This link will tell you about the tiering of emissions and their timeline for being implemented:
http://en.wikipedia.org/wiki/United_Sta ... _standardsThere have been emissions standards for diesels in on-road vehicles since at least 1970, when the EPA was formed.
Any on-road application manufactured in the U.S. must have an engine emissions certificate label somewhere on or near the engine at the time of production.
If you look closely at certain equipment, like the grid heaters and intercooler on the 91-1/2 trucks, you come to realize they are also "emissions compliant" equipment, due to the fact that they both are designed to enhance engine performance while lowering emissions.
In fact, a grid heater cycling to aid air temps while the engine is running is an emissions tool as much as it is a starting aid.
The intercoooler is aimed at reducing air temps when the engine is warmer, to enhance power and help emissions.
Keep in mind, as part of the emissions layout, the intercooler was added onto these engines, while the number of injector holes increased (for cleaner atomization) and the injecton pump's dispersal rate (ccs) was decreased.
All of this was in an effort to reduce emissions for a new set of (pre-tier) standards that went into effect on January 1st of 1991.
Ever wondered at the "half year" models?
Most of these came about in response to both government safety standards, and EPA emissions standards.
Mark.