For my son's recent school science assignment & for my own personal interest, we ran some tests using an old OS35 engine (cross flow) & varied the castor oil % in the fuel - the ratio ranged from 40% right down to 10%. The purpose of the tests was to see what would happen to the general performance, power output & operating temperature of the engine. The engine was fitted with a control line venturi & a MA 10 x 4 prop with spinner. The engine was placed on the test stand, standard muffler fitted & run in our back yard.

We monitored the head temperature by attaching a thermocouple probe to the rear left hand head bolt (when viewed from the rear of the engine). The probe was then connected to the multimeter.

As we were concerned about seizing the engine on the lower oil %'s, we did the 40% test first then worked our way down in 5% increments until we arrived at the 10% mix. The dear old engine was then ran at FULL POWER for a total of 3 mins on each fuel/oil mix & recorded the head temperature every 20 seconds. Ambient air temp was around 16C or 61F.

Briefly, the operating temps ranged from about 185C or 365F for the 40% oil mix & peaked at about 265C or 510F for the 10% test. At this lower %, the engine was clearly HOT HOT HOT - even after only 3 mins - and your nose could easily detect that "hot metal" type of smell.

As other readers have noted, the tests can be a bit meaningless comparing one make of engine to another. However it is useful for ongoing monitoring of a particular engine & also if you like to experiment with different fuel mixes - eg: varying the % of oil & altering the mix of synthetic/castor oil within a fuel mix.

If there is enough interest, I will publish in full my findings/results on RCU for information & comment.