Author Topic: Aircooled cylinder head temps?  (Read 1444 times)

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Offline John Orchard

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Aircooled cylinder head temps?
« on: December 10, 2022, 11:28:24 AM »
Anyone know what temps a 125cc aircooled cylinder-head should run at? I just ordered a go-kart sparkplug sender & gauge, so I will soon find out :-) It would be good to have an idea before my first run with it :-)

Thanks, John.
Johnny O - Tahition_Red factory rider.

Offline pokey

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Re: Aircooled cylinder head temps?
« Reply #1 on: December 10, 2022, 03:14:35 PM »
Not sure that will give you the answers your after. An EGT probe is the go and should be sited as close to the Ex  port as possible as thats the hottest position of an engine and temps over 600c are quite common. The head and piston crown are a lot cooler thus not an accurate  gauge for real engine temp. best of luck with the run.

Offline John Orchard

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Re: Aircooled cylinder head temps?
« Reply #2 on: December 10, 2022, 07:21:30 PM »
Not sure that will give you the answers your after. An EGT probe is the go and should be sited as close to the Ex  port as possible as thats the hottest position of an engine and temps over 600c are quite common. The head and piston crown are a lot cooler thus not an accurate  gauge for real engine temp. best of luck with the run.


This is a racing kart sender & gauge, goes under the plug, in a twostroke, the plug is the thing that channels the heat away from combustion, a 'hot' plug transfers the heat to the head quicker, I'll see how it goes and report back, thanks :-)
Johnny O - Tahition_Red factory rider.

Offline PeterC

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Re: Aircooled cylinder head temps?
« Reply #3 on: December 11, 2022, 09:02:16 AM »
Keep us informed. Interesting area of tuning
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Offline skypig

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Re: Aircooled cylinder head temps?
« Reply #4 on: December 11, 2022, 11:19:34 AM »
I remember being horrified at the reading I got when I put an under plug sensor on the Honda CT110 engine in my Z50. Part of a Trail Tech “Vapour” install.

Offline James P

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Re: Aircooled cylinder head temps?
« Reply #5 on: January 20, 2023, 08:21:31 PM »
I monitor both CHT and EGT on my bikes. When I researched the topic of temperature monitoring about 15 years ago, the consensus seemed to be that EGT was useful for tuning (carb jetting etc.), while CHT was better for running.

I set up my thermocouples the same way on each bike; The CHT thermocouple is attached to a ring terminal under the spark plug seat, while the EGT thermocouple is screwed into a welded-on boss on the exhaust header pipe, within 100mm of the piston skirt. As Pokey mentioned, putting the EGT thermocouple closer to the piston skirt will give a higher temperature indication. However, it takes some experience to interpret readings.

For CHT, I have adopted an absolute limit of 250degC. If it looks as if the CHT will reach that figure, I change riding style or let the engine idle until the temperature reduces. Of course, some lower limit may apply if the thermocouple is located in a different position. However, the ring-under-plug-seat location seems to be fairly standard and seems to allow the most plausible case for comparison between different engines. I have found that my air-cooled engines generally operate in the 125-200degC range for varied types of riding, whereas the water-cooled ones generally operate in the 100-130degC range. The usefulness of CHT on a water-cooled bike is that the gauge should respond a lot quicker than a water temperature gauge (although obviously a lot of off-road bikes don't even have those!).
I have found EGT to be a much less definitive parameter than CHT and it seems difficult to apply an absolute limit. There are a lot of factors which will affect EGT (although not necessarily indicating a problem), so the best suggestion I can offer is to get some experience with it on a known 'good' engine and see how it varies with carb jetting, fuel and oil types/mixtures, riding conditions etc. Once you have a good general idea of how it varies and the figures which are obtained with normal running, it will be a lot easier to spot trouble from air leaks, dodgy fuel and the like. Although the thermocouple is mounted within 100mm of the piston skirt on each bike, I haven't been able to achieve exactly the same distance in each case, due to the proximity of frame tubes, radiators, slip joints and other obstructions.
All of my experience is with two-strokes - temperatures normally obtained from four-stroke engines (particularly EGT) may be different.

Most of my bikes have a Westach 2DC1-2 dual CHT-EGT gauge. These aren't perfect, but are fairly robust and the system is self-powered (i.e. no battery - the analogue gauge just responds to the voltages generated by the thermocouples). I would probably find a digital electronic gauge easier to read, but these are frequently more fragile and need batteries (or rectified supply from a magneto) to operate. Also, I haven't yet found one which offers the simplicity of the Westach system.

Admittedly, one problem with monitoring temperatures and other engine operating conditions is that you may have to take your eyes off the road/trail/track/course to look at the gauge(s). This is not usually a problem on roads or smooth trails, but could spell disaster in other circumstances. Mounting the gauge(s) in an appropriate position may lessen the risk. 

With some experience and care, monitoring CHT and/or EGT should give sufficient early warning of impending melt-down to allow large-scale engine damage (due to overheating, whatever its cause) to be avoided.

Regards,
James

Offline John Orchard

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Re: Aircooled cylinder head temps?
« Reply #6 on: January 20, 2023, 10:26:33 PM »
Hi James, thanks heaps your reply, gives me something to think about.
Johnny O - Tahition_Red factory rider.

Offline Hoony

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Re: Aircooled cylinder head temps?
« Reply #7 on: January 21, 2023, 02:18:48 PM »
hi James, as a sparky what type of thermocouples are you using (J, K or other) can you post some pics.
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Offline James P

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Re: Aircooled cylinder head temps?
« Reply #8 on: February 06, 2023, 10:37:21 PM »
hi James, as a sparky what type of thermocouples are you using (J, K or other) can you post some pics.

Hoony,

The thermocouples I use with the Westach system are 712-4WK (CHT, Type J https://www.westach.com/product-page/712-4wk) and 712-4D6K (EGT, Type K https://www.westach.com/product-page/712-4d6k). The instrument is a 2DC1-2 (metric scale https://www.westach.com/product-page/2dc1-2). Of course, the instrument is calibrated for the aforementioned types of thermocouple.

The first system I ever tried was from MGL Avionics - it was very good, but had a lot more features than I really needed. It required a fair bit of 'setting up' compared to the much-simpler Westach system (e.g. had to solder thermocouple wires into a multi-pin plug connector for the instrument) and it also required a DC supply. I thought that the instrument was a little more fragile than it should be for motorcycle use (so I mounted it on Mackay Multicushions), but I imagine it is fine for panel-mounting in aircraft (for which it was designed). It used Type K thermocouples for both CHT and EGT.

The Westach system is much more 'plug & play' (all plug connections ready-to-go, no soldering) and is self-powered. I made some bosses (OD 15, ID M8x1.0 thread, length 12) which are welded to the exhaust front pipe in an appropriate position to take the EGT thermocouple. The thermocouple itself has an adjustable sliding fit so as to locate the tip shallower or deeper into the front pipe, as the user sees fit.
The CHT thermocouple ring is 1mm thick, so it spaces the spark plug 'out' by the same amount. However, this should be nothing to worry about if the plug thread depth in the cylinder head is correct in the first place.

Regards,
James