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Dodge - Plymouth - Chrysler 426 Hemi: the Elephant Engine

by Steve Boelhouwer

Hemi. Few words in the automotive world are more instantly recognizable than this legendary word. From its original stock car roots to its eventual domination of the world of drag racing, the 426 Hemi has left an indelible stamp on automotive history.

"When we got the green light to go ahead and adapt the hemi head to the big B engine, we realized that one day it would be something revered, that it would be something that everyone would look back on as something very special indeed." - Tom Hoover

Chrysler made their first engines with hemispherically-shaped combustion chambers in the 1951 (following their aircraft engines), but these early motors ranged from 301 to 392 cubic inches). They were called the "Red Ram," "Firedome," and "Firepower," depending on brand; horsepower peaked in 1958 with a dual four barrel version of the 392 rated at 390 hp, which would soon be outperformed by a 413 Wedge. Today, these motors are difficult to find, and those which aren't in restored vehicles are most often found in fuel dragsters and funny cars.

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The original Hemi engines by Curtis Redgap

In 1963, Chrysler approved a program to build a car that could win the 1964 Daytona race. Tom Hoover said that he and Don Moore used Chrysler's expertise in Hemi designs rather than working further with the Wedge, because the Hemi was the cheapest and surest way to win.

The new 426 Hemi had a standard RB engine block, with "hemi" heads - reversing the progression of the original Hemi into [cheaper] polyspherical and then wedge-head engines.

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You Chrysler guys may remember the Race Group's flat-crank Hemis in Room 13. If your feet hurt, 5 minutes in the hall outside would do wonders! - Pete Hagenbuch

In a 1994 Mopar Muscle interview, engineer Tom Hoover said, "We knew with the power level we could expect, we could provide performance and win races for minimum expenditure. You could continue to hone and evolve the Wedge forward, but the results would be limited. The cost effective way to make a real impression at Daytona was to take advantage of the A311 Indy program background, and adapt it to the race 'B' engine."

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When the 426 Hemi was introduced in 1964, it was strictly a racing engine. On February 23 of that year, four Hemi-powered Mopars swept the Daytona 500, finishing 1-2-3-4. It caught the racing world by surprise, and prompted NASCAR to impose stricter production rules on Chrysler. Instead of producing a few blueprinted Hemi motors each production year, they would have to produce several thousand and sell them in street-legal cars.

Chrysler didn't throw in the towel on the hemi after this (although they did sit out the 1965 season), and the Street Hemi soon showed up in 1966 B-body Dodges and Plymouths. (There was still a race version; a December 16, 1964 service bulletin, showed the 1965 Plymouth Belvedere with the "Super Commando 426," rated at 425 hp @ 6,000 rpm and 480 lb-ft at 4,600 rpm with 12.5:1 compression.)

The hemispherical chamber, as the name implies, is a portion of the sphere. ... With this we can bring the spark plug into the center of the chamber, which is an excellent position for the spark plug [and allows] excellent breathing of the air past the valve seat. This combination of the excellent breathing, of the inlet and the exhaust makes for a very high volumetric efficiency of the engine. ... We knew that we could not just put a race engine into production on the street, it just wouldn't work. But we were able to make a modified version of the engine that became our street Hemi.- Willem Weertman

The street version was created with a lower compression ratio (from 12.5 to 10.25:1), milder valve timing, different intake and exhaust manifolds, and cast iron heads, instead of aluminum, for long term reliability.

During its eight year production life, the street 426 Hemi saw updated camshafts (more duration was added in 1968, and a hydraulic bumpstick was used beginning in 1970) but few other changes. Chrysler kept the engine's horsepower and torque ratings, 425 hp at 5000rpm and 490 foot-pounds of torque at 4000 rpm, when ratings switched from gross to net. Four bolt mains were standard on every 426 Hemi, street or race.

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Unlike earlier performance engines, the Street Hemi came down the line complete with exhaust manifolds; the Cross-Ram engines had to have their manifolds fitted later. The combustion chamber had been tilted to prevent the exhaust rocker from being too long, resulting in a narrower engine, so they could do a standard body drop in the Dodge Main factory.

In 1965, the A-990 racing Hemi was launched, using aluminum heads and magnesium intake manifolds to cut weight.

Given their high cost (which included modifying cars to handle the power), not many 426 Street Hemis were made. Steve Boelhouwer wrote that there were 741 "race Hemis" in B-bodies in 1964-67, and 155 in 1968 'Cudas and Darts. Street Hemi production (probably inaccurate) appears to have totaled just 10,904 from 1966 to 1971, with the highest numbers by far in 1966 (3,350).

The motor was essentially ruled out of NASCAR in the 1970s, and emissions laws, production costs, and insurance surcharges ended the street version after 1971.

Willem Weertman wrote, "There had to be ... [a] decision whether or not to continue with that level of engineering work, or we should say 'the Hemi has done its job, let's discontinue it now, we don't want it to ever get a flavor of being a mild-mannered engine.' So, in 1970, we said, 'let's stop production.'"

It still dominated the top drag racing classes more than twenty-five years later. Aluminum versions power virtually all top fuel dragsters and funny cars (including those with other brands painted on the top), and are often used in drag boats and "monster" trucks. Restored Hemi muscle cars carry astronomical prices.

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In 1993, Mopar began manufacturing a new Hemi block, and since then almost all of the other Hemi parts have also been re-introduced.

Hemi engine valves

by retired engineer Pete Hagenbuch

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I was involved in the development of the Street Hemi, which came out in the 1966 model year. The only goal was that it perform well with a new camshaft, designed with hydraulic tappets in mind. When I say "perform," I mean [with a] minimal reduction of engine safe operating speed due to the hydraulic tappets. I seem to remember we got at least 6400 rpm.

As to the remainder of the Hemi world, I served by providing advice and consultation on matters having to do with valve train. And since this was as far as 40 years in the past I claim only slight memory of anything involving numbers. The nominal stem diameter was 5/16;" normal valves were 3/8". I remember the valve head diameters as about 2 1/4" intake and 1 7/8" exhaust. I am quite sure that the fillet between stem and underhead was hand spiral polished. I remember that scuffing of the valve tips was not uncommon, though never a serious problem. The scuffing was a product of the small tip area of the skinny valve stems.

Twin spark plug Hemi

1970 Street Hemi
Horsepower425 @ 5,000
Torque490 @ 4,000

A small number of special twin-plug heads were produced for the 426 Hemi, long before Chrysler engineers started using dual plugs on the 5.7 Hemi. James Schild provided a 1970 press release showing the twin-plug heads; engineers managed to insert the extra spark plugs (shown on the left-most cylinder below) without interfering with the rocker arms or pushrods. He wrote, "I found a part listing in the 1973 Mopar Performance catalog for a dual plug head, Race Hemi, P3690038, and aluminum valve covers with part number P369232 for the 8 or 16 plug heads." These might or might not be the ones pictured below.

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Tested but not produced

The Chrysler 300 with 440 Hemi

by Curtis Redgap

At one point, serious consideration was given to having a Hemi 300 engine, solely for Chrysler 300 letter cars. When the 413 Wedge was launched, purists argued that the 392 Hemi turned just as much power, and more power was available but not used before the Hemi was tossed out. The 413 Wedge was the better torque producer, along with a hydraulic lifter set up for the valves for less intense maintenance. Until you got past 100 mph, the 413 was marginally faster than the 392 Hemi.

Team Crew Photo caption Photography
One of the spinoffs from the A990 racing Hemi (built and tested but not produced) was the 300 Hemi model engine, meant for the 300 letter series sport sedan cars. 440 cubic inches were obtained by boring the 426 block to 4.32 inch from the 4.25 inches. The 3.75 inch stroke remained the same. It was designed to accept the two bolt main bearing cap structure, but avoid the expensive bake-out heat treatment.

This engine was slated to receive one of the first triple carburetor setups, as well as a 268° hydraulic camshaft later to be seen on the 440 Six-Pak cars. It would have used the cast iron heads, so the valve size remained the same with an intended 10.25.1 compression. Of the many 426 Hemi engine variants, this was the only one that got two main bearing cap bolts.

Following are some of the specifications from the white paper, written for John Wehrly by Bob Rodger, father of the 300 letter cars, and by product planner Robert Cahill.

Mr. Wehrly had joined Chrysler in 1962 and quickly moved up to become supervisor of racing engine development in 1971. In 1964, Mr. Wehrly was in Hemi development for the Daytona 500. In 1965, after NASCAR moved to ban the Hemi and Chrysler staged a complete boycott of the NASCAR circuit, Bob Rodger and Bob Cahill, sensing that NASCAR would not relent, moved to push the 426 Hemi engine into a regular production model available for the street to anyone with the cash to buy it. The letter follows:

Description: Because of continued requirements for an ultimate performance drag strip and street usage type engine with expanded usage, the following change is the engine lineup has been agreed upon after discussion with the affected areas. Please release a hemispherical combustion chamber engine for "B" Series with the following general characteristics:

  1. Intake manifold to have two four barrel carburetors.
  2. Cylinder block to maintain cross tie bolt bearing caps.
  3. Cast iron exhaust manifold.
  4. Solid lifters are acceptable but not preferred. [Hydraulic lifters were not achieved until the 1970 engine - ed.]
  5. Pistons - forged acceptable and thermally controlled preferred.
  6. Manifolding and camshaft to be designed to give the best highs speed power possible while still maintaining a reasonably drivable vehicle for summer and winter.
  7. Automatic and four speed [manual] transmissions required. 4-speed to receive development priority.
  8. No air conditioning required for "B" series.
  9. Limited warranty is acceptable for the "B" series.

Projected production volume expected is between 5,000 and 7,500 cars, which will take effect in the start of the 1966 production year. This engine to replace the eight barrel wedge requested in Production Planning Letter of 8/5/64.

The letter was dated January 6, 1965. On January 12, 1965, an engine production letter was issued for the A102 engine. It listed all the items needed to bring the race Hemi engine to specifications for use on the street.

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Mike Sealey wrote: There is a 1966 Chrysler 300 out there with what appears to be a factory-installed 426 hemi, something most historians say never happened in C-bodies. According to Chrysler historian Jeff Godshall, there was last minute discussion of not dropping the letter series 300 after the L in 1965; a plan was drawn up for 500 1966 300-M models powered by the 426 Hemi, but was withdrawn when marketing research suggested there might be a problem selling even that small number for the price Chrysler would need to charge.
The prototype still exists, and while it is not badged as a 300-M, numerous trim details are different from the garden variety 1966 300 Sport Series. It's known among 300 fans as the "1966 300-M(onkeypuzzle)."

Dual overhead cam Hemi

A pair of 426 dual-overhead cam Hemis may have been produced in 1964 to counter Ford's response to the 1964 426 Hemi, the 427 single overhead-cam engine, but when NASCAR ruled against Ford's engine, there was no need for the overhead-cam Hemi. Recently, famed engine builder Larry Shepard told us that he has the A-925 cylinder head and other related parts, purchased from the late Dan Napp.

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An article by Tom Shaw in Mopar Muscle said the DOHC Hemi project was coded A-925, and that it had an unusued contingency plan of using two cams between the heads, with four pushrod-activated valves on each cylinder (which went back to very early hemispherical-head engines). Development mainly focused on an engine with aluminum heads, dual overhead cams, and, again, four valves per cylinder, with pent-roof chambers. (Chrysler had been working with four valve per cylinder engines for a never-completed Indy run in 1963.)

The dual-plane intake manifold had eight runners per side and were made of magnesium, with a single four-barrel carburetor, as required by NASCAR. The cams were driven by a cog belt, with external cog wheels at the front of the heads. Because the cams were directly above the valves, valvetrain mass was low, so the engine could rev to a high-for-the-era 7,000 rpm redline.

Tom Shaw wrote that no DOHC Hemi ran under its own power; they were driven by an electric motor to check the valvegear. Research stopped in 1964 when NASCAR banned the SOHC 427 and Chrysler's race Hemi. One Chrysler DOHC Hemi reportedly still exists.

Jon Field wrote that there was a third Mopar DOHC Hemi made, which he owns -a 301 cid aluminum-block-and-head engine with twin cams, two cam covers on each head (the plugs are between them), hydraulic tappets, brass valve seats, and four Weber two-barrel carbs (165 cfm each). He wrote that the oil pan holds 10.6 quarts, and that the engine has stainless steel headers, and an aluminum intake; he said it was functional and runs on regular gas. We don't have verification, or information on whether it's a Chrysler effort or an aftermarket modification.

Chrysler alumnus and historian (of The Ramchargers) David Rockwell and historian Stewart Pomeroy agreed told Mopar Action that the A925 engine program was a fake, meant to impress Bill France into banning Ford's SOHC engine. The one engine was run by an electric motor. The story goes that Chrysler racing chief Ronnie Householder learned about the ban from Bill France, and the engine was then destroyed.

Thanks to Steve Boelhouwer, who created Allpar's first Hemi page in the 1990s. You can also view his main Hemi page, or his Wedge engines page. This page has been greatly expanded from the base he created.

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