The 426 Hemi Dual Overhead Cam Engine – The “Doomsday Hemi”

I’m often asked about the 426 Hemi and its specifications. One topic coming up is about an overhead cam design. Let’s answer, is the 426 Hemi DOHC?

The 426 Hemi is not an overhead cam design engine. A 426 Hemi dual overhead cam engine was designed as a prototype in 1964 but was never completed or placed into production. The overhead engine was discontinued due to Nascar’s ban on non-production engines.

Even though, a prototype of this engine exists today and the design is interesting. This article will examine why Chrysler designed the engine, its features and why it never saw production.

The 426 A 925 Hemi DOHC
The 426 A 925 Hemi DOHC

I was lucky enough to see this engine in person and learn more about it at a Carlisle event. If you thought seeing a Hemi was impressive, this engine is a little extra special.

The following specifications and facts were learned from my personal experience and research, Mopar books and articles.

Why Chrysler Designed a Dual Overhead Cam 426 Engine (DOHC)

With the introduction of the 426 race Hemi into Nascar, its dominance had Ford very concerned. It was painfully obvious the Hemi designed 426 was superior.

In response to the Hemi, Ford built an overhead cam engine more powerful than the 426 Race Hemi. It was the 427 SOHC engine , aka “The Cammer” or the 90 Day Wonder.

Chrysler knew the 427 SOHC was superior to the Hemi and would produce more horsepower, over 600 hp.

The Ford engine wasn’t approved as of yet, but just in case Chrysler got busy with an answer of their own. Chrysler did what they always do, fighting fire with something much greater.

In response to Fords overhead cam engine Chrysler started to develop the 426 Hemi dual overhead cam engine.

It was estimated the new DOHC engine would produce over 700 hp.

A video about the 426 DOHC engine.

The 426 Hemi DOHC Engine

In response to Ford’s new project, two different configurations were designed during the drawing board process.

The first used two cams mounted in the valley of the block. The drivetrain included pushrods and lifters. There were three rocker arms to operate four valves. Two for the intake and one Y shaped rocker for the exhaust valves.

This design never left the drawing board. It was a potential backup plan in case the overhead design was rejected.

426 Hemi overhead cam engine parts
426 Hemi overhead cam engine parts From Mopar Muscle Magazine

The second project did progress past the drawing board and was designated A-925. Many people call it the A-925 or the “Doomsday Machine or Hemi.”

The prototype engine used an actual 426 Hemi block. Each aluminum head featured dual overhead cams for a total of four camshafts.

Overhead cams removed the bulky and horsepower robbing valvetrain components like the rocker arms and long pushrods.

The heads featured four valves per cylinder in a pentroof combustion chamber.

426 Hemi DOHC cylinder head and front cog
426 Hemi DOHC cylinder head and front cog

There was a pair of external cog wheels, one in front of each head. They would be driven by a crank-driven cog belt, similar to ones used for superchargers.

The external cog wheels spun a series of gears inside the heads which spun the cams.

This new head design allowed for higher rpms and was more efficient.

The engine would have a single four barrel carburetor mounted on a magnesium intake manifold. The manifold had eight runners on each side.

One carburetor was used to satisfy Nascar requirements but two four barrels would be used for the street version production cars.

426 Hemi DOHC intake manifold and valve covers
426 Hemi DOHC intake manifold and valve covers

Moving past the drawing board, foundry work was conducted by Alcoa. They reportedly cast two sets of parts.

The engine had a crankshaft with no connecting rods or pistons. It was driven by an electric motor for testing purposes.

The electric motor was able to spin the valvetrain and cams at high rpms for extended periods of time.

The lifter bores were left unmachined and a portion of one valve cover was cut away allowing visual inspection while the assembly was spinning.

There were some early issues like any new design but solutions were planned to fix them.

Find out the all differences between a Race and Street Hemi in my article, The Difference Between a 426 Street Hemi and a 426 Race Hemi.

What Happened to it?

Unfortunately, this is where the whole A-925 project was halted. Nascar rejected Ford’s 427 SOHC and Chrysler stopped developing their engine. They didn’t see a need to continue on at this point.

The A-925 never ran under its own power. It has been reported Chrysler sold selected items from the project to recover more than salvage value from the parts.

One of the prototypes survived and is now featured at car shows. It is reported the current owner is the 3rd owner of this prototype since it left Chrysler.

The 426 DOHC on display at a car show.

One prototype bare cylinder head showed up on Ebay a few years ago and was sold by Larry Shepard of Shepard’s Automotive.

By the way, Shepard’s Automotive specializes in Mopars only since 1951, especially Hemi engines. You can take a look at the website here.

426 Hemi DOHC cylinder head for sale on Ebay
426 Hemi DOHC cylinder head for sale on Ebay
Front of the DOHC cylinder head where the cog attaches
Front of the cylinder head where the cog attaches
426 Hemi DOHC cylinder head intake ports
426 Hemi DOHC cylinder head intake ports

It would have been interesting if the DOHC Hemi was approved and made its way into street cars. The famous street Hemi would have been inferior and we’d be looking back on Mopar history in a much different way.

Any questions or if you have more information you’d like to contribute, send us an email found on our contact page.

Read More 426 Hemi Articles!

Why the 426 Hemi is Called the Elephant Engine

Why the 426 Hemi was Banned From Nascar

Hemi Engines Made Before the 426 Hemi

The Carburetors on a 426 Hemi

Transmissions a 426 Hemi Used

The First Year of the 426 Hemi

The Last Year of the 426 Hemi

Article Resources

Similar Posts