Bore and stroke size, rod angle, piston speed etc. If you change any of these, they have a knock on effect (no pun intended ) of their ability to be resistant to det or not. You cannot compare different engines to their ability to run high compression - it absolutely has to be like for like - just because it is an in-line four of the same capacity, doesn't mean that it will respond in the same way to a compression hike. A good example of this is the Evo engine, which due to it's configuration, can take a much higher compression as standard (8.5:1 compared to the Cossies 8.0:1), as well as modified.