I run a pair of the collector mufflers with 2-1/2" inlets, 5" outlets and they take a little edge off. I run 4.60's and will run open collectors later this year to see any improvement. I like running mufflers and had borlas before these. Prefered the borlas, less sound, and roudn shape added clearance, but the flowmasters are unique sounding and I ended up shortening them for more clearance. I was looking for the least restricitive muffler to meet a rule requirement adn feel the flowmaster collector met that. Freind of mine has the meth muffler they make and they do make a difference for sure on his 14:1 compression 500ci engine. Check them out as well, they still make them just dont promote them as much.