This is a topic i've been trying to hunt down for darn near 20 years. This may be completely new information to people here, or it may be old hat, but i'm going to share what I know. In the past I posted on a few hot rod forums with no real interest, maybe i've finally found the right forum.
Somewhere possibly as early as the late 60's, and as late as the early 1980's (there were still advertisements in Hot Rod magazine that late because thats when I got into the magazine) had an interesting little anomaly... a camshaft kit, intended for high compression "musclecar" engines of the 1960's that in one fell swoop would convert them to get up to 30mpg despite their full frames and non overdrive transmissions, while still maintaining some measure of performance as well. It also let them run on pump gas despite coming with up to 12.5:1 compression ratios. The two brands that stand out that I think had it were Crower and Isky in specific (with Isky doing it first, and only Crower selling it later). Apparently nobody has made cams like that since the mid 80's.
They DID actually work, somewhat. With everything in google scanned now we might even be able to find it again... Hot Rod magazine at some point did experiment with one. A small block 350 chevy motor, TH350 transmission (no OD, no lockup), all in a hardly aerodynamic 1930's Ford or something. The engine was built with somewhere from 12.5 to 14 to 1 compression ratio yet I believe run off 87 octane gas just fine, and pulled down 25mpg despite the aerodynamics of a brick. For reference 15mpg would be what i'd expect normally. Sticking in my memory was "power below 2500rpm was notably lacking", implied worse than almost any other small block chevy cam tested, I think cruise rpm on the highway was around 2500rpm, and a switch pitch torque converter (the hot thing at that time) let it accelerate better when it needed to by setting it to high stall to get out of the hole. Oh and it was carbureted and with iron heads.
So what was the magic? Apparently waiting an extremely long time to close the intake valves - keeping them a BIT open for awhile beyond BDC, the strategy was actually to bleed off pressure in the cylinder. Compressing 43.75cid of 87 octane mixture at 12:1 would normally detonate to hell - the cam apparently left the valves open, the mixture traveled back up the intake to some degree, and so in effect a smaller air-fuel charge was in the cylinder to be compressed leading to less heat and such. The 'backcharge' actually helped keep the fuel/air mixture atomized somewhat more than not letting it in in the first place. The high compression gave it notably increased efficiency, the large cylinders vs the amount of charge in the cylinder let it expand more, there was more surface area to absorb the heat, and thus apparently yes it actually WORKED. Sort of like forcing part throttle openings at lower speeds, even if you were at full throttle (if you ever drove a moderately high compression engine on pump gas, they usually wouldn't ping at part throttle, only if you floored it) yet once you reached 2500rpm the pistons were moving fast enough and the valve opening small enough/restrictive enough that it no longer bled off much pressure - allowing it to create adequate power at higher rpm's with a fuller charge. It was not a hot rod cam, but it was an adequate performer apparently my memory wants to say maybe 200-250hp gross, definately no more and better than some anemic stock engines of the time with their low compression. Hot Rod guessed that with slower driving or a more aerodynamic form it actually would give 30mpg.
So why would such a technology be thrown away? By the mid 80's gas was cheaper again, plus I have a guess it had something to do with lower power density. Roller cams became the hot ticket in 1985, aggressive profiles not previously possible allowed higher torque at low rpm's, allowing lower cruise rpm's with the overdrives then coming out. Driveability was a little funny, the engine didnt' feel like a V8 under 2500rpm, it had the economy but to perform at all had to rev. It was probably more suited to modding down an engine still in the car whereas new vehicles would just be designed to run with a smaller engine to do the same job with it's benefits of less weight and less friction and similar.
The problem is that all the above is just guesses and analysis from talking to a handful of others that remembered the cams. I have little hard data other than that one article, some remembered ads, and a few discussions with people that at least had seen one. Was something valuable thrown away? Could the data still have some useful application today? I'd like to know personally. That it seems to be almost unknown has been bugging me for over two decades. Does anyone want to help me explore this a bit more like tracking down articles about it and sharing for discussion?