[There are a lot of people on this forum that are A LOT smarter than me. I still scratch my head from time to time thinking about my results, I probably just built a poor system. These are the results from my 605 Hemi on my dyno. My dyno exhaust system is two 5" pipes attached to two 5" diesel mufflers. I have adapters that slip fit over the header collectors so the system is sealed tight while making pulls. The headers on the motor may be a bit small for the application, they are 2-1/8x36x3-1/2, bolted to 18" 3-1/2" collectors.
Through the dyno exhaust it made 816.9hp@6059&752.6ftlbs@5383. I tried everything I could think of and this was the best it would do. Finally I I unhooked it from the dyno exhaust turned it and pointed the open collectors out the door. I made 3 pulls like that and it netted 852.8hp@5782&818.4ftlbs@4818. Finally I hooked the street exhaust up to it 3-1/2" pipes with X pipe hooked to 3-1/2 Ultra Flows. It killed the hp, 735.7hp@5405&748.5ftlbs@4274. I unhooked the exhaust and it went right back to the previous numbers.