In this installment, we'll take a peek inside the Chrysler Hemi siblings - the garden-variety 5.7L, and the rip-roaring 6.1L version found in SRT8 products.
Inevitably, someone will ask, "are these engines really Hemis?" First, we should define the term Hemi - it's short for "hemispherical combustion chamber", and it relates to the distinctive half-spherical combustion chamber shape used in certain Chrysler V8s back in the 50s and 60s. The 426-cubic-inch second-gen version dominated stock-car racing and still has its place in drag racing; it wasn't really a great street engine, but regardless it built name recognition like few other engines.
OK, so are these really Hemis? The modern rendition uses two "squish pads", one each to the front and rear of the chamber; this helps keep the air-fuel charge closer to the center of the chamber and encourage additional turbulence. Ideally, a combustion chamber would resemble a sphere, as this results in the least surface area for a given volume (that means less heat rejection to the coolant) and the smallest burn distance for a spark plug. Domed pistons in a hemi chamber results in a thin skin of charge; imagine a golf ball sitting next to an orange rind and you get the idea. The squish pads improve the situation significantly, with the only drawback being semantics regarding the trademark name. Squabbling about the chamber shape neglects the most important attribute of the Hemi design - the valve placement. We'll get to the importance of that in just a moment, but suffice to say that this new Hemi deserves to carry the name.
There's nothing too special about the engine when viewed from this angle. We can see the size of the valve covers, which are somewhat larger than typical for an OHV engine. We can also see the coil packs mounted on the covers; normally, each coil also sports a plug wire that jumps to a cylinder on the opposite bank. Each coil drives two plugs in series, and each coil fires every revolution of the crank (instead of every two revolutions, as would be typical for a four-stroke engine). This yields a pair of spark plugs for each cylinder that are fired both on the compression and exhaust cycles - likely a necessity to improve the ability to light off the sparsely-distributed mixture in the Hemi combustion chamber.
Note the complex intake manifold geometry. The large plenum and long runners help maximize the torque across the rev range and help the engine to come on strong right off idle without excessively compromising upper-end power.
Ah, yes - the above three shots show the reason that Hemis have built a reputation for horsepower. The valves are arranged perpendicular to the cylinder bank, which puts the intake valve at the inside of the cylinder and the exhaust valve at the outside. This means that air has a nice straight shot from the intake runner into the port and right down into the cylinder, instead of going through a tight radius and across the back of the valve as is the case with a traditional "wedge" chamber.
The angle of the valves allows them to travel towards the center of the cylinder and away from the cylinder walls. This helps prevent the phenomenon of "shrouding", where a portion of the valve becomes ineffective. Due to this, the flow of Hemi heads is quite impressive across the full range of valve lift.
Here's the domed pistons that are required to achieve the 9.6:1 compression ratio with the relatively large combustion chambers. Unfortunately, this peek into the lifter valley doesn't show the hardware responsible for the engine's Multi-Displacement System that deactivates four of the eight cylinders during part-throttle operation. Oil pressure is used to control a lost-motion system in eight of the sixteen lifters; essentially, a pin is pulled that allows the lifters to collapse during cylinder deactivation.
If it has an orange block and black crinkle-finish valve covers, it must be something special...
Here's the high-perf 6.1L version of the Hemi. The first thing to note in the picture above is the sweeping long-tube headers; such geometry was traditionally not found on factory exhaust manifolds, but modern sheetmetal fabrication techniques have achieved wonders. Chrysler claims that they're worth an additional 12 HP over the 5.7's cast manifolds. Not quite so visible is the extra 3.5mm of bore that gives the 6.1L its extra displacement.
In the above two pictures, note the substantially different intake manifold geometry. The runners are shorter, with a longer cross-section; that's a classic recipe for moving the powerband higher in the rev range. Think of this as a modern interpretation of the classic tunnel-ram intake.
The above three pictures show just about everything one needs to know about the Hemi's ability to make power. The airflow paths into and out of the cylinders are smooth and sweeping, with generous cross-sections and no dramatic change in direction. This engine was built to breathe. The 6.1 gets larger intake ports, and the large bore enables the use of larger valves.
Note the flat-top pistons. Due to the increase in displacement, it was necessary to eliminate the dome found on the 5.7's pistons in order to keep the compression ratio down to a reasonable 10.3:1.
We can also see the odd lifter angle of the Hemi, as well as the toothed reluctor wheel used by the crankshaft position sensor. What we can't see are the details of the 6.1's camshaft, which utilizes more lift and duration to make the most of the high-flowing intake and heads.
Barely visible here are the oil squirters that keep the pistons cool. This is certainly not a typical feature for a mass-production domestic V8.
This particular cross-section of the head shows the twin spark-plug bores.