Mac McKellar built the most advanced American V8 of 1963 — and GM wouldn't let him use it. McKellar was Pontiac's chief engine engineer, the man who designed the camshaft profiles that made the 389 and 421 Super Duty engines dominate NASCAR and NHRA. His name was literally on the camshafts — "McKellar #7," "McKellar #10" — grinds so famous they were sold over the Pontiac parts counter by name. But McKellar knew pushrod engines had a ceiling. At high RPM, the pushrods flexed. The rocker arms deflected. The valve float that killed a pushrod V8 above 6,500 RPM was a mechanical limitation, not a tuning limitation. The only solution was to put the camshaft directly above the valves and eliminate the pushrods entirely. In early 1963, working with John DeLorean's blessing at Pontiac's engineering center, McKellar completed a DOHC 389 V8 with 32 valves, belt-driven quad overhead cams, cross-ram intake manifold, sequential fuel injection, and output that Pontiac never officially rated but that experts estimate exceeded 500 HP.

The engine used Gilmer-type reinforced rubber timing belts — the same technology McKellar would later bring to the production Pontiac OHC Sprint six-cylinder in 1966. The belts were lighter than chains, quieter than gears, and didn't stretch under sustained high-RPM use the way roller chains did. The DOHC heads placed two intake and two exhaust valves per cylinder at compound angles in hemispherical combustion chambers — each valve actuated directly by a cam lobe without the mass of a pushrod or rocker arm between the cam and the valve. The reduction in valvetrain mass allowed the engine to rev past 7,000 RPM without the valve float that limited pushrod Pontiac V8s to approximately 6,500 RPM. McKellar's team also experimented with SOHC configurations, three-valve heads with both possible arrangements — two intake and one exhaust, and one intake and two exhaust — and two-valve SOHC designs that could use production Pontiac intake and exhaust manifolds for easier packaging.

GM chairman Frederic Donner killed it. In early 1963, Donner issued a corporate memo reiterating GM's ban on factory racing involvement and adding that under-the-table participation — which every GM division had been practicing — would no longer be tolerated. The DOHC 389 went back on the shelf. McKellar continued experimental OHC V8 work quietly: a simpler SOHC 421 with 16 valves making approximately 620 HP with Tri-Power carburetion by late 1963. A 24-valve SOHC version of the 428 in 1965. McKellar showed the experimental engines to Hot Rod editor Eric Dahlquist in 1968 — the article made the March 1968 cover — but none of the OHC V8s ever reached even limited production. Without racing, Pontiac had no business case for a DOHC V8. The growing safety lobby had made GM management terrified of selling powerful engines. The OHC V8 technology was redirected into the production Sprint six — a single overhead cam on a six-cylinder instead of four cams on a V8. McKellar's V8 vision was scaled down to a six-cylinder compromise.

This is the engine that gets me upset. This is the motor that was once in Mac McKellar's Grand Prix. Now it's gone - stolen - and it has not resurfaced except for a bizarre picture of it one an engine stand from a Facebook posting over a decade ago asking anyone if they knew what it was. We did and responded and the picture was quickly taken down.

This engine was a retirement gift from GM to Mac. It was very temperamental to start.

At one point, Mac took the GP to a shop, and in a bad decision, agreed to have the DOHC taken out and a regular Pontiac V8 put into his GP. The mechanic had the DOHC motor out and re-sold within 24 hours. Mac changed his mind about the regular Pontiac motor but the DOHC was already gone. So Mac was swindled and it's in the wind to this day.