Originally Posted by Sakib
Next thing I want to know is how I can evaluate, before purchasing, what "semi-tuned" headers are better than others, from a technical engineering standpoint.
Probably the most common example for 3.7L Mustangs are the BBK and Mac shorty headers. Each makes different compromises, and it's impossible to say for certain which set of compromises is "better" without doing a direct performance evaluation of each. However there are a few things that can be seen from the pics available online.
-Obviously unequal length
-Consistent bend radii
-Medium length collector
-Closer to equal length
-One pinched bend per side
While I don't proclaim to be an expert on tuning headers, I do have a few opinions on the physics behind them. First is that, especially on V6 and cross plane crank V8 engines, header tube length tuning does not work like most people think it does. As such, I'm not convinced that equal length is as critical as it is given credit for on a street car, but I would still give the nod to Mac on this item. Second is that smooth bends will out flow crimped bends, in which regard the BBK is definitely superior. Third is that a collector volume and geometry is important. Without having seen a well lit photo into the collector end of each, I don't know which is actually fabricated better, but BBK looks to have the likely advantage. Ultimately if it was my money on the line, I would make my gamble on the BBK over the Mac for shorty designs...But (not butt) dyno testing could just as easily prove me wrong too, as there is often more going on than meets the eye.