I had a 350 camaro (about 300hp) with 2 5/8" single exhaust with crossflow muffler. Lost over .3s seconds in the quarter when I "uncorked it." This was back to back runs (same track, same afternoon). Really depends on the design of your exhaust system and engine.
A 440 might repond better to open headers if he's running 2.25" duals without a crossover and/or mufflers that don't flow well. You also mentioned track prep was better. That also played a large factor in dropping the 1/4 mile time. Not only that, but if the track prep was poor before, the loss of torque running open headers probably helped it ease out of the hole better.
The two cars I was refering to when I raced my Brother were both SBC 350's and I had a short system with one muffler exiting in front of the back wheel for ease of removal at the track. Neither me or any of my mates have had a car go slower uncorking the headers. Once we are sure we have maxed the car out it always releases more power you don't have to have a restrictive exhaust!
As for the 440 track prep would account for 1/2 second the rest of the power would have come from uncorking the headers!
Let leave it there as your flogging a dead horse!
67 Firebird 350, TH350 auto Convertible, B&M 3000 TC, 4bbl Edelbrock Performer carb and intake Black Jack headers,disc brake conversion. 10 bolt chevy axle with 3.73's and B&M Rachet shifter.
