metals is another thermocouple junction, generating an associated voltage.
    This includes thermocouple leads to (probably copper) patch leads to instrument terminal posts and internal instrument wiring.
    So long as all of these junctions are at similar temperatures, the parasitic thermocouple voltages will all cancel out, just leaving the much hotter actual thermocouple head voltage to be read.

    So, in summary, I think Westach wants you to keep the sensor leads as long as possible to keep the external connections well away from the heat source.

    Dave

  8. #18
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    Exhaust Gas Temps

    Progress report:

    I had limited time with the 'plane at the last visit but it was long enough to confirm that the main jets were a standard 165 size and the needle clips were in the standard 3rd groove. I had time to put the clip of one of them into the 2nd groove and start the engine, with an immediate improvement (in my case a rise) of EGT reading on that cylinder even at a lowly 3000rpm. Obviously I shan't fly the aircraft until I've adjusted the other needle clip also. A bonus of the brief twiddling was that for some unknown reason (perhaps the needle fitting more snugly in the needle jet?) the idle was as smooth as silk at 2000rpm instead of the usual shaking that encourages one to throttle up a bit!

    Also, I've realised that I was talking twaddle (not unusual!) in a previous post about a cold day producing low EGTs....in fact, of course, with colder, denser air producing a leaner mixture, the EGTs should be higher (duh!). Encouraging to think, though, that the EGTs will be lower in the heat of Summer - if we get one this year.

    Can't wait to reset the clip on the other carb then fly her!! I'm hoping for EGT readings of 550 on each cylinder but I won't be moving the needle clips further if I don't get them.....enough is probably enough and it'll be interesting to check plug colour afterwards....I shall report back!

    I also checked the length of the sensor cables and they seem to be about a metre in length - certainly not shorter.

    Phil.

  9. #19
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    Exhaust Gas Temps

    Hi again all,

    Well, today I dropped the needle on the second carb by one notch (as per the first carb) and test flew her....disappointingly, the EGTs are both still well under 500 degrees.....about 480 in fact. Sadly I was distracted by an unassociated electrical problem and I forgot to check the plug colour before putting her away.

    Does anyone think it's worth reducing the main jet from 165 to 162?

    I'm beginning to think that the gauges are under-reading!

    Phil.

  10. #20
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    Exhaust Gas Temps

    Dont forget Phil...the type of thermocouple we use is only accurate at 24c oat...below that the ind temp reads higher......above lower...prob has more of a bearing on variations on the gauge reading than the cold dense air.

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