The First Modifications

Gregg was friends with many of the engineers and technicians who worked on the SD-455 program, so there was no shortage of helpful input and information available to him. "Those guys knew what worked from actual testing and I decided to incorporate some of those modifications on my own engine. I immediately replaced the Shaker's block-off plate with an open mesh screen."

Within its first year Gregg used a friend's chassis dyno to finely tune his SD-455's carburetor and distributor. His friend, Ed Blackerby worked in the Engineering lab and carefully assembled a centrifugal spring and weight kit that precisely tracked the unique curve Pontiac found worked best with the SD-455. "The rear wheel horsepower jumped when we installed the curve kit into the distributor and advanced the initial timing to 20 degrees BTDC (from 10)," says Gregg.

For the next few years Gregg drove his Super-Duty Trans Am in this configuration, accumulating about 12,000 miles in that time. As a single guy with some disposable income, Gregg had ordered a number of new Trans Ams in subsequent years, and even drove company cars later on, so that allowed him to keep his SD T/A a low-mile original. "I always wanted to see how much potential the SD-455 had, so I decided that if Pontiac wouldn't build my proposed Trans Am, I'd do it myself," he remarks.

Subsequent Modifications

In 1980, Gregg and John Crawford, an experienced Pontiac technician, removed the Trans Am's front end and began disassembling the engine for significant modifications. "The plan was to increase engine output, while maintaining durability by using equipment proven on the 600hp SD-455 mule developed in the Pontiac Engineering dyno lab. I wanted to improve high-end horsepower and increase the redline by 1,000 rpm while reducing low-end torque to minimize wheel spin and improve launch characteristics."

The SD-455 was sent to Leader Automotive, where George DeLorean (brother of former Pontiac General Manager John DeLorean) prepped and machined the block. Gregg added a 4-inch-stroke, forged-steel crankshaft that was left over from the SD-421 days. "The short-stroke crank reduced displacement to 433, but it would allow the engine to operate reliably at a much higher rpm. The box it came in had ‘Smokey' written on it suggesting it was originally destined for Smokey Yunick," he adds.

While the SD-455 was apart Gregg also incorporated a special coolant design that had been developed by Louie Clements, a GM technician at the Desert Proving Grounds in Arizona. It seems that GM was using production SD-455s to conduct its high-speed tire tests and the continuous high-speed operation in very hot temperatures caused the engines to burn exhaust valves in as little as 5,000 miles.

"Louie theorized that the intake valves are sufficiently cooled by the incoming air charge," Gregg explains. "Using threaded inserts he developed a way to divert the coolant, creating a uniform flow around the exhaust valves. The modification resulted in much greater valve life and greatly reduced coolant temperature, which cured engine overheating at 110 to 115 degrees F ambient air temperatures. Louie won a major award from GM for this concept and went on to develop a similar design for the Chevrolet small-block."

Gregg obtained a print of Louie's coolant-system modification and had George DeLorean incorporate it onto his SD-455 so he could run more aggressive spark timing and a higher compression ratio together. "George machined my block and heads to match the print and the engine was able to run 20 degrees of initial advance, the aggressive distributor curve, and a compression ratio of 9.5:1 without fear of detonation or overheating."

The original SD-455 heads were port matched and all traces of casting flash removed. It was topped by a single-plane Holley Street Dominator intake manifold and 750-cfm four-barrel. Gregg replaced the original 744-spec camshaft with an aggressive 290-degree hydraulic grind capable of running to 7,000 rpm. He also added 1.65:1-ratio rocker arms, pushrods, and other valvetrain pieces from the R/A-IV.

A key part of the build was a set of 180-degree headers that John Crawford helped him create. "Herb ran them on his Trans Am race cars and the NASCAR Grand Am. I loved their distinctive high-pitch sound. At Daytona you could pick our Pontiac out of the pack by its exhaust note. I installed a set of custom-fabricated units and one of Herb's deep sump cast-aluminum oil pans on my Trans Am early on but eventually removed the headers because of ground clearance issues."

While Gregg never measured the output of his modified SD-455, his Engineering experience tells him it probably generates 500-525 gross hp. Though several horsepower shy of the 600hp dyno mule, Gregg's engine is capable of operating reliably on pump gas at 7,000 rpm, and that's the exact result he was after. He made a significant number of changes to the suspension, along with a few to the body and interior to create a Trans Am that coincides with his original proposal.


Today Gregg's Trans Am has less than 13,000 miles on the odometer and is driven sparingly. As a rolling testament to the enthusiasm surrounding the second Super-Duty era, a look at the documents and information he provided to us illustrates the SD-455 program was something very special at Pontiac. With the enthusiasm surrounding it at the time, no wonder it's held in such high regard by enthusiasts so many years later!