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426 Hemi engine information page

426 Hemi

Hemi. Few words in the automotive world are more instantly recognizeable than this legendary term. 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 V 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 produced their first engines with hemispherically-shaped combustion chambers in the 1951 (using their experience in aircraft engines), but these early motors, ranging from 301 to 392 cubic inches), were primarily passenger-car motors. 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. 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, running on alcohol.

street Hemi

Developing the original Hemi engines by Curtis Redgap

In 1963, Chrysler executives 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.

hemi heads

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.”

426 HemiWhen 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. This single event 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 “ordinary” production vehicles. Chrysler didn’t throw in the towel on the hemi after this (although they did sit out the 1965 season), and the end result was the Street Hemi, which first appeared in 1966 B-body Dodges and Plymouths.

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 largely by using 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 reliability.

Little changed inside the 426 Hemi throughout its eight-year production life; camshafts were updated (more duration was added in 1968, and a hydraulic bumpstick was used beginning in 1970) but little else separated the model years. Chrysler never changed the engine’s horsepower and torque ratings, 425 hp at 5000rpm and 490 foot-pounds of torque at 4000 rpm, even when ratings switched from gross to net. Four bolt mains were standard on every 426 Hemi, street or race.

One advantage of the Street Hemi over earlier performance engines was that it could come down the line complete with exhaust manifolds, while the Cross-Ram engines had to have their manifolds fitted later. He and Frank Bilk had tilted the combustion chamber to prevent the exhaust rocker from being too long; this resulted in a narrower engine, so they could do a standard body drop in the Dodge Main factory.

hemi heads

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

1970 Street Hemi
Horsepower 425 @ 5,000
Torque 490 @ 4,000
Compression 10.28:1

Given their extremely high cost (for Street Hemis, that included modifying the car to handle the power), not many 426 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 (and any Hemi production numbers are likely inaccurate due to factory record problems) appears to have totalled just 10,904 from 1966 to 1971, with the highest numbers by far in 1966 (3,350).

Although the motor was legislated out of NASCAR in the 1970s, and emissions laws, high production costs, and the insurance surcharges stopped production of the street version in 1971, the motor 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.

1966 chrysler street hemi engines

Although it’s been a quarter of a century [when Steve first wrote the article for Allpar] since the last classic Hemi-powered car rolled off a Chrysler assembly line, and the chances of finding a Hemi car in a used car lot or junkyard are slim to none, all is not lost for the modern-day Hemi enthusiast. 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

I was involved in the development of the Street Hemi, which came out in the 1966 model year. The main (and 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

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.

1970 twin plug Hemi heads for Ramchargers racing team

Ending the Street Hemi

“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.’” — Willem Weertman

Tested but not produced

The Chrysler 300 with 440 Hemi

At one point, serious, and I mean earnestly serious, consideration was given to having a Hemi 300 engine. This would have been an exclusive engine just for the Chrysler 300 letter cars. With the introduction of the 413 wedge, it was argued by purists that the 392 Hemi turned just as much power, and a lot more power was available but not utilized before the Hemi was tossed out. In all fairness, 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.

One of the spinoffs from the A990 full-racing Hemi program that was 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.

race group 1978This engine was designed to accept the two bolt main bearing cap structure, but to avoid the lengthy and expensive bake out heat treatment. It 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 heavy into the 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 ever relent, moved quickly 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:

The above 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.

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, last minute reconsideration was given to discontinuation of the letter series 300 after the L in 1965. A tentative 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 a number for the price Chrysler would need to charge.

The car, which appears to have been a 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 "'66 300-M(onkeypuzzle)".

Dual overhead cam Hemi

A pair of 426 dual-overhead cam Hemis might have been produced in 1964 to counter Ford's response to the 1964 426 Hemi, the 427 SOHC, but when NASCAR ruled against Ford's engine, there was no need for the overhead-cam Hemi.

Neither of the DOHC Hemis were ever placed in a car; one was destroyed, the other moved to the Kansas City area. (source: Muscle Car Review. Thanks, Stéphanie Dumas.) 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.

Hemi engine layout

An article by Tom Shaw in Mopar Muscle went into more detail. The DOHC Hemi was project A-925, and it would need to be much more powerful than Ford's SOHC 427, but still rugged enough for racing, and conformed to NASCAR rules. Shaw wrote that they considered one using two cams between the heads, with four valves on each cylinder were operated by lifters, pushrods, and lifters — an unused contingency plan. Development focused instead 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 (Chrysler was into efficient and innovative intakes) and made of magnesium, with a single four-barrel carburetor, as required by NASCAR.

The cams were driven by a cog belt, using 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.

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 (at least) Mopar DOHC Hemi made — and that he owns it, 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 says the oil pan holds 10.6 quarts, and that the engine has stainless steel headers, and an aluminum intake; it is apparently functional and runs on regular gas. We don't have any information on where it came from and whether it’s a Chrysler effort or an aftermarket modification.

Chrysler alumnus and historian (of The Ramchargers) David Rockwell (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.

Modern Hemis 5.7 Hemi • SRT 6.1 and 6.46.4 Truck Hemi • Supercharged “Hellcat”
Classic Hemis 426 HemiOld Hemi History • 392 Crate HemiDevelopmentPlane Hemi

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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