Performance gains? Don't kid your self folks, Ma Mopar made these things for that very reason. The tube length's are equally tuned at 21 inches. That accounts for them being oversize.The rest of the wild "H" pipe set-up that went along with the manifolds provided for a total tuned length of 42 inches. The pipes were paired together, hence the term "TRI-Y", to pair the exhaust event's of the odd and even cylinders to provide maximum cylinder scavenging of the exhaust gases. Basically a cast iron set of actual tuned headers. Yes it is more technical than I can remember off hand but cool none the less!