W72 Info (source http://www.iwaynet.net)

Horsepower was finally making a comeback in good old America, and assigned to lead the pack was the 1978 Pontiac Firebird Trans Am's W72 optioned engine- ( coded WC when bolted to the 4-speed and X7 if you needed an automatic ) the 220hp T/A 6.6. Good old cubic inches were finally saved from the Government's noose and an engine worthy of the Trans Am moniker was available. 

In 1978, you couldn't "chip" an engine to make more horsepower and "cat-back" exhaust systems were unheard of. If you wanted an EPA legal power boost, you had but one choice- visit your Pontiac dealer. Some pretty neat features went into making of a T/A 6.6. First there was the carburetion, fattened up slightly for little more oomph. Next was a bit more spark advance, giving the GM high energy ignition more opportunity to burn that fuel bonus. As any hot rodder knows, the camshaft is the lifeblood of performance and the T/A 6.6 had one with an additional 20 degrees of duration. To help finish the cycle, a new low restriction dual resonator exhaust system was added, replacing the single muffler used on all other Trans Am's. ( The dual splitter tail pipes were also fitted.) Keeping the oil pressure from dropping to zero on when turning quickly to the right or left, a windage tray was added. To allow more air into the engine, a air cleaners with a larger snorkel was added. These items were in addition to 1977's compression ratio increase of 8.1:1 (versus 7.6:!) by bolting the 350's cylinder heads on the 400 engine. Just to make sure everyone knew you had a T/A 6.6, the callouts were affixed to the shaker and chrome valve covers completed the look.

Although 220 horsepower may seem wimpy compared the the hairy chested Ram Air III's, IV's and and SD-455's, but remember this was power achieved with little valves, low profile cast iron intake's, plain old exhaust manifolds, and the power restrictive catalytic  converter. The numbers for the W72 were 220hp @ 4000 rpm and 320 lb/ft of torque peaking a good bit further up the rev range at 2800 rpm than the garden variety 400. When you figure this all this was available for a mere seventy-five dollars , it is no wonder production was limited. The key to 1978's best all-around performing car was no doubt this one option (as long as you didn't live in California).

WS6 Suspension

First off, on automatic cars you got the obligatory stiffer rear springs (25-lb/in higher deflection rating) that in Detroit-speak means "tuned". The rear stabilizer bar grew to .75" versus the .62" of the non-WS6 car and all stabilizer bars received polyurethane link bushings versus the softer rubber bushing of all other Firebirds. The shocks were valved a little tighter for greater wheel control, and a new steering gearbox with a ratio of 14:1 replaced the 16:1 variable ratio unit of lesser TA's (is there such a thing?). Finally where the rubber meets the road (literally) a P225/70R15 steel belted radial of Goodyear or Uniroyal origin was mounted on a set of 15 x 8 inch wide snowflake wheels. The wheels are easily recognized by the deep dish look versus the flush appearance of the standard T/A snowflakes.

If you are looking at a T/A and wondering if you have the WS6 suspension, there are two ways to find out, an easy and a hard. Unfortunately for me, I found out about the easy way only after doing things the hard way. The hard way was to make a pair of gauges, preferably out of metal, of .62" and .75" respectively, crawl under the car and see which one just barely fits the rear stabilizer bar. This is what I did, suit jacket, tie and all. Pretty dumb when you consider if I would have been thinking all I would have had to do is open the left door and check out the tire plaquard on the door. A non-WS6 car will have a GR70x15 listed as the tire sizes while the real deal will list the P225/70R15. If a door has been replaced or someone has taken the time to counterfeit a sticker, then maybe the hard way isn't so dumb after all. By the way I have a WS6 car.

The WS6/W72 combination quickly established the 1978 Trans Am as the "King of the Hill", long before there was such a thing a Corvette ZR-1. Magazines loved the car and the Spring 1978 issue of Road Test magazine squared off a Z28, a Corvette, and a Trans Am in a street fight. The T/A was clearly chosen as the easiest car to drive fast.