There could be only one car I could put in here…
1978-1989 Gatz Valerian Mk4 600-6TDE (VSX40/D) (1986 second facelift shown)
This, folks, is insanity and ingenuity wrapped up in one oddball malaise-era premium muscle car. The Mk4 Gatz Valerian (which by now had become pretty much a rebadged and re-tuned KZNG Statesman coupe), although smaller and lighter than the Mk3 that it replaced, was still a bit too big and gas-guzzling for the 1980 CAFE standards. Obviously, since Gatz only had its big V8s and V12s and KZNG’s many straight-sixes on standby (and because Gatz refused to use anything smaller than 6 liters in their biggest model), they decided to go the wild route. This meant swapping the mighty KR-60-O/TDE 6-liter turbodiesel pushrod mechanically-injected straight-six from KZNG’s pickup trucks into a new trim of their halo GT/muscle car, with a specially-built 4-speed manual to handle the torque.
The combination, called the 600-6TDE, proved to be an instant success with those who still wanted to buy a Valerian, but didn’t want to have to pay out of their pockets for single-digit gas mileage or lose the performance of the V12s. It was plenty powerful for the time, with almost 500 pound-feet of torque at 2,600 RPM and almost 300 horsepower at 4,000 RPM. It could crack 135 miles per hour and reach 60 miles per hour in less than 8 seconds - seriously quick for a 3,700-pound muscle car. It wasn’t that bad-handling, either, with the Valerian’s all-around double-wishbone suspension and clutch-pack LSD (swapped to a geared LSD for the '82 first facelift, then a viscous LSD in '86) giving it decent cornering grip and driveability. It got even faster once the KR-Series received EFI for the 1986 second facelift, providing it with a mild boost in power, fuel efficiency, and torque.
But after the release of the ZD-Series quad-cam all-alloy twin-turbo V12s in 1985, the TDE no longer had much of a use in Gatz’s lineup - for the elite minority, at least. It was now too heavy for the corners, too slow for drag racing, and not as economical of a choice as it had been before '85. Neither did it have enough prestige for most Valerian buyers, as it was now viewed as only a poverty-spec eco model for those who couldn’t afford a “proper” model. Nor was it satisfactory for Gatz’s chairman, Jordan Marshmallow, who found a diesel model to be harmful to the Valerian’s reputation. A shame, really, for the 1986-1989 final edition proved to be a fine choice in the lineup with over 500 pound-feet of torque and over 300 horsepower. It cracked 144 miles per hour and hit 60 in 7 seconds, all while still getting 17 mpg from its multi-point-EFI-equipped engine.
Yes, Gatz’s V12s were so bad for fuel economy that the only option they had in the late-1970s to have a hope of achieving CAFE fuel standards was to stick a gigantic truck engine in their luxury coupe. Ironically enough, this car was actually more fuel-efficient than the 3,000-pound, 4.6-liter-V8-powered '78-80 Verno by about 1 mpg (and the Verno used a SOHC 3-valve engine with aluminum heads for light weight). Also, yes, I’m now the madman who stuffed a 6-liter straight-six turbodiesel monster in a muscle car.