HEMI

This simple word inspired awe and fear from 1966-71, and still does today.  An original Hemi vehicle from Dodge or Plymouth is worth twice as much, or more, than its non-Hemi version – and might be a major star at Barrett-Jackson, a top auto auction. 

First of all, what is a Hemi?  Usually it refers to the 426 Hemi V8 used in Dodges and Plymouths from 1966 to 1971.   The Dodge models included the Charger R/T and Daytona, the Coronet R/T, and the Challenger; in Plymouth this would be the Barracuda (“Hemicuda”), the GTX & Road Runner, and the Superbird.  Plymouth did not make an equivalent of the Charger. 

Ok, fine.  Still, what does it mean?  It means hemispherical combustion chamber.  Each cylinder of the engine could be visualized like a grain silo you see at a farm: a tall cylinder with a hemispherical top to it.  The top has two circular openings, the valves. 

Four-stroke cycle.  Bear with me, this may be interesting (maybe not).  The piston goes up and down the cylinder, lubricated with oil.  Freudian, no?  Sometimes a piston is just a piston.

Intake.  The piston goes from its highest position (top dead center – TDC) to its lowest position (bottom dead center – BDC) in the cylinder.  The intake valve is open.  The piston’s downward movement sucks in the air-fuel mixture waiting outside the intake valve, which ideally fills the combustion chamber and cylinder.

Compression.  Both valves close, and the piston rushes up from BDC to TDC.  In doing so, it compresses the air-fuel mixture.  The ratio of this compression is the “compression ratio”.  The higher the compression ratio, the more power the engine can make, but it also makes the air-fuel mixture hotter and more likely to spontaneously combust without the spark plug’s assistance.  Highly volatile gas has a LOW octane rating, whereas relatively less volatile gas has a HIGHER octane rating.   This is why performance cars typically require 93 octane while economy cars can run on 87, and why running the wrong octane is a bad idea:  87 in an engine needing 93 may result in detonation, while 93 in an 87 engine is wasted.

Firing.  The spark plug fires, igniting the air-fuel mixture.  This is not an explosion, but rather a controlled burn which spreads out from the spark plug like a blooming flower.   Both valves are closed.  This is the stroke which produces the engine’s power: the piston goes back from TDC down to BDC.

Exhaust.  The exhaust valve opens, the piston goes from BDC back up to TDC, and the exhaust gases are pushed out of the combustion chamber.  Bye bye!

In a HEMI engine, the combustion chamber is hemispherically shaped, the intake valve is canted towards the intake side, the exhaust valve is canted towards the exhaust side, and the spark plug is close to the top of the combustion chamber between them.  Thus the valves are lined up parallel to the flow of air/fuel/gases through the combustion chamber.

Aren’t all engines set up this way?  Actually, no.  Most use a “wedge shape”.  The chamber shape is raised on one side and slants down to the other.  Think of those soft cloth caps working class men wore in the early twentieth century.  Nowadays pretentious people like Samuel L. Jackson wear them backwards.  Anyhow.  The valves are side-by-side, perpendicular to the flow of air-fuel/gases, with the spark plug coming in from the side rather than the top.

Looking at it from the outside, the HEMI engine has its spark plugs coming in the top of a large valve cover, whereas a wedge engine has the spark plugs coming into the side of the engine.

Power.   It’s not surprising that Hemi engines do in fact make a good deal of power.  What’s counterintuitive, though, is that the wedge design is remarkably competitive.  So much so, that the Hemi is the exception and not the rule.  Why is this?  In other words, why doesn’t everyone use a Hemi design?

            First, the Hemi head is much larger and more complex than a wedge head.  This makes it heavier and more expensive.  It makes the engine much wider and more difficult to fit in to an engine compartment.  Hemi cars needed special modifications and different suspensions and chassis stiffening.  Putting a Hemi in a car was a major undertaking.

            Second, as noted above, the wedge design works very well.   This includes all GM engines, all Ford engines (though the Boss 429 could be considered a hemi design), and the 440 V8 from Mopar.  With three two-barrel carburetors, the so-called 440 Six Pack was actually very competitive with the 426 Hemi on the street, and many enthusiasts considered it a better street engine than the Hemi. 

            In fact, the current run of GM performance engines in Camaros and Corvettes still keep the wedge design, although the LS combustion chambers are much different than the prior wedge heads.  The same holds true with Ford V8s, even in Shelby format.  While undeniably impressive, the Hemi design is not the only one which makes serious power. 

First Hemi.  From 1951-58 Mopar made the first generation Hemi engine, a 392.  This can be distinguished from the second generation by having a rear-mounted distributor the way most other engines are; the 426 has a front-mounted distributor, as all Mopar big blocks do.  The 392 was dropped, the 426 picked up later, and nowadays the 392 is esteemed only by those preferring the 50’s cars and insisting on authenticity.

Second Hemi.  The legendary 426 Hemi:  from 1966-71.  If someone refers to a classic era Hemi, they probably mean this one.  The Hemi was also used in NASCAR at this time; but the NASCAR engine only had one 4 barrel carburetor, while the street hemi had two – “dual quads”.  The street hemi also used mechanical lifters, which made it an especially noisy engine; to quote Muscle Car Review on a Hemi Road Runner, “even blindfolded you know it’s not a Lexus.” 

Street racing the 426 Hemi.  By now many of us may have seen the YouTube video which claims to show a Challenger Hellcat being blown away by a Tesla electric car at a dragstrip.  Of course, the Hellcat owner redlighted, i.e. immediately disqualified and thus no reason to blaze down the track.   Hemis are good engines, but a poor driver is always fair game for competitors with fast cars and better skills. 

            Jim Wangers, famous for promoting Pontiacs in the 1960s, gave some pointers in his book Glory Days.  What works well at the dragstrip might not translate well to the street: the 426 Hemi had a bad reputation for fouling spark plugs at idle and low speed.  Tactic?  Spool around in traffic for awhile before the race begins, so the Hemi driver’s engine is fouled.  Also, break off the race sooner rather than later, claiming victory; do not let the race go on further, as the Hemi WILL catch up with a vengeance.  This is how Royal Pontiac’s 421 4-speed GTO vanquished a Hemi Road Runner. 

NHRA.  The Hemi never died in drag racing.  Current top fuel dragsters run supercharged Hemi engines with nitromethane fuel: 8000 HP, with the supercharger draining off 850 HP by itself.  The cars go from 0-60 in half a second, cover the quarter mile at 4.4 seconds at 335 mph.  That’s fast.

Modern Hemi.  In 2003, after 32 years of dormancy, Mopar reintroduced a hemi V8 engine, in 5.7L displacement.   The new version has an aluminum block, aluminum heads, electronic fuel injection, and electronic ignition.  No more dual quads!   The modern Hemi came in Jeep Grand Cherokee, Ram pickups, the Dodge Durango, the Magnum (an oversexed station wagon) (2005-2008), the Chrysler 300M, the Charger (2006-present) and Challenger.  Like the drag racing versions, the modern version has two spark plugs per cylinder.  They’ve also introduced MDS (Multiple Displacement System), which is a cylinder deactivation system supposed to improve fuel economy without sacrificing power.

SRT8.  This is a 6.1L version of the engine in Chargers and Challengers.  The 370 HP of the 5.7L is bumped up to 425 HP.

Hellcat.  Very recent – and very powerful.  Finally the factory put a supercharger on a Hemi, and the result is 700 HP.  Be careful.

Crate Engines.   Mopar actually sells 426 Hemis (and an improved, larger version, the 528 Hemi) as crate engines.  They’re brand new and have a warranty, but they are very expensive.  Potential uses:

1.   Replacing a valuable original Hemi engine;

2.   Replacing a non-Hemi engine (e.g. 383 or 440) in a non-Hemi car, e.g. Road Runner or Charger which was originally available with a Hemi;

3.   Here’s the fun part:  putting in a small, light car like a Demon or Duster which never came with a Hemi.  Don Yenko was terrified by 427 Novas, imagine how terrified you can get with a Hemi in a Demon.