There was no "IndyCar" racing in 1909, however you define the word, so it's a bit pointless discussing engine rules for that year. What has become known over time as IndyCar racing started in 1916, when the rules stipulated a maximum swept volume of 300 cubic inches, roughly 5 litres. Some of the engines developed 100 bhp and more. Engine capacity dropped successively over the following decade or so, but power output rose to about 150 bhp, and over 200 bhp with the use of superchargers. In 1930, the displacement limit was raised to 6 litres, but other restrictions meant that the cars were down to about 200 bhp again. The most powerful cars prior to WW2 ran in 1937, the last year of the 6-litre formula, when most of these restrictions had been lifted, and the Sparks (or: Sparks-Thorne) 6-cylinder entry that year easily developed more than 300, perhaps 350 bhp. From 1938 onwards, international Grand Prix rules prevailed.
After WW2, with unblown 4.5-litre engines, power output steadily rose again to about 350 bhp, and a slight reduction to 4.2 litres in 1957 didn't change that much: by the sixties, 400 bhp were available for most cars, and shortly thereafter supercharged and/or turbocharged 2.8-litre engines raised the benchmark even further. It's been said that some of the 1970s engines put out four-figure numbers, but difficult to say how much of that was available on the track. Bit by bit, horsepower was then ruled in by restrictions to the boost pressure, i.e. pop-off valves.
Edited by Michael Ferner, 17 February 2014 - 17:56.