I remember seeing a film on TV as kid, either in the late '70s or the early '80s. I saw it on Dutch TV, but it most likely was a subtitled English language film.
A kid had his favouriteT-shirt end up in some kind of laboratory, where it is experimented on. I remember an image of a "scientist" brushing it lightly with some sort of flame.
The T-shirt had an image of a tiger on it.
The kid retrieves the T-shirt, which now gives him superpowers when worn. He uses that to fight some bullies, who may have had something to do with the T-shirt ending up in the laboratory.
He competes in a running competition, but the T-shirt starts to backfire, slowing him down and maybe even catching fire. He has to take it off. I'm not sure, but I think he wins the race without the T-shirt, on his own strength.
What is this film?