The SX was a Cutlass Supreme option package called the Y79 Performance Package and was offered only in 1970 and 1971. At the beginning of the 1970 model year, the SX was simply the newest version of 1967’s Turnpike Cruiser option and the “economy” 442 of 1968 and 1969. The Y79 package initially gave you:

- L33 455 cubic inch big block with a 2-bbl carburetor
- M40 automatic transmission with OG performance code calibration
- Dual exhausts
- 442 exhaust trumpets and the obligatory notched rear bumper
- Highway gears for economy
- SX emblems

The Y79 package was available on both the Cutlass Supreme Coupe and Convertible, but not on the Cutlass S, the Cutlass Supreme 4-door, or the station wagon.  Additionally, you could add options from the list of available equipment such as a console (the Supreme had buckets standard), sport steering wheel, Rocket Rally Pac, and the FE2 Rally Sports Suspension.

Here is a copy of the factory SX Conversion sheet.

What wasn’t available on an SX? Here are some of the key items:
- Sport Mirrors
- 3.91 Gears
- 3.42 Gears
- M20 or M21 4-speed tranny
- Heavy Duty Cooling (the SX could not be had with either a 3.42 or 3.90 rear, so no heavy duty cooling)
- W-25 Forced Air Induction
• W-26 Dual Gate Shifter
• W-30 Package
• Y73 Hood Stripe

  Bulletins flew out of Lansing during the year changing what was available on an SX. The major changes during 1970 were announced by the two bulletins below:
Sept 5, 1969 – Bulletin introduces the W-32 455 and W27. The W-32 engine was an option on the SX and was the same engine as in the automatic transmission 442. This gave the SX its “W-Machine” status. W27 was the Aluminum Rear Axle Carrier and Cover available with W32. Production records show that no W27 rears made it into Cutlass Supremes, so obviously no SXs had it.

- Feb 24, 1970 – Bulletin announces the L33 2-bbl 455 is dropped in favor of the L31 4-bbl 455. The L31 was the basic 88/98 4-barrel 455.

So during the course of 1970, there were three 455 engine options for the SX: the initial L33, the W-32 442 engine, and the L31 that replaced the L33.

Given the addition of the W-32 engine, one could now start with a Cutlass Supreme, check off Y79, W32, and FE2 and voila! You had all of the performance features of a 442!   This was also the time when insurance companies were putting the kibosh on performance.  With an SX, you could have 442 happiness in a Cutlass Supreme without the 442 unhappiness of higher insurance rates.

The 1971 SX was basically the same as the ’70, but with only one engine for the entire year; the L32 4-bbl 455, now with lower compression and its net horsepower rated at 250. This engine was only available in the SX and the Vista-Cruiser wagon, and was also the only way to get a 455 in a Cutlass Supreme Coupe in 1971.

All 1970 and 1971 Cutlass Supremes had the same basic VIN codes, so identifying a real Cutlass Supreme SX can be difficult.


Feature Article from Hemmings Muscle Machines

December, 2005 - Jeff Koch and George Mattar

As the 1960s closed, automakers were using two approaches to save performance-hungry customers' insurance money: One was to develop small-displacement cars with performance characteristics, like the powered Dodges and Plymouths, the Heavy Chevy, the Rallye 350 and Pontiac T-37. The second was to offer a big-inch V-8 in an intermediate bodystyle, but without the VIN that signaled "high performance" to your insurance agent.

In 1970, Oldsmobile blew one right past the insurance industry by stuffing its torque-monster V-8 into a Cutlass Supreme SX. By not giving it a unique VIN, new car buyers in 1970 and 1971 avoided paying increasingly high insurance premiums. And in stark contrast to the budget muscle cars, the SX went all out with plush, comfortable interiors, air conditioning and power assist for just about everything.

Built only in 1970 and 1971, the Cutlass Supreme SX offered muscle car power, without air-induction hoods and body stripes. Hinting at performance were the chrome exhaust trumpets through cutouts in the rear bumper (ala 4-4-2), but they were about the only sign of the iron being pumped under the flat hood.

About the only other outward signs of an SX are the emblems on the front fenders under the "Cutlass" script, and front fender braces on hardtops (the braces were deleted on convertibles).

When the Cutlass Supreme SX debuted, its buyer had a choice between the standard engine, L33 in late 1969 until February and March 1970, and the optional L32. Mid year, the L31 engine replaced the L33, which is the same engine available in the Delta 88 that year. The L33 455 had 320hp, a two-barrel carburetor and was dubbed the "turnpike cruiser." With 500-lbs.ft. of torque, and a 10.25:1 compression ratio, it was an axle-twister and would be one of the last of the real killer big-bore V-8s.

There was one optional engine offered during the entire 1970 model year on the Cutlass Supreme SX: the W-32, which gave a buyer the 455, also with 500-lbs.ft. of torque. Like the more powerful W-32, the L31 had 365hp, but it had more docile characteristics, such as smaller intake valves (2.00 inches in contrast to 2.072) and a turnpike-like standard gearing at 2.56:1. All engines used "E" cylinder heads.

In 1971, only engine code L32 could be ordered, the 455 V-8 with 320hp, 8.5:1 compression and 460-lbs.ft. of torque at 2,800 rpm.

All SXs used the Turbo Hydra-Matic 400 transmission, mandatory with the Y-79 Performance Package-manual transmissions were not available. The standard Turbo Hydra-Matic 400 carried an OD code. For the W-32, the Turbo 400 had an OG code designating a transmission with firmer shifts. If a console wasn't ordered, the SX had a column shifter. The standard SX rear gear was an ultra-tall 2.56:1, but ordering the W-32 also bought a 3.08:1 rear end.

Total production numbers were low, with 9,374 hardtops and convertibles built. The rarest Cutlass Supreme SX was a 1971 convertible; just 357 were made.

One of the biggest tip-offs to a fake SX is that those cloning a car place the "SX" emblem in the wrong spot. A correctly located "SX" emblem has its left edge directly under the second "s" in Cutlass, not centered, Peters said. Some SX owners report an "X" in yellow grease pencil on their car's firewalls. Initial research shows that cars going down the assembly line had this mark, but has been proven only on a handful of cars.

Sure, the prospect of driving what amounts to a 4-4-2 for more reasonable money sounds enticing, but the proof is in the driving. Inside feels more spacious than a comparable Dodge Charger, despite the two being comparably-sized mid-size platforms. Shut the door with a solid thunk: nothing tinny happening here.

Take stock, and you wonder if this is really a HMM kind of car. It's plush, it has faux wood on the interior, A/C, tachometer, comfy bucket seats, console and floor shift... nothing terribly sporting going on here, but it is luxurious. This would look at home in something from the post-muscle, "personal-luxury" era of Cutlass. Even when you hit the key, and the idle settles into a low rumble, there's nothing to tip its hand that there's fun to be had. Stealthy.

  Drop the chrome lever down into D and start to roll, though, and you quickly realize that the game is afoot. The 455 has enough luxury accoutrements to haul around that it's not going to be at its absolute screaming 4-4-2 W-30 best, but you can float off on a wave of torque that you can control. Press the gas like there's an egg between your foot and the pedal and takeoff is effortless and smooth-the sort of acceleration that wouldn't startle your elderly aunt as you drive to lunch on Sunday. Give it the boot and you'll chirp the tires (briefly, especially if you power-brake slightly) and get going with the kind of grunt you expect from a muscle car-the sort of move that would have your aunt screaming for you to stop and whacking you on the head with her purse. A slightly steeper gear would get things moving more quickly, but would also alter the slightly soft character of the rest of the car. Still, all the sound deadening in the world won't quell the throaty, guttural aggression coming out of those pipes at full chat.

The ride is that perfect alchemy of sport and comfort. It's smooth but isn't seasick floaty and it provides surprising stick in the turns for something this big and heavy-though no one will mistake it for a Corvette. Steering has minimal slop, turn in is predictable and the power steering even allows a modicum of feel to be transmitted through the column. The front end will lift in corners if pressed even slightly (anyone who's watched a stock A-body mid-turn, with its tortured inside-wheel, front-end geometry in play, will marvel that it doesn't simply tip over), but inside the car it feels like you're hunkering down into the turn. Do brace yourself with your leg, however, as the bucket seat will offer no assistance in keeping you upright.

What shines through most in a spin behind the wheel of the Cutlass Supreme SX is just how right GM had things in the early 1970s. They were on top of the world, and cars like this were the reason why. Kind of makes you wonder just what happened. The SX, and the Cutlass in general, proves itself to be the quintessential American car of the '70s, although it took half a decade after the SX for America to launch it into the stratosphere as America's best selling car.