The Gen III Hemi platform has been proven to be an extremely capable engine family, powering everything from Ram trucks to the first seven-second Stock Eliminator effort. While the Gen III Hemi’s mechanical design is extremely robust, it also benefits from the same nostalgic brand recognition as the vehicles it powers.
Jason Fenske of Engineering Explained has recognized the awesomeness of the Hemi as well, and produced a video in his “The Best Engines” series dedicated to it. If you’ve followed EngineLabs or Engineering Explained for any amount of time, you know that if something receives mechanical praise from Fenske, it’s usually well-deserved.
The Nitty Gritty
So, what makes the Hemi – The Hellcat Hemi, specifically – such an awesome powerplant? For one, it was the most powerful engine in the Chrysler line at 707 horsepower, until the Dodge Demon came on scene and bumped it. However, there is still a price-to-performance ratio the Hellcat Hemi has, which is still hard to touch within the Mopar lineup.
The 6.2-liter roots-supercharged V8 is a unique animal, producing 707 horsepower at 6,000 rpm and 650 lb-ft of torque at 4,800 rpm, with a 6,200-rpm rev limit. “650 pound-feet at 4,800 rpm is certainly impressive,” says Fenske of the Hellcat’s torque rating. “However, one of the little known facts is that the engine is making 411 lb-ft at only 1,200 rpm. That means it makes more torque at 1,200 rpm than most cars on the road make, at any RPM.”
It comes from the factory with a cast-iron block, with the entire rotating assembly beefed up to withstand all the power. Aluminum Apache cylinder heads, with the namesake hemispherical combustion chambers top off the shortblock, and the Gen III’s dual spark plugs are retained. “Because of the two-valve design, the spark plug can’t go into the center of the head, so they have [a spark plug] on either side of the valves,” Fenske explains.
The 103.9mm bore and 90.9mm stroke (approximately 4.090-inches by 3.579 inches) measure out to an actual displacement of 6.166 liters, or about 376 cubic inches. A healthy factory cam, with 0.561 inch of lift on the intake side, and 0.551 inch of lift on the exhaust side control the non-typical valves. “[The Hellcat engine] has sodium-filled exhaust valves to keep them cool, and hollow-stem intake valves, to reduce the reciprocating mass,” says Fenske.
With the 9.5:1 static compression ratio, and 11.6 pounds per square inch of boost from the supercharger, pressures in the cylinders can get pretty high, especially by production engine standards. “Peak cylinder pressure of the engine is 1,600 psi. That means that every piston has about 21,000 pounds of force pressing it down on the compression cycle,” says Fenske.
However, where the Hellcat Hemi engine design really shines is in the intercooler setup. Sandwiched in the plenum between the supercharger discharge and intake ports are four air-to-water cores, rated at about 1,060 cfm of airflow, which keep the charge temperatures in check. With an ambient air temperature of 100°F, the post compression temps hover near 250°F, but are brought down to only 140°F once they pass through the intercooler.
In order to make 700-plus horsepower, there is a serious fuel demand placed on the Hemi’s 600 cc/min (57.1 lb/hr) injectors. Fenske puts that into perspective, saying, “At wide-open throttle, the 600cc injectors can empty the Challenger’s fuel tank in just thirteen minutes.”
The fact that the Hellcat Hemi engine program has undergone over two million hours of dyno validation at the Mopar factory, and the fact that each engine undergoes a 42-minute validation run before going into a vehicle, also adds significantly to the reasons why it is such an awesome engine platform.