I’ve just been playing around with the General scenarios and for some reason the heavy cast pistons aren’t appearing in the piston choice list. i havent looked at all of the scenarios but i’ve quickly checked a few of them and they arent appearing in any of them. They do appear in sandbox mode though
I think the general scenarios are currently running on “old” code where there have been no heavy cast pistons available. The scenarios have to be rebalanced for the current sandbox code, but this is not top priority for the beta team. There are more important, Car Designer related things to do right now. 
To avoid posting another thread I’ll post in here since the bug I just found is kind of similar. In the I6 challenge turbo action the game seems to be taking into account the size of the intake, which I’m 99% sure shouldn’t be taken into account.
That’s intended, but for balance of the Car Designer, most of of the size-dependend scenarios are kind of broken right now (because of rebalancing the stroke-to-height dependency). They will be rebalanced and properly working in the next update.
When the engine is naturally aspirated the area taken by the intake isn’t taken into account for the engine size though
edit
looking at the size comparisons using other fuel systems (namely carburettors) and their various intakes, i think there is just something wrong with the calculation of engine size and intakes when a turbo is being used. somehow the intakes for the carburettors add significant length to the engine when visually they don’t add any length at all.
I’ll provide screenshots of what i mean if you want