Well I can tell you what I know for sure… First of all, any tuning will be within the limits of the engine designer as it would be silly to design a completely different system for that…
Timing you can adjust, Atleast on engines without VVT. Also you can adjust Cam profiles. There is no Air/Fuel ratio, nor is there programable ECU. However, There is a Power/Emissions(and Economy) Slider that basically covers that sort of thing.
We have yet to actually test the car designer and suspension bit, but from that Developers previous comments we can assume there will not be that amount of detail like Toe camber and all that. It will probably be something much simpler like a Stiffness setting.
But, We can buy cars and rebadge them. We can buy Engines and Chassis and stuff like that. But what Is not quite clear is how much we will be able to tune these parts…
I imagine there must be some kind of system, I have some thoughts myself on the matter. In the case of Engines for example, I think there could be 3 stages of tuning. Each stage has a certain amount of man hours allocated for dissasembly and reassembly. Then material costs and other costs could be calculated depending on how many Parts are changed.
Stage1: This is just fuel system and Exhaust tuning. You could change the fuel system parts, Power/emission slider, Resonance, Exhaust parts and stuff like that. Aside from the base Man Hours cost, you would have to pay to manufacture any new parts and man hours to make them.
Stage2(+stage1): This is top end Tuning. You could change the Cam Profiles, Valve Timing, Head Material, Compression, Valvetrain and stuff like that. This is a bit more tricky. For example you could adjust the Cam profile which Require new cams and the cost/hours associated with that. However, If you wish to change from SOHC to DOHC for example, you would have to pay to manufacture a new Cylinder Head. Compression could be modified, but I think there should be a Limit. There is only so much of a change you could make by Thicker/Thiner head gaskets and Shaving the head.
Stage3(+stage1 &2)**: This is Bottom end tuning. You could change almost anything except for the Block size/Material. But there would have to be a limit to the adjustment to Bore & Stroke. You could bore it out within the limits of the block, and put a bit longer stroke in, But You couldn’t really De-Bore an engine… Well, I guess if you put sleaves in maybe, but then you’d need to change the Cylinder head too, Doesn’t seem worth it IMO. Also you couldn’t de-stroke an engine without greatly lowering the compression ratio. And since theres no way for the designer to simulate that change, Thats best left out.(and Im not sure giving your workers a hacksaw and telling them you want Xcm off the top of that block is such a good idea )You could however change the Internals to better quality ones, and increase/decrease the compression more than the Stage 2.
Edits: I forgot about increasing the conrod length to make up for the loss in compression (Do’h) So I guess de-stroking would work.
Just my ideas anyways, Im not sure how it will be handled in the game, but I would love to hear the Devs opinions on this.