Pontiac’s 428 was a sneak attack in the muscle car horsepower wars

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Photo by Karissa Hosek/Auctions America

It’s the end of April, or, as we like to call it at Hagerty, Engine Week. We’re celebrating some of our favorite big-inch V-8s from the peak of the muscle car era. Today is 4/28, so it’s time to pay homage to Pontiac’s 428.

Intended for full-size cars, Pontiac’s 428 has plenty of muscle car history, even if it took some aftermarket wrenching to make it so.

GM had an internal policy that limited mid-size cars to engines no larger than 330 cubic inches.  Pontiac ignored that policy in 1964 when it created the GTO and installed a 389-cu-in V-8 into the LeMans. For 1965, that policy was loosened, with a 400-cu-in cap this time around. That allowed the Olds 4-4-2  to pack a 400 V-8 for 1965, while Chevy brought the 396 big-block to Chevelles starting the same year.

The GTO continued packing 389 power until the debut of the 400 in 1967, but there were larger engines in the Pontiac stable that could have given the Goat even more power. The 400 shared the same 3.75-inch stroke of the 389, but increased the bore from 4.0625 to 4.120 inches. The 421 used the same 4.120-inch bore but used the 4.0-inch crank from the 421 it replaced. Like the 421, the 428 used a large-journal crankshaft, but externally it was the same size as all Pontiac V-8s of that era (the later 301 used a shorter deck height).

Spurred on by the allure of lower elapsed times and increased traffic to their showrooms, Royal Pontiac in Royal Oak, Michigan, installed 428 engines normally found in the Bonneville/Catalina and Grand Prix into Firebirds and GTOs.

The 428’s extra displacement brought extra power, with 360- and 376-hp variants in 1967 supplanted by 375- and 390-hp variants for 1968 and ’69. A contemporary Pontiac 400 V-8 Ram Air III in the GTO was rated at 366 hp in 1969, and the elusive Ram Air IV was rated at 370 hp. Of course, Royal Pontiac didn’t settle with the factory rating and tuned the 428 to around 425 hp. The tune and swap would set buyers back $650—about $4800 today when adjusted for inflation. Not cheap, but it was an easy way to make sure you had one of the quickest cars on the street. Even with the hood propped open, nobody would be the wiser unless they spotted the sticker on the valve cover.

Royal Pontiac’s engine swap wasn’t a new trick for Pontiac. A 421 was secretly swapped into a Grenadier Red 1964 GTO that Car and Driver magazine famously pitted against a Ferrari GTO.

We’re not saying that you have to build a 428 for your 1967–69 GTO or Firebird, but if your search to return your car to numbers matching comes up short, you’ve got our blessing to drop in a 428. You don’t even have to change the fender emblems. It’s what Jim Wangers would have wanted.

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