In order to enforce its obligations under the
US Clean Air Act, the EPA had to take some tough stances. Unfortunately, some of these directly impact the legality of adopting alternative fuels in highway bound vehicles.
The concept here was to prevent people from removing the catalytic converters and other modifiers to their cars performance or to negatively impact the manufacturers pollution control systems. These intentions are reasonable but they impact our ability to legally modify our cars unless the cars are tested to meet standards (tampering enforcement memo). The net effect is that while we can modify our cars without limitation, we must test the resulting modifications to ensure that they meet the standards now and can be expected to meet the standards for the life of the car. While that sounds good, that battery of tests can cost thousands of dollars, effectively barring the conversion. There could be certified kits for car modification, but they must be certified for each model year and significant change (engine size or the like) requiring the creator of the kit to undergo extensive testing - read huge investment. The effective result is to prevent modifications like boring the man jet to allow alcohol use in a gas engine. The EPA enforcement group
For a reasonable intent, the EPA also has taken a tough stance on the management of fuel additives for on-road vehicles. The end result is that all fuels and fuel additives targeted for on-road use by gasoline or diesel burning engines must be registered and tested to ensure they do not pose a negative threat to the environment. It is not clear whether they would, but it can certainly pose a risk for home brewers of Bio-Diesel(there are more than 25 registered Bio-Diesel variants as of this update) and those who use alcohol or hydrogen additives in cars.
Think twice about buying a manual explaning how to make all these changes without considering the EPA's opinion on the matter.