Hemi Engine Evolution: The Only Constant is Change, Especially With the Nitro Hemi

       nitro engine goes boom
When a nitro engine goes boom, it really goes boom. The risk of an unplanned significant event comes with the territory.

Prior to its untimely demise, Orange County International Raceway (OCIR) in Irvine, California was famous for a number of things, including its tower, track surface and events featuring 64 Funny Cars. These events would often last well into the early morning hours, largely because when 64 Funny Cars showed up, you could pretty well count on almost as many oil downs. The unplanned depositing of petroleum, petroleum byproducts and exotic metals on the racing surface was so frequent these events soon became known as the Oildown Nationals.

Keeping a nitro-burning engine from self-ventilating is difficult even for the well-funded teams. That was a natural consequence of squeezing several thousand horsepower from an engine that was originally designed to produce a few hundred.

The earliest nitro-burning engines began their lives pushing Chrysler, DeSoto and Imperial passenger cars along the nation’s highways. Although smaller versions were sometimes sacrificed to the racing gods in the name of nitro-fueled acceleration, the most popular FirePower Chrysler Hemi was the 392-ci model that was introduced for the 1957 model year. (The least popular early Hemi was the 1952 De Soto engine that displaced a mere 276 ci.)

Affectionately known as Whale motors, early Chrysler Hemi engines were used in Top Fuel dragsters and Funny Cars into the ’70s, but by the end of the decade, the Whales had been replaced by Elephants, the moniker given to engines derived from the 426-ci second- generation Hemi. Introduced in 1964, the late-model hemi was initially designed for racing and was inherently stronger than the 392-based Whale motors. It also had a larger bore than its predecessor (4.25 inches compared to 4.00 inches) and cylinder heads that provided enhanced air flow characteristics.

Although the 426 Hemi was designed for racing, it wasn’t designed to have a supercharger bolted on top, nor for the after-effects of igniting a mixture of compressed air and nitromethane in the intimate confines of its combustion chambers. Consequently, by the middle of the ’70s, competitive Top Fuel engines were equipped with few, if any, original equipment parts; the use of aftermarket components designed for the singular purpose of surviving the forces of nitromethane combustion became a necessity.

Even with engines built of components created specifically for the conversion of nitro into noise and power, engine failures continued, although at a much-reduced rate. Any time internal combustion engines are operated on the ragged edge of sanity, memorable failures will always be an unfortunate consequence. However, as component manufacturers have continually enhance their products, and as technological developments have enabled racers to get a better handle on individual causes of failures, the frequency of catastrophic engine events has continued to lessen.

Conceptually, one of the most significant improvements in engine component construction is recognition of the fuel racer’s mantra that “you need mass to save your ass.” Improved metallurgy is certainly another advantage, but regardless of material strength, if there’s not enough mass, a component will have a short life cycle.

According to noted racer and crew chief Dale Armstrong, “Over the years, wherever there’s been a weak link, it’s been addressed. Pistons, wrist pins and connecting rods are all made from better material and more of it. Connecting rod big ends are now so wide you can barely get them into the cylinders. Crankshaft mass has also been increased, and combined with better metallurgy and heat treating, cranks now last a lot longer than they used to.”

Computer-based data acquisition systems, combined with recognition of the importance of better record keeping, has also enabled racers to replace components before, rather than after they’ve failed. Although component life is always a matter of conjecture, barring unforeseen circumstances, it’s now considerably more predictable than in years past. Armstrong notes, “Even with all the improvements, cranks only last about eight runs.” Aside from planning component replacement based on the number of runs, a crew chief can also analyze data from each run to determine whether a crankshaft or other components should be replaced sooner rather than later.

The same type of mass and metallurgy enhancements that have contributed to more dependable internal components have been applied to cylinder blocks and heads. Aftermarket manufacturers initially developed improved castings that provided greater strength than their stock counterparts. Castings, however, have inherent strength limitations, but it wasn’t until CNC equipment became reasonably affordable that alternatives became economically viable. Billet aluminum is inherently stronger than cast aluminum and when advanced CNC machining techniques were developed, blocks and heads machined from billet material became the obvious next step in the evolution of Top Fuel engine development. CNC machining transformed some operations that were once somewhere between difficult and impossible into routine procedures and that further enhanced the desirability of billet blocks and heads.

Improvements in magnetos and fuel systems are other factors contributing to improved engine life. According to Tom Prock of Venolia Pistons (former Funny Car pilot and crew chief for Tom McEwen), “A number of years ago I had to grab a coil wire and the current drove me to my knees. If you had to do it now, it would probably stop your heart.” That’s quite understandable considering that current magnetos put out about 45 amps, about the same as a small arc welder.

All that spark energy enables engines to use more fuel with greater efficiency, leading to both higher horsepower and improved dependability, providing fuel flow is adequate. Armstrong notes that “pedal fests” (on-off-on throttle operation) disrupt the fuel system and can lead to a variety of failures. Fuel pumps have also been dramatically upgraded throughout the years to provide reliable flow under a wide range of engine demands. Prock notes that current pumps, which look like pieces of jewelry, put out about 100 gallons per hour. That level of fuel flow is vital to keeping an engine together, especially at mid-track where fuel demand is especially high.

Electronics have also played a major role in the evolution of nitro-burning engines. Data-logging equipment that records numerous aspects of engine and vehicle operation during a run provides information that is invaluable for tuning and diagnostic work. In addition to mechanical operation, data-logging equipment also provides insight into track conditions and driver actions.

One of the continuing problems faced by nitro racers is head gasket failure. NHRA dictates that the original 426 Hemi head bolt spacing has to be maintained. That spacing is relatively wide considering the forces that head gaskets have to contain. But with the mandated use of engine blankets, more commonly known as diapers, when a head gasket or any other component fails, most, if not all of the debris is contained, as opposed to being deposited on the track surface.

Mechanical, metallurgical and technological improvements have certainly contributed significantly to the extension of Top Fuel engine life, but perhaps of equal importance is the decrease in run time. As Armstrong notes, “These engines only have to run less than four seconds at a time.” 

Drag Car Engine
Current editions of nitro-fueled engines are worlds apart from their predecessors of just a few years ago. In addition to dramatic improvements in blocks, heads and internal components, fuel motors now sport magnetos that generate enough energy to light up a small city and fuel pumps that could supply a fire hose. Blower technology has also advanced to the point of your analogy of choice. 

Drag Car Engine
Blocks machined from billet aluminum have become the standard for nitro-burning Hemi engines. Billet material is inherently stronger than castings.

Drag Car Engine
As with blocks, cylinder heads machined from billet aluminum are now standard-issue nitro Hemi parts. CNC machining makes it all possible.

Drag Car Engine
Billet crankshafts are nothing new, and the current generation has benefited from lessons learned throughout the past 20-plus years. Improvements in materials and processing have enhanced strength and durability.

Drag Car Engine
Another example of “mass saves your ass”: connecting rods, pistons and wrist pins are significantly stronger than in the past as a result of improved metallurgy and increased mass.

Drag Car Engine
Durability doesn’t win races unless it’s accompanied by a sufficient amount of horsepower. That’s achieved in part through the use of large diameter valves, roller lifters and camshafts with generous amounts of lift and duration.

Drag Car Engine
Like just about every other component of a current nitro-burning Hemi, the rocker arm assemblies are works of art. The unique rocker geometry demanded by the hemispherical combustion chamber configuration makes rocker design especially challenging.

Drag Car Engine
As the old saying goes, you can’t tell where you’re going unless you know where you’ve been: instrumentation that monitors the exhaust and other engine functions provides valuable insight into the engine’s combustion efficiency.

Text by Dave Emanuel and Photos by Auto Imagery

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