Alcohol adds oxygen to the fuel. In CA we have about 2 percent oxygen by mass (about 10 percent ethanol by volume) in our gasoline year round. The "rationale" is that it reduces HC/CO during warmup (prior to the system going into closed loop when it runs slightly rich), but the reduction is marginal and even CA has asked the EPA for a waiver, but it's turned into another federal farm subsidy program, so no dice.

During closed loop operation the system just adds more fuel to consume the onboard O2. Oxgenated fuel costs more to produce and has about 2-3 percent less energy, so it reduces fuel economy by about the same amount for no meaningful improvement in emissions, but the farm lobby is strong and has Congress in their hip pocket.

Most HC failures (assuming the test report data indicates that the fuel system is properly functioning in closed loop mode) are due to the cat bed being too cold, which reduces oxidation reactions, and it's more of a problem as catalysts age and need to be hotter to achieve close to 100 percent of potential oxidation reactions than when they were new. Retarding the ignition advance map increases EGT and helps keep the cat bed(s) hot.

Fortunately our CA emission reports show O2 content in percent, and it's an excellent diagnosic tool. You want to see that number be 0.0. If it's 0.1 or more it means the catalyst is not hot enough to consume all the available O2 in oxidation reactions, but even with O2 readouts at 0.0 KE systems can read over 50 PPM HC on the loaded tests.

Bottom line is that adding alcohol to your fuel is of little value, and 50 percent alcohol will probably push the Lambda system beyond the limits of its control authority. What you need is a REAL HOT catalyst bed. Manage your emission control test so this is achieved.

Duke