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

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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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    Nice article about A body and 428 conversion. Just so happens that I’m restoring a 71 T-37 and my 428 is at the machine shop after sitting in a feild for 30 years.
    Articles like this keeps my vision alive and friends confused.

    I have 68 firebird 400. Only the rear end has the right #’s. So I am installing a 68′ high output 428 w/670 heads, roller cam and rockers, solid timing,fuel injection and MSI.

    I picked up a 69 GTO, RAIII car, phs documented, restorable car, super solid. Rod knock. Pulling motor down, ends up a YH 428 69 motor, havent verified vin on block, could motor be original? Why pull a RAIII block for 428, assume they blew it up.

    Pontiac was known to play fast and loose with the rules. When they debuted the GTO to the world in 1964 they showed up with 2 cars. The press car had the advertised 389 while the test car had a 421 Super Duty beast. Of course the 421 car came with hood pins and locks that they forgot to bring the key for. They also would put whatever engine you wanted in the car if you had the right connections.

    I have a 1979 Pontiac TA with a 428 ci HO Tri-power in it. The TH 400 Trans. is like new and just remanufactured better than from the factory. B+M shifter and shift kit. Gear Vendors over/under drive. Herb Adams suspension all around each corner with full roll bars in the cab, Engine compartment and Trunk. Motor is rebuilt and continues to be at this time improved. All the options you can put on this vehicle are present. I have the cruise control, but have yet to install it completely. Tilt steering, 4-Wheel disc, rear window defroster, electric windows, Trunk release, power steering. Electric door locks and Louvers over the rear window made of Aluminum. Extra insulation under the carpet, side panels, and all door panels and inner panels. I created custom inner insulated panels for the T-tops to keep the sun out and then upholstered them with the same vinyl ceiling head liner Black Landu material. The inside is Black Vinyl as well. For all the details follow me on the PY forum under Tradermike2012.

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