Hemi (from "hemisphere") is a design of internal-combustion engines in which the cylinder head's combustion chamber is of hemispherical form. The chamber design puts the intake/exhaust valves in-line, rather than side-by-side, allowing for better flow of air through the head. The spark plug in the center of the chamber makes for better ignition of the fuel/air mixture. These aspects help make the hemi-type engine more efficient and powerful.
Although hemispherical cylinder heads have been used in some engines for a very long time, the Chrysler Corporation has made the most effective use of them, producing them in the greatest numbers and with the most publicity, and thus the word Hemi generally refers to the Chrysler-built engines.
The hemispherical cylinder head increases the engine's efficiency through reduced thermal energy loss and increased airflow through the engine, but drawbacks such as increased production cost has meant that it has been a rare design.
Three generations of hemi engines have been produced by Chrysler, the first (the Chrysler FirePower engine) in the 1950s, the second from the mid 1960s through the mid 1970s, and finally in the early 2000s.
FirePower
Hemi's heritage in the automobile industry began in 1951 when Chrysler introduced a 180 hp (134 kW) Hemi V-8 with a displacement of 331 in3 (5.4 L). Starting in 1951, Briggs Cunningham chose to use the Hemi, and in 1953 the Hemi powered Cunningham C5-R won first in his class, and his cars finished as high as third place overall at the 24 hours of Le Mans.
This engine is covered in the Chrysler FirePower engine article.
Classic Hemis
The classic Hemi engines of the 1960s featured true hemispherical heads, and splayed (perpendicular) valves.
426
Later years saw several improvements, including the development of a 426 in3 (7 L) Hemi in 1964, which earned recognition when it placed first, second, and third in the 1964 Daytona 500 NASCAR race. This famous engine was sized for NASCAR with an oversquare 4.25 in (107.9 mm) bore and 3.75 in (95.2 mm) stroke like the Chrysler RB engine big-block. NASCAR effectively legislated the Race Hemi off the track.
There were many differences between the Hemi and the Wedge-head big-block, though, including cross-bolting and a different head bolt pattern. The 426 Hemi was produced from 1964 through 1971, and new crate engines and parts are available today from Damlier-Chrysler Corporation. The Street Hemi version was rated at a very conservative 425 hp (317 kW) with two Carter AFB carburetors
This engine was used in the following vehicles:
Modern Hemi
The modern Hemi is not really a hemi at all - it has a polyspherical combustion chamber similar to that in many other modern engines. A true hemi head was considered in the design phase, but it was rejected and only the perpendicular valve arrangement remains.
5.7
The 5.7 liters, was released in 2002 on the Dodge Ram 2500 and 3500 pickup trucks to replace the Magnum 5.9 engine. The buying public was enthusiastic about the engine, and snapped up Rams and Hemi-powered Dodge Durangos at an amazing rate. Chrysler reacted by putting the 5.7 liter Hemi in the 2003 Dodge Ram 1500, 2005 Chrysler 300C and Dodge Magnum, and the 2005 Jeep Grand Cherokee.
The 5.7 Hemi in the Ram delivered 345 hp (257 kW) and 375 ft.lbf (508 Nm), but 340 hp (254 kW) and 390 ft.lbf (529 Nm) for the 300C and Magnum. It is a 90-degree V8, 2-valve pushrod design displacing 5654 cc (345 in³), with a bore of 99.5 mm (3.9 in) and a stroke of 90.9 mm (3.5 in). The block appears to be based on the Chrysler LA engine as it has the same stroke.
A new displacement on demand technology called Multi-Displacement System (MDS) is used in some versions which can shut off a bank of cylinders under light load to improve fuel economy.
The 5.7 liter Hemi is made at Chrysler's Saltillo, Mexico engine plant.
The Hemi was on the Ward's 10 Best Engines list for 2003 through 2005.
This engine is used in the following vehicles:
6.1
Chrysler is developing another modern Hemi, this time at 6.1 liters to replace the Ram Tough V10. The engine is bored-out to 103 mm (4.1 in), but many other changes were made to allow it to produce 425 hp (317 kW) at 6200 RPM and 420 ft.lbf (569 Nm) at 4800 RPM. The engine block is different, with revised coolant channels and oil jets to cool the pistons. A forged crankshaft, lighter pistons, and strengthened connecting rods add durability. A new cast aluminum intake manifold is tuned for high-RPM power and does not include variable-length technology. Chrysler's Multi-Displacement System is not used.
Applications:
See also
External link
- Mopar's infamous Hemi http://www.thehemi.com/
- How Stuff Works/Hemi page http://auto.howstuffworks.com/hemi.htm
- New Hemi technical article at AutoSpeed http://www.autospeed.com/cms/A_2092/article.html
Last updated: 02-02-2005 05:26:41
Last updated: 05-03-2005 17:50:55
