This discussion brings to mind one of my favorite jokes. What does 80 yo pussy taste like? Depends... There is no definitive answer as to which system works best for every engine... PERIOD. tti can claim 15hp or whatever. I've seen cars slow down using an X. Also seen them pick up. For tti, that particular set up liked the X, much like Myron's race car liked the X system. It is totally combination dependent. Most engines that are street driven, aka 400-450hp or less tend to like an X or some type of crossover.