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If you break engine tuning down into: Intake/Fuel System/Ignition/Exhaust/Cams/Valves/Displacement/Forced Induction(if equipped)/Cooling System
All with a couple upgrade stages of their own, you open up a lot of freedom for people, without it being too hard to manage. You can get the same PP rating as someone else, yet have a different powerband that may be more suited to your personal preference. Speaking from experience, it's pretty easy to deal with in Forza.
PD is usually pretty decent at explaining how some parts work, so this is another opportunity for them to spend a little more time educating players on how cars work.
Just for engines, my categories would be:
Cylinder head
- Valvetrain (sizing, type)
- Porting + head work (flow and combustion efficiency, CR)
- Camshafts (lift and duration, timing)
Internals
- Crank types (lightweight, durable, stroker etc.)
- Rods and Pistons (lightweight / low friction, durable, bore, CR etc.)
- Ancillaries (reduce friction, and improve durability)
Induction
- Plenum volume / presence / type, pulse plates etc.
- Runner lengths + diameters (torque curve shaping)
- TB location / number, sizing (plus restrictors)
- Filter, silencing + feed location
Exhaust
- Manifold configuration (runner groupings, collector types etc.)
- Runner lengths and diameters (torque curve shaping)
- Tail exit location, cats, muffler
- Material (changes tone, affects durability e.g. turbo manifolds)
Forced Induction
- Superchargers (centrifugal, various positive displacement, pulley ratio)
- Turbos (turbine and compressor sizing, blow-off, by-passes, screamer pipes etc.)
- Configuration (twin-charge, sequential, parallel, compound etc.)
- Intercooling ("size" i.e. duty, type)
- Boost control (mapping)
Fuel and Ignition
- Fuel metering (carbs jetting, injector "sizes" etc.)
- Ignition (advance, coil upgrades etc.)
- Mapping (reliability, torque, economy)
- Firing order
Misc.
- Cooling and lubrication (mostly durability)
- Etc.
Then there are the other mods, like a completely revamped RM menu of options for a start, or more sensible suspension and gearbox options.
Then it would help to group these upgrades together to offer generic "kits" that do a certain thing (e.g. NA screamer, drag beast, endurance, rally turbo etc.), slightly tweaked to suit each car (e.g. turbine sizing). The real optimisations would come from diving in there and getting some dyno time in. Not even Forza has enough detail, not the sort that matters anyway.
Obviously each part would have its own weight and performance and durability benefits that either need to be learned or tested. This detail would really help the rise of the online tuners and form a proper community, whilst the creation of an abstracted level of control would still involve the casual player. (e.g. sliders to control the power band and effect those necessary changes on the parts installed, or install the required parts to suit - well, get close at least; the real optimisation must be tested - at a cost).
Ideally, this tuning would be performed on a per-engine basis, rather then per car. Obviously the same engine in a different car might have packaging constraints that limit the plumbing options, say, but I'd suspect there are substantially fewer unique engines (/ families) than there are cars. Then each engine could be given characteristics that more-or-less mimic their real world counterparts, (which would require some serious research; maybe each engine could just be slightly different and respond to upgrades differently, without overall being any "better" or "worse").
Further, they could bring back the tuners, only this time they specialise in different things, e.g. strictly NA stuff, or vintage stuff etc. and the kits they sell reflect this, although again the kits should be generic whilst the individual parts can be tweaked to suit should you want to.
So, yeah, pipe dream!