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.
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.
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."
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.
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.'" https://www.allpar.com/corporate/bios/weertman.html
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.
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
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|