These are a couple of old videos that I did before I started restoring the car but it gives a good comparison between Flowmaster 40s and the Dynomax VTs. The car is a 65 GTO with a 69 428 bored 30 over, E heads ported, Ultradyne 280/288, Edelbrock intake and 800 carb.
Flowmaster 40s
https://www.youtube.com/watch?v=xjqr3KNkLqo
Dynomax VTs
https://www.youtube.com/watch?v=mT1ee2kQyDw
I really loved the VTs because of how they would quiet down at highway speeds but the clanking at idle drove me crazy. I have since replaced them with Dynomax Ultra Flow SS and these are a very good choice, sorry no video but they are between the VTs and 40s on the highway.
This last video is the engine after I rebuilt it, I had to add it in for anyone interested. It is after we did the break in run and were adjusting the idle and seeing how it responded. Same engine bored 40 over, RaceTec flat top pistons (raised compression to a hair over 10:1), more work to the heads with some milling, nearly zero decked the block (not quit), Edelbrock RPM intake matched to the heads, 1976 Quadrajet (thanks B-Man) rebuilt to match the engine, Lunati VooDoo 60903 cam, Harland Sharp Roller Rockers, Butler Performance Pushrods, 62 SD421 connecting rods, and the lifters are Hy-lift Johnson/Made In The USA lifters from Butler Performance.
https://www.youtube.com/watch?v=UKi96nU0W0o
Dale