(first posted 11/17/2011. Several commenters referred to this yesterday, so let’s trot it out of the CC garage one more time)
It may seem like during the late sixties Detroit’s power train engineers were all just focused on ever higher maximum performance and had given up any interest in fuel economy. There was a little pocket of engineers at Oldsmobile who seemed to think that efficiency still might be of some possible utility, even then. Their efforts resulted in the interesting (and obscure) Turnpike Cruiser package the Olds engineers came up with in 1967. I’d almost forgotten about it, but CC reader MarcKyle64 sent me a link to a 1967 Car Life test that explains it in great detail.
I’ll give you the Cliff’s Notes version here, but before I do, let’s jump ahead and explain why I’d forgotten about it and you may have too. It obviously didn’t click (as in sell); folks just weren’t interested in what was actually a simple but effective engineering solution to tailoring existing technology for the conditions American cars were commonly used. Buyers would rather spend the couple hundred dollars it cost on vinyl roofs or the 442 package; gas was cheap; image was more important.
The essence of the Turnpike Cruiser was the acknowledgement that a detuned large engine can power a car more efficiently than a higher-revving smaller engine. In German, this was called the drosselmotor, or throttled engine, which in actually is a misnomer. It does involve a relatively modest carburator, but most of all, a camshaft with an extremely modest duration, yet fairly high lift. The goal is to create a very low and rich torque curve, which combined with a very low (numerical) axle ratio assured that the engine would run at low rpm, but at relatively greater throttle openings, lowering both engine friction losses as well as pumping losses.
Frankly, the classic VW air-cooled boxer motor operated on this principle, as well as other in Europe. The most extreme example was BMW’s eta motor, which made a mere 128 hp from the 2.8 l six, at a diesel-like 4800 rpm.
The Olds engineers started with a 442 400 cubic inch engine, specifically for the large valves, which help compensate for the short valve opening duration. The cam had a 250/264 (intake/exhaust duration degrees) grind, and a relatively large but two-barrel carb. But the real key was in reducing effective gearing. The result was 300 hp (290 in 1968) at a relatively low 4600 rpm. Torque was a very healthy 440 lb ft at 2400 rpm.
The really critical component was the new three-speed Turbo Hydramatic transmissions. Not that it had an overdrive ratio, but by combining it with a super low 2.41 to 1 rear axle ratio, the THM in effect became a two speed with an overdrive third. Top speed in first was 60 mph, second top at 100 mph. The big 400 ran at 1850 rpm at 60 mph, very close to its torque peak, which corresponds to an engine’s most efficient speed.
The result were steady state speed fuel economy readings of 21.04 mpg @ 50 mph; 19.26 @ 60 mph; 17.22 mpg @ 70 mph, and 15.26 @ 80 mph. Yet 0 to 60 came in a very respectable 8.2 seconds (all in first gear, presumably).
And there’s more: the Turnpike Cruiser only came with the excellent 4-4-2 suspension package, with heavy duty springs and shocks, and a rear anti-roll bar. Radials were optional. What’s Detroit coming to? Stuttgart?
It appears the TC package became available as an option in the spring of 1967, and shows up in the 1968 brochures, but no more thereafter. Of course, it was a preview of things to come in the mid seventies, when detuned engines, lazy axle ratios, and overdrive transmissions enforced a similar regime. The only problem was that they often didn’t perform as well as the TC. Weight and emissions controls were doing their part to assure that. We could call the Cutlass TC a prophet, which are usually never appreciated in their time.