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I was thinking about the possibilities of offering some more valvetrain options last night while I was playing. Allowing the option to Increase valvespring pressures will decrease high rpm valvefloat and allow for higher revving but decrease reliability. The option of mechanical lifters could also allow for higher revving capability, but increase maintenance as they require constant adjustment compared to hydraulic lifters. Also, and option for flat tappet lifters/roller lifters could be interesting, flat tappet will be cheaper to produce but have higher friction and wear vs. roller lifters.

Valvespring pressure increases with the cam-slider setting. Don't forget that the engine designer is considered feature complete (game mechanics), but not content complete (engine types, aspiration options).

An early (2 years or more ago) version did have a choice of lifters but it just didn't end up being an interesting gameplay mechanic. As Rob says, it's automatically assumed that bigger cam = stiffer springs