Inside the 455 Super Duty

Skip McCully's Views
In addition to component development, Skip McCully's duties on the Super Duty project included tuning the SD-455 for maximum reliable performance while remaining compliant with federal exhaust-emissions standards. To find the optimal amounts of fuel and timing for the SD-455, McCully and an engineer from Delco worked together using an engine dyno with full control over fuel metering and ignition timing to test several combinations, until they found what produced the most power under full throttle conditions. The production settings of the carburetor and distributor were altered slightly.

McCully stated that to prevent failure from detonation-related issues such as poor fuel quality and incorrect octane, total timing was reduced 1 percent from the point where the engine made peak power. For example, if a respective engine made peak power at 38 degrees BTDC, McCully and the Delco engineer would reduce the total by roughly 3.8 degrees (or approximately 4 degrees), then use that point as the factory spec. He further added that initial timing and primary jetting were primarily set to comply with emissions and the rate of distributor advance was a compromise of performance and emissions. And since additional advance at part throttle typically generates excessive hydrocarbons under light load, the vacuum advance unit was limited to provide enough advance to benefit, yet remain emissions compliant.

As many enthusiasts already know, the original SD-455 camshaft had the same specs as the 041 used in the R/A IV, and McCully confirmed several preproduction engines were built with it. His personal test vehicle, a one-off black Trans Am pilot car, was powered by an SD-455 with the 041-spec cam and a functional Shaker hoodscoop. In addition to normal driving duty, the car was also part of a test fleet that traveled to the Desert Proving Grounds in Arizona for factory testing. The joke within the caravan was that the SD-455 would require frequent stops for refueling, slowing progress. But McCully suggested the group forge onward and that he and the SD-powered Trans Am would catch them on the top end. To the surprise of the those traveling in the group, however, the SD-455 Trans Am averaged the second-best fuel economy the entire trip, only to be outdone by a much more sedate cruiser!

Once at the Proving Grounds and without any tuning after the long trek, the SD-455 Trans Am with its automatic transmission and moderate rear gear consistently ran the quarter-mile in just 13 seconds. If that weren't enough, the car was taken directly from the track to the emissions-testing station where the large-cammed SD-455 passed below the federal standards but without enough "cushion" to satisfy the emissions engineers. This cushion was a safety margin implemented by the emissions engineers between the engine's actual output and the federal standards, to ensure that 100 percent of the engines in that group were compliant once sent out the door.

To adhere to this margin, a camshaft with less duration and overlap was used. In place of the 041-style camshaft with 308/320 degrees of advertised duration, 0.470 inch net valve lift, and 87 degrees of valve overlap went a cam with valve timing identical to the 744 grind. Best known for its use in R/A III manual transmission applications, the 744 cam offered 301/313 degrees of advertised duration, 0.407 inch net valve lift, and 76 degrees of valve overlap. With less aggressive valve timing, power output was affected slightly--but not all negatively. Horsepower dropped from 310 with the 308/320 grind to 290 with the 301/313 cam, but torque increased from 390 lb-ft to 395 lb-ft, respectively.

Consequently, McCully strongly stated that under no circumstances were any SD-455 engines with the 041-spec cams ever released to the public, and any rumors of production engines shipped that way from the factory are false. He added that all the pilot cars and any of the test cars in the Pontiac press fleet with the noncompliant SD-455 were returned and destroyed. Not only were these cars typically handbuilt without a valid VIN, they also were not emissions or safety certified. Pontiac could have faced severe consequences from the federal government and potential legal suits if anyone was injured in an accident had they been released to the public.

Once the SD-455 was certified with the new cam, it was ready for production. But just when it appeared the car might finally hit the streets, it was further delayed by a recertification mandate affecting every Pontiac engine when the EPA found that part-throttle emissions were noncompliant. In March 1973, a running change was made, eliminating the timed solenoid that gave full vacuum advance and disabled the EGR valve in high gear after a specified amount of time. Only after this was corrected and the SD-455 recertified was the engine finally released for production in approximately late April 1973.

Pontiac sales literature from 1973 states the SD-455 was optional on Firebird, Grand Prix, LeMans, and Grand Am models. Already delayed once by the cam change and then again with the midyear solenoid change and recertification, it would have been cost-prohibitive to recertify the SD-455 for each model with such a late market introduction. Since the Firebird was gaining popularity as Pontiac's premier performance car, the SD-455 was limited to just the Firebird Formula and the Trans Am. And because of drive-by noise standards and to save costs certifying the engine twice, the engine was certified only once with a sealed Shaker scoop in place. Because of this, the only Formula to receive the Trans Am Shaker scoop were those '73-'74 models powered by the SD-455.

When asked how he felt the Super Duty 455 was compromised for production, McCully simply replied, "Compression, camshaft, jetting, and vacuum advance." He then stated that ideally he would have preferred to have the compression ratio closer to 10.25:1, a camshaft with 041 valve timing, slightly richer carburetor jetting, and as much vacuum advance as the engine would tolerate at light load for maximum throttle response. But none of that was possible. Overall, however, the SD-455 was much the way Pontiac envisioned it, and its performance speaks for itself. McCully also said excitement about the project within Pontiac was extremely high. From the engineers' perspective, it was an opportunity to showcase their ability and produce the best Pontiac engine up to that point.

Nunzi Romano'S Influence
Another interesting point mentioned by McCully was the close working relationship he had with Pontiac performance legend Nunzi Romano of Nunzi's Automotive in Brooklyn, New York. Romano's reputation for making significant amounts of power using stock Pontiac components continues even today. McCully said he took notice of Romano's achievements and would routinely contact him for his opinion on a variety of projects, including the Super Duty 455. Oftentimes he would send Romano experimental pieces for testing and review and some of that input was used on the production pieces.

When asked about his involvement, Romano recalled discussing with McCully the oiling system and reliability of the cam gear with the 80-psi oil pump. Romano said the drag created by a high-pressure pump often led to premature cam and distributor gear failure. He said to prevent this, a 0.030-inch hole was drilled into the oil plug inside the block, which shot a continuous stream of oil directly on the constantly meshing gears. Though effective, the hole was small enough that a piece of foreign material or sludge buildup could potentially plug the hole and prevent the lubrication, which could cause cam or distributor gear failure over time.

Romano also recalled at least two Super Duty Firebirds in his shop for engine rebuilds because of oil-pressure-related issues. The extreme pressure generated by cold oil and the 80-psi pump blew the oil filter off the housing, which led to engine failure from lack of lubrication. To prevent this from happening after the rebuilds, he substituted a 60-psi pump and tightened the internal clearances--which not only seemed to remedy this problem but also the cam and distributor gear issues. Romano said because of the excessive piston clearance, the SD engines were quite noisy when cold. He even received calls from dealers and customers concerned that their SD-455 had problems internally, but he assured them it was normal.

Overall, Romano feels the SD-455 was an excellent engine capable of making plenty of power, and that it was obviously designed with total performance in mind. He said the cylinder heads are among the best Pontiac ever produced and that little port work is needed to make them any better for the street. Romano currently has four Super Duty Firebirds in his private collection. His Buccaneer Red '74 has been featured in HPP several times. It has generated quarter-mile trap speeds in excess of 105 mph in stock trim with sharp tuning, and 108 to 110 mph with headers, increased ratio rocker arms, a high-volume fuel pump, and a pair of slicks.

Conclusion
Ultimately, someone somewhere may disagree with any part of the information presented here, but considering that both McCully and Hicks were in the middle of the project making the decisions, we feel they are excellent sources with qualified credentials. After talking with them, we were not only confident we could lay some of the SD-455 mysteries to rest but we could also reveal and provide accurate insight on how the Super Duty evolved. Like its predecessors, it was a foregone conclusion that the Super Duty was going to be something special, but no one had envisioned SD-455 excitement would be as high today as it was more than 30 years ago. After hearing more about its development and production from a few of the engineers who were there, hopefully the Super Duty 455 saga will continue for years to come.

Special thanks to Mike Hicks, Skip McCully, and Nunzi Romano for their assistance.

*Pontiac service bulletins courtesy of General Motors