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In order to find out what type of oil contributes the most to engine combustion I lit three engine oils on fire and the group 2 oil (highly refined petroleum oil) burned the most. It was SynPlus by Amalie. The next best was Bel-Ray Mineral Oil which was mostly group 2 but had less solvent (highly combustible) in it (which is known by the oil density) than SynPlus. The worst was Motul 800 Road Racer with ester synthetic. SynPlus has a flash point temp of 190C and Motul has 274C and Bel-Ray had 73C so if that was very important then Bel-Ray would of burned the most but it didn't. So I think this test shows group 2 oils to be the most combustible. Here's the oils with folded paper napkins as wicks:  From the time of first lighting the oils till they burned out the three oils maintained the same relative levels of flame.  When I road my 100cc street bike with a 50/50 mix of Motul and Bel-Ray Mineral Oil at 25:1 fuel/oil ratio I had better acceleration than when I used only Motul at 41:1 fuel/oil ratio. So my road test and the burning test are in agreement. The ester synthetic of Motul contributed less to combustion. Someone tested different oils on a 2 stroke airplane engine mounted in place [link] and the oil with ester synthetic (Bel-Ray H1-R) gave the least engine power which was evident by a lower RPM. He had to enrichen the jetting to get the RPM up to what the other oils allowed. Also the engine temperature was higher before the jetting was enriched. The tester also theorized that the ester didn't burn like other oils. The graphic below can help understand how a slower burning oil can change the air/fuel ratio to be leaner. Understand that a burnable oil is also part of the "fuel" that has to be kept close to a ratio of 1 to 14.7 with air. This graphic shows a 14.7 ratio with a fully burnable oil being used.  So if the oil is not burnable then the air/fuel ratio leans out to 15.2, and if the oil is half as burnable then that gives a 14.95 ratio which is also lean. I always run my 100cc street bike a tad on the lean side and it was fine with semi-synthetic oil mix but when I changed to just Motul at 41:1 fuel/oil ratio it experienced a 4 corner seizure. I took it apart and the underside of the piston had a little bit of black oil burned onto it which means it got too hot due to the lean mixture. Here's its pictures:   Here's the spark plug (light chocolate brown) and piston skirt with seizure marks:    New piston/rings were recently installed in the iron lined cylinder that was rebored to fit them. This seizure happened at around 300 km after the rebore. Afterwards with the premix the same, I cleaned up the piston and cylinder and enlarged the main jet and consequent riding was without incident. So I want you to know that if you switch from a mineral oil or semi-synthetic to a full ester synthetic then enlarge the main jet one size unless you have stock jetting and are at or near sea level. Stock jetting is typically a tad rich to lessen any possibility of seizing. |