actually, i'm not sure how we got here, but to extend the digression for just a moment, this gentleman's agreement actually hid a technological war that was brewing between the major japanese auto manufacturers. w/rt the engine, everyone was trying to make bets on what would be the most economical with fuel, cleanest exhausts, while providing the most useful power and providing the most durability.
Honda's supercar, the NSX, made the least power because Honda was betting that naturally aspirated variable valve timing was the best approach. they used a revolutionary cam changeover mechanism to make the car breathe at high RPM, while making the engine sip at low RPM. They were right. the technology that Honda named VTEC revolutionized motor design up to today, when every manufacturer now has a form of continuously variable valve timing. The conservative nature of the change in motor combined with natural aspiration kept the motor durable and the exhaust clean. Many NSX engines are still running to this day without a rebuild, and vtec motors kept them ahead of the game in air pollution regulations for a couple decades.
Honda's handling contribution was to design supercars as if they were formula karts. that carried through every super and hypercar to the current day, but didn't contribute much to consumer cars.
Mazda's supercar, the 3rd generation RX-7, bet that turbocharged wankel (rotary engine, derived from one offshoot of aircraft motor) would fix the wankel's fuel economy while maintaining the wankel's durability. Wankel engines utilize a rotational design to divide the combustion chamber dynamically and minimize both the number of rotating parts (1, the crankshaft) and surfaces under combustion pressure (just one wiper per chamber). naturally aspirated, this design had been proven extremely reliable, efficient per combustion chamber size (ie, small, light), but also hilariously inefficient with fuel, and with very very dirty exhaust gases -- they had a lot of trouble tuning the air intake to keep the engine stoichiometric. it turns out that turbocharging made all of this worse. the increase combustion pressure destroyed their chamber seals within tens of thousands of miles, their continued challenges with air intake timing made fuel efficiency even worse, and the chambers, because of the limited lifetime of their seals, now had to have incredible amounts of oil injected into them, making the exhaust gas slightly cleaner than a coal furnace.
the ony thing they were right about was the incredible efficiency per volume of their motor. they made more than 300hp _to the wheels_ with a 1.3l motor. there were two turbos, but only the larger one was significant for peak power.
also because of that, their cars were hundreds of pounds lighter than the competition. this never caught on in the wider market, as cars have been getting larger and heavier for the last 30 years because of feature creep. they had to be rescued by ford at the end of the 90s, so take what you want from that.
NIssan's supercar, the r34 Skyline gt-r spec V used a twin turbo setup on a medium to large displacement (2.6l) inline 6. twin turbo meant each turbo fed 3 cylinders. Nissan's bet was actually that microprocessor calculated fuel injection on turbocharged engines could increase efficiency without damaging exhaust gases too much and make the motor able to produce at-the-time v8 levels of power from mid displacement 6 cylinders. Their durability bet was that an the more balanced inline 6 dual overhead cam topology would last much longer than pushrod and carb fed v8 topologies. in few words, they were right. the rb26dett, as it was known in that car is still famous today for making hundreds to thousands of horsepower with incredibly high induction pressures and little to no increases in displacement. also, does your car have microprocessor controlled fuel injection? yes, it does. manufacturers recognized immediately that DSPs were the way of the future for fuel and air control in motors.
Nissan also made a bet on DSP based AWD drivetrains that didn't really pan out until the late 2010s with honda, so we'll ignore that for the time being.
Mitsubishi mostly followed Nissan, but with V layout 6 cylinders and rally homologation 4 cylinders. their primary contribution was complex center differentials for awd/4wd cars which i can talk about in another post. like subaru below, they used the world rally series as a testbed for AWD drivetrains. they mostly went under in the late 2000s because they didn't take the same leaps as others, and they never managed to make a reliable center differential.
no one bothered to rescue mitsubishi after the 90s. they made some money through an OEM agreement with GM and then pretty much died after after that ended. they are on life support now.
Subaru made front mounted flat motors like a reversed Porsche, but otherwise had unremarkable engine technology. Much like Mitsubishi, their primary contributions were handling-based, using the world rally series as a testbed for fully mechanical AWD drivetrains that were inexpensive and relatively fuel efficient.
toyota made a very reliable DOHC V8, but otherwise played it very very safe during this era. they did enter an awd turbo into the world rally series, but the interesting tech died on the vine and toyota decided to make a million camrys and corollas based on the exact same 2.4l 4 cyclinder for the next 30 years. they did take a bet on electric combustion hybrids and continuously variable transmissions, but that wasn't until the late 2000s.
what did make it? as far as the I6 twin turbo topology, well, almost all BMWs today are driven by turbocharged or twin turbocharged I4 or I6 designs that have completely replaced their V8 designs. And jumping back to variable valve timing for a second, BMW has been a leader in continuously variable valve timing (which they call VANOS) since the early 2000s. almost all motors are now turbocharged small displacement, variable valve timing, overhead cam motors with complex engine control units derived from these technological leaps in the 90s.
there are, however, no wankels on the market.
the handling wars are still very much in play, so i won't comment much further unless people want me to.