I think I remember someone doing something similar regarding austin 7 based racing which I found on some site which was basically a write up of archive from the '60s. The motor concerned is a 10hp rated 747cc side valve (or in americanese, flathead) 4 cylinder.

The only problem was if the drilling was too off centre things parted company and large clouds of smoke and much havoc ensued.

There was also an unusual review of an old (1911 or so) racing car with a huge motor and an exhaust twist. There was a remotely operated valve half way between the manifold and the primary silencer that opened the exhaust to atmosphere when it was operated and apparently gave a useful hike in power.. something for the 4-strokers to ponder maybe?

jemma xx