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2018 Q4 with Fiamenghi Ti exhaust, Race Mod, and Tecnico wheels.
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Not quoting the nested long message above...

I did not understand why Fabio Fiammenghi went went with the nearly symmetrical design. One consideration was keeping valve closed sound level within relatively strict European in town noise limits. I do know that the assembly is really light. I think a titanium downpipe would be a massive smog liability and at risk for catching fire and/or melting. I have been discussing a titanium mid pipe, with resonator and crushed section delete. I dunno what that will do to sound levels.

I have Tecnico 19x8.5 in a square setup. I am eyeing the John4C Teklas, as my purchased used and kind of unsightly now Tecnicos are not as strong.
I have Michelin PS A/S 4 245 35R19 non-RFT tires, which I was pleased to find are remarkably light.

I more-or-less melted my first set of Ceika light weight rotors. Too much enthusiasm I guess, I did push them harder than I ever pushed the OEM rotors. Calaveras road in Milpitas California is brutal on brakes. The speed limit is 25MPH and consider that a challenge--hairpin after hairpin with less than 100 yards between. The ring is a little lighter than OEM but the aluminum hat is a lot lighter. The car already can over heat the brakes, so I don't think lighter is much of an option. IIRC there is another mfg that offers slightly heavier 2 piece rotors that feature rings that are about the same weight as OEM.

I was told that the smart battery controller is in the BCM. It gets data about the battery via a serial bus and sensor on the battery. Maybe it is possible to put a battery "piggy back" on that serial bus to convince the controller to fully charge an LFP battery? Also need to avoid charging the battery when the battery is too cold.

I was told that the OEM front seats weigh 35 pounds each, plus or minus only a couple of pounds depending the version. Articulated seats have a heavy mechanism regardless of the material. Note how Alfa 4C seats are fixed to save weight, like a race car. Air bags add to the total. All of that can be removed, but then you really do not have a civil street car any more.
The rear seat bottom cushion does not weigh much. I do not know about the rear seat backs. The fold down arm rest certainly feels heavy. Note that the rear seat backs are re-enforced to keep objects that are in the trunk in the trunk in the event of an accident.

The seat in my backhoe weighs about 120 pounds (I recently had to remove it for a repair, so I am painfully aware of this), but it can rotate 180 degrees, has spring loaded shocks, and more weight is a feature in a backhoe (mine does not have the optional 1000 pound extra weight, but it does have the standard front counterweight).

Why would I want to spray highly flammable methanol on the inside of my hot brake rotors?

There are several fairly large fasteners on the engine that are suitable to swap for aluminum or titanium (e.g. big bolts on accessory belt idler, A/C mount, alternator mount), but finding a manufactured fastener that will fit can be a problem. Make them myself is not really an option as 7075 Al and grade 5 Ti both require butter smooth rolled threads to be reliable. I suspect 1-2 pounds worth. As you said, it is a game of small gains in many places.

All else being the same, the lighter weight machine will be better/more fun to "toss around". My local truck scale says my Q4 weighs 3600 pounds with 3/4 of a tank of gas.

Specific and/or absolute elastic modulus does not tell the whole story behind the use of light weight materials. A thin flat sheet of material changes stiffness by the 4th power of the thickness and strength by the 3rd power, but weight only by the 1st power. Thus compare

X thick 120ksi steel: weight A, stiffness B, strength C.
2*X thick 70ksi aluminum: weight 0.7*A, stiffness 5*B, strength 4.6*C

Obviously if the steel was adequate the twice as thick aluminum is overkill. Also there are much stronger steels (mostly used in tools and knives), but they quickly keep stupid difficult to work with.
The above is really useful relative to replacing those soft steel brackets. Use modestly thicker 6061 (45ksi) aluminum (much easier to bend than 7075 70ksi aluminum) and retain all of the strength while ditching most of the weight.

There are other complications in the general case such as temperature sensitivity (aluminum likes it cold, steel likes it hot), buckling (mostly an issue for very thin super strong materials), formability (the higher strength versions of whatever base metal is usually brittle) and on and on.

Of course a hub is not such a simple shape and will not have such a large advantage for lower density materials. Also, if there is no room for extra thickness the whole project may be a bust.
 
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