In 2012 when E85 (85 per cent ethanol, 15 per cent petrol) was available within easy reach of my regular commute at the time, I converted my road car to use it. Having learned what I needed to, when I finally finished building my race car I converted it as well.
I also found it genuinely surprised many enthusiasts how easy the conversion was. In fact, it was less of a conversion and more of a tuning exercise, and no more difficult than the performance-chasing methods they were using themselves with premium unleaded. Nor did it take any more effort, or cost me any more money than what they were doing.
So, what are the steps to converting a vehicle to run on (mostly, or even solely) biofuel?
If you have a flex-fuel vehicle such as some VE Commodores and a modest list of others, the process of converting to E85 is this:
That's it. Flex-fuel vehicles already have a sensor in the fuel line to measure the percentage of ethanol going to the engine, engine management maps to feed about the right amount of fuel, ignition timing, and any other parameter they may control (such as cam timing), and oxygen (O2) sensors in the exhaust so they can fine-tune as needed. That means they can also easily go back to 100 per cent petrol if it's ever necessary (which at this point in the 21st century it really shouldn't be, but here we are).
For 4-stroke petrol engines the process of converting to E85 is this:
Now, that first step can be achieved in a number of ways, but it's all that's strictly necessary. Anything else is for optimisation.
Racing enthusiasts have long been able to get E85 carburettors. Racing fuel cells are aluminium or plastic as well, so they don't need to worry about their old steel tank corroding if the fuel is allowed to attract excess moisture (racers also tend to run the tank low or drain it anyway for best fuel quality, while most road cars have had plastic fuel tanks since the 1980s or earlier).
For EFI there are a couple of options. One is replacing the injectors with a set that has a flow rate 25 per cent higher. Another is raising the fuel pressure to increase the flow (using an aftermarket fuel pressure regulator and an online calculator). The third method is to simply lie to the engine management system and tell it that the injectors are smaller than they really are.
For any of these, the O2 sensor is measuring a lambda number, the target for which does not change when you change the fuel. The air:fuel ratio (A:F) changes, sure, which is why we are delivering more, but the voltage the ECU wants to see from the O2 sensor stays the same (so I leave petrol A:F numbers in the tables, and lie about the injector size).
For all of these, you need to ensure that the fuel pump (and peak delivery of the injectors) is up to it, and that your rubber fuel lines are flex-fuel compatible. If they're not they're probably due for replacement anyway (they're actually meant to be replaced every 10 years, but very few people do).
Now for diesels. The process for converting an old pre-DPF (diesel particulate filter) model to biodiesel is this.
Again, that's it. Diesels can run on a wide variety of oils. Just do some reading and you'll be amazed how versatile they are (as was the intention when they were invented at the end of the 19th century).
The only reason DPF engines have any issues running biodiesel is because they deliberately burn some fuel in the exhaust every now and then to clear the particulates out of the filter (literally burning them off), and some common biodiesel oils don't burn hot enough. I think that's a wasteful method anyway, and eventually someone thought of injecting urea (AdBlue or similar) instead.