I measure the surface temperature at the 6 exhaust headers of my XK120 with a laser thermometer and get strange results. The average temperature of the three front headers is around 240 degrees C (with some variation) while the back three temperature is only around 160C. This seems bizarre. Has anyone else observed these anomalies? And if so, what causes them?
My engine is in very good condition - good compression with good ignition - and I carefully tuned carbs using lifting pin tests and tachometer.
Suggest you check your spark plugs for correct color, Front carb could be weak causing extra heat in cylinder, rear carb could be rich which cools the cylinder due to extra fuel.
Check your main jets in both carbs for equal height.
Also consider perhaps your “laser” thermometer uses infrared sensors which measure blackbody radiation. Blackbody radiation is highly temperature dependent and can be a great way to get temperatures, particularly of hot materials, but also is highly dependent on the surface material where the measurement is taken. Iron has emissivity varying according to its condition from 0.14 to 0.95, a huge variation. Coatings on the exhaust surfaces and different materials can make a substantial difference in the temperature readout from such a thermometer. IR thermometers are very good but they require the user to understand how to recalculate the temperature based on the material surface measured.
i posted in response to someones else a while back, but cant find it,afaic, the manis were around 240
…160 seems too low. The laser should be accurate enough to be in the ballpark if used consistently
i am chasing a weird fluttering noise in my motor
what I DID notice is front carb runs cooler than rear, for no apparent reason, and I was really surprised how cool the carb bells are ie ~40C after 30min drive
1/ You are only measuring the exterior of something which was hotter when driving but is now cooling down and there is no reason to think that heat dissipation is identical per cylinder/header.
2/ The relevant hotter cylinders may have a retarded ignition relative to the cooler ones
3/ The relevant hotter cylinders may have a leaner fuelling relative to the cooler ones
The result may be real. There is no reason why it shouldn’t be as the combustion chambers may not be the same size, the compression ratio may be different or different cylinders may simply have different breathing. Layer on top of that different spark and fuelling at it’d be a surprise if they were all identical to each other.
I don’t think 160’c at idle is particularly low, rather 240’c at idle is probably high. I use exhaust gas temperature “EGT” sensors on the inside of my cylinder block mounted right next to the exhaust valves and I datalog the temperatures when out driving in the car.
Other than the fact that it’ll show up a large difference if there is one, a laser thermometer at idle is a pretty blunt tool by comparison to measuring the EGTs in real time.
You can quite easily have a 70’c difference between cylinders on a running engine which presents no symptoms of any fault. I’m not sure I’d have learnt much from such an IR thermometer result.
If you want to look at some datalogs to see how these things move around, then feel free to get in touch.
You’d probably want to look at the heads on the bench to make sure the probes sit out of the way of the valve stems, but in principle that sounds very promising.
The first picture is of the v12 HE head as viewed from underneath. The pocketed exhaust port is on the left and the probe is visible ~1cm from the valve stem. All twelve are identical
The probes are JB welded into bolts of the appropriate thread and screwed into the heads. They are mounted recessed into the heads only because I have other goodies sitting just above them and space is at a premium. In your case, you could mount them on the inlet manifolds if those are already suitably threaded. Milling, drilling and tapping the heads makes this a heads off job in my case. The 6 cylinder heads and manifolds will be different but the same principles apply.
EGT probes produce only a very weak (a few mV’s) signal and need circuitry to amplify it to something readable. I made my own electronics which interface with my fuel injection via CAN.
Phillip I’ve been using infrared thermometers to help in tuning the for the last 20 years. I’ve found on various E Types that the temps of cylinders 2 - 5 runs in the 500 - 600 degree F range when optimally tuned, and the front and rear run a bit cooler - in the high 400 degree range. When I was running Webers that allowed individual barrel tuning I could get the temps very close and the car would idle better. In my experience differences here are the result of mixture strength. Cooler means richer, or alternatively you have something wrong the the combustion in the last three cylinders. I can’t comment on the accuracy of infrared thermometers but you are using them for comparison only so it repeatability that counts. If your temps are more or less accurate it would appear to me that all cylinders are too rich, with three being richer that the other three.
I’ve seen the same uneven temps on several Jags and it’s normal. The centre is also where the heaviest crazing of enamel often happens.
I believe it’s similar to why it’s cheaper to heat a town house in the middle of a row than the ones at either end. The central house and centre manifold runner both receive heat (or lose heat more slowly) from the heated neighbors either side. Where the same heat energy flows into three siamesed exhaust branches, the one that loses heat the slowest runs hotter. #2 & #5 are those branches.
I also measured the long headers on a triple-Weber D replica. The differences were much smaller as each header is separated from the ones alongside except at the flange.
You may have equalised the mixture settings using the lift pin, but have you equalised the throttle openings? Sound to me as though one carb is open more than the other.
