Thursday, April 9, 2015

CHT Installation

After my recent experience with the cracked compliance fitting and the resulting lean burning, I picked up a dual CHT or Cylinder Head Temperature gauge. The thermocouples are installed under both of the spark plugs in place of the normal gasket. Since I had to remove the plugs to install the thermocouples, I got a chance to check for evidence of rich or lean burning. Both of the plugs showed no signs of either and were a darkish tan. Thermocouples generate a voltage proportional to the temperature of the wire junction (dissimilar metals) which is attached to the ring now installed under the spark plug.

I ordered the gauge with a handlebar mount and the thermocouples from Aircraft Spruce, a mail order company specializing in small aircraft accessories and instrumentation. This gauge shows the temperature of the right cylinder on the right scale and the left cylinder on the left scale. The gauge requires no power as it simply measures the thermocouple voltage. It is a non-temperature compensated gauge which means that the scale is only "correct" when the ambient temperature is +75°F. For other temperatures, you simply need to add or subtract. E.g. at 0°F the gauge will read 75°F high and at 100°F it will read 25°F low.

Another use would be to determine if I really need to use 91 octane gas. Once I establish a baseline temperatures, try lower octane gas and see how much the temperature increases. Now it's time to get some baseline temperatures. I.e. go for a ride...

Thursday afternoon update - After quite a bit of running around this morning, the right head is consistently about 25°F cooler than the left side. Also, I usually upshift if I'm just cruising at a constant speed as the engine sounds "busy" just running at 4,000 rpm in second or third gear. But you can see the head temperature slowly start to creep up if, for example, you are going 30 mph and upshift to third (about 2900 rpm) from second (about 3900 rpm). In second gear, the head temperature is below 300°F. After upshifting, it'll slowly creep up to 400°F. So the lesson learned is to keep the revs up even though the engine sounds "better" at the lower rpm.

8 comments:

Lucky said...

Just gotta say, that's super cool. I'll be interested to hear what happens when you try using a lower octane fuel.

redlegsrides said...

A couple of Uralisti I ride with have this, they seem to be happy with them. Having a baseline would be nice. Sounds like your plugs are telling you things are good.

Canajun said...

Also looking forward to hearing about your results.

RichardM said...

So am I. I had been curious what the difference is. I can't hear any pinging when running lower octane fuel and given the modest compression ratio (8.6:1) I wasn't sure why the factory called for 91 octane fuel. Possibly for cooler combustion temperatures, slower burning possibly resulting in lower performance and lower gas mileage.

RichardM said...

I used the parts list that Darrell put out there. The initial run showed corrected temperatures (OAT 31°F) of ~330°F for the left and ~300°F for the right. About what was expected. I think the right may have better airflow across the head due to the sidecar.

RichardM said...

It's another tool to try and guess what's going on inside of the engine. I only have 5 months of warranty left so I can't really rely on that long term.

Chris said...

Cool post Richard. I kind of want one on my rig now.

RichardM said...

Way back when I was learning to fly, the plane had a cht installed. The instructor had explained how to use it to adjust the air-fuel mixture control. He emphasized how important it was for both economy and reliability to keep it within the 300°F to 380°F (temp corrected) range. Above 400°F was too lean and, if sustained, engine failure was probable. Not a good thing in a single engine airplane. I wish everything had a mixture control!