Re: 100:1 Mix
The answer to the question as to why some two stroke engines seem to thrive on Opti-2 at 72:1 or 100:1 and why other engines overheat and seize on it as mine did seems to lie in two things: displacement to horsepower ratio and how the engineers took that into account when designing the engine's heat dissipating characteristics. China girl engines have two displacements and horsepower ratings and if I remember correctly the 48 cc engine has a 1.75 rating and the 66 cc has a 2.75 rating. So they would have a 27.4 and 24 ratio, respectively. My CAG engine that seized up was 47 cc with a 4.2 horsepower rating, a ratio of about 11.2 to one. A 47 cc engine that puts out 4.2 horsepower will produce more heat than the same size engine that puts out 1.75 hp. One of the factors that engineers look at is how much heat must be dissipated to keep the engine from overheating, and they take two factors into account with an air cooled engine: the surface area of the cooling fins and the cooling effect provided by the oil in the fuel as it passes through the engine. A higher horsepower engine, such as the CAG I have (which can produce up to 10 hp out of 47 cc), has a lot more heat to dissipate than the weaker china girl engines, and thus is more sensitive to the fuel to oil ratio and fin surface area. In addition, if the engineers kept the fin surface area down to make the engine more compact, they would be relying more on the oil to carry that excess heat at a specified fuel to oil ratio of anywhere from 20:1 to 40:1. When you raise the fuel to oil ratio above that you are in danger of making the engine overheat and seize the piston, as happened to me. Bottom line: follow the engineers' recommendations regarding fuel to oil ratio and you cannot go wrong. If their design relies too much on the oil in the fuel to dissipate heat and you reduce the amount of oil in the fuel, you may just experience a locked up engine. You could only find out the design specifications about a particular two stroke engine by contacting the engineers that designed the engine and getting a reply from them, or by trial and error as I did. Can we have the input from any engineers on this?
1997 Associate in Applied Science - Automotive Technology, Cum Laude, Southern Illinois University
1997-2003 ASE Master Technician