Originally Posted by martyhalmi
what would the difference between long and short tube headers be? and what do high flow cats do?
Basically in a nutshell the longer the header the more power that it's going to make. When building headers you want the tubes to be equal in length as that way the exhaust can exit the tubes and into the collector where it scavenges the exhaust and actually sucks it out v.s the motor trying to push it out, and the longer the tube the better.
I am sure there were others before this, but they really came into their own when CARB came out in California for emissions and long tube headers became illegal, so to skirt the new emissions laws in California people started coming out with short tube headers which are much shorter then long tube. That way they were able to get some of the benefits of the headers but still remain legal for emissions, but they don't make near the power that long tube headers make.
Keeping emissions in mind, the faster and less restriction you can get to get the exhaust gasses out in general is going to make you more power and the government made catalytic converters mandatory on vehicles, which for the most part are very restrictive to flow. Some states like here in Arizona and Nevada and others when it comes to emissions do not check to see if you have cats on your car during the emissions test, they just check to make sure you check engine light is out, but many others do a visual check and sniff the exhaust electronically. So to combat that companies started producing high flow cats that were way less restrictive to exhaust flow which kept you making good power but also kept you legal when it came to emissions, and also are a good way to quiet your exhaust down a bit if you don't like a pretty much open exhaust system.
Hope that helps.