Well going from a 2.5 pinch to a 4'open collector I seen a 12 horsepower increase on the dyno.. which was a significant find more than any other single change that we made including from small carburetor to Big carburetor.. but the smaller collector accelerates the car so much faster it actually provides a better time slip. Now keep in mind this is on a fairly heavy two speed small block.. mph is definitely down with the smaller collector