After all the discussion on oil and cylinder head temperatures on Kohler K-series engines, I decided to conduct an experiment (I see Matt did a similar test – nice job!). I found some spark plug thermocoupled gaskets I had stashed, along with my digital thermometer.
The grass was in need of mowing so I set up the test to monitor spark plug temperatures during the cut. Here are the specifics of the test:
Ambient temperature – 82 deg F
Fuel used – 91 octane, non-reformulated gasoline
Engine – Kohler K301 AQS with modifications
Compression ratio – 7.45:1 (stock – 6.5:1)
Spark plug – Autolite MP 4316 Platinum
The thermocouple was installed and engine started and warmed up for several minutes to check for a temperature rise. The digital thermometer was rubber banded to a Pilots Pal knee board and strapped to my leg:
I had planned to carry the camera along and photograph the display, but this proved too cumbersome (and it looked dorky as hell).
Prior to mowing, the governed no-load engine rpm was verified to be at 3600 with mower deck engaged (as shown on my instrument panel mounted tachometer). The grass was fairly thick and high and I made several laps while watching the temperatures. All total the engine ran for about 10 minutes before the thermocouple wire broke, ending the test. The highest temperature I saw was 509 F, considerably higher than what I thought it would be. This was during the maximum load points where rpm dropped to around 3500. I know the engine is not running too lean, but I pulled the spark plug to do a visual check. The insulator appears a charcoal grey, indicating high speed mixture is just about right.
This plug temperature is higher that I thought, by about 80 degrees F. My engine is running 1 full value higher compression ratio and this could contribute some. Do Quietline tractors with AQS engines run hotter than older, uncovered engines do? I don’t think this is the case but I will repeat the experiment on my 107 just to verify. I also found an oil temperature thermocouple but will have to make up a special oil plug to adapt it.
In closing, I found the picture of the oil comparison test we did at Tecumseh, running straight 30 wt mineral oil endurance without an oil change. Only make-up oil was added during the test. The engine ran full load, 3600 rpm for slightly over 100 hours whereupon the connecting rod broke due to crankpin bearing seizure. Upon teardown, the cause of failure was immediately apparent – the oil had turned into tar. The sister engine with Mobil 1 synthetic was still running happily.