Incidentally I don't believe Uncommon Engineerings horsepower claims. So far as I know the only engine they ever got to run, scattered after about 10 seconds.
Every engine has its built in limitations. You may be able to get a 50% HP gain and retain enough drivability and reliability for daily road use. Or double horsepower for short bursts as in racing.
The highest horsepower flathead 6 Chrysler car engine was the 120HP 1954 Windsor.
A 50% hp gain would be 180 total, a doubling would give you 240.
I believe either of these would be possible given a sufficient investment in time, talent and money. But to go substantially farther would be impractical if not impossible.
