My turbo K4 specifically says to use nothing but 87. I thought, how odd. Nope, they tuned the Smartstream 1.6T to run very conservatively stock to use 87, any other fuel is wasted.

It doesn't have another map for a higher octane, unlike Mazda which has some sort of black magic to push timing until it micro-knocks and then pulls back timing. Kinda cool.

Shame on Hyundai / Kia. JB4 time soon!

Octane rating is just a measure of how fast petrol burns. Most gasoline is hexane, 6 carbons, octane is 8 carbons, burns slower. But modern fuel doesn't actually add octane, they use ethanol to slow the burn rate.

No idea why it costs more or is called "premium", it does not cost more to make, nor does it produce more energy.

I avoid cars that require expensive petrol.

Cars with electronicaly adjustable timing will detect if there is knock caused by pre-detonation/pre-ignition and advance or retard the timing to compensate. So cars that can do both octanes will have the timing more advanced when premium gas is present allowing for more power and efficiency. More timing with lower octane gas will explode earlier than expected causing pre-detonation/pre-ignition.

Engines that have premium requirements usually have higher compression, forced induction (turbo super, charger, ect) or even a more aggressive cam setup that advances timing where variable timing can't compensate or it doesn't have variable timing. So they need higher octane in order to ensure the engine isn't igniting at the wrong time, which over time damages pistons, rings and other components.

Also if your car is not designed for both, you are just throwing money away getting premium if it requires regular. Best case no difference or in the case of my wife's car would actually cause a check engine light.

To be fair, yes, the weight might not even have THAT much to do with it. The funny thing is, my car is over 1800 kg. It's an Audi A6 wagon. The SQ7 TDI which has tons more power and torque in a much heavier car, is more efficient. It just sprays the fuel quite differently and does a ton of other magic.

But the real thing isn't that weight makes them more efficient, it's more that they have so much torque that towing a heavy load doesn't affect them nearly as much as a petrol engine. For more efficiency these days we have VNT turbos so you don't need a ton of load to get boost (and thus a bunch of extra air). The cool thing is that you can add any amount of air, diesels run hot when rich, not when lean. So you can go for like 30:1 AFR for efficiency. Hell you could go 200:1 AFR in some circumstances (when fuelling is pulled due to letting off the pedal, but boost doesn't get reduced as quickly)

Newsflash: all combustion vehicles do that

Have you looked into a standalone system like Holley or similar? Seems like they make it easy now at least from the YouTube videos I've seen 😅

Is 1991 modern?

Higher octane without an engine that can take advantage of it will just lead to poor mileage. In my case, I can increase the boost on the turbo and make considerably more power out of the same parts, but you'd be more limited when it comes to a naturally aspirated engine. There is more to worry about and it's not that simple but E85 has a huge scene within the tuning community.