Sure it was asked but why wasn't this in gm design to begin with? Doesn't effect mpg or emissions......does it?
Not at all. It's just PCV (Positive Crankcase Ventilation), which is just a means for the air that builds up in the crankcase to be vented off so that it doesn't build up pressure and blow out seals and gaskets. The air contains combustion blow-by which includes fuel/hydrocarbons and engine oil, so it can't be simply vented into atmosphere. It's routed back into the intake so that it can be drawn into the combustion chambers where it's burned off. The problem is that there is excessive oil in this blow-by- too much to remain as a mist/vapor and be burned off. So it makes it's way through the entire system and puddles up everywhere. The catch can simply filters the oil from the PCV air. It gives it a place to "puddle up" (collect inside the can) instead of puddling up inside the intake manifold. The catch can does not impede the flow of or add anything to the PCV air, so it doesn't increase the engine's emissions. If anything, it would reduce some of the emissions (albeit very very slight) because of the engine oil that is no longer being burned off in the combustion chambers. The PCV system has virtually nothing to do with the engine's efficiency, so fuel economy is unaffected. Although, on a minuscule, essentially immeasurable level, the PCV system would increase economy because the unburnt hydrocarbons in the blow-by are passed by a second time where they are burned off. This would help the engine achieve and maintain the target A/F ratio without that extra amount of fuel having to come from the injectors. Again, it's an extremely small and negligible amount.
The catch can is really more of a "band-aid" to a minor design flaw by GM. But, pretty much any engine, regardless of manufacturer, has oil mist in the PCV stream and could probably benefit from a catch can. GM's flaw is that the LS engine has more than what can easily be burned off. The other maker's engines pass too small of an amount of oil to be collected- not enough for the maker to include an OEM catch can which would be yet another maintenance item for the consumer.
