Chrysler Hemi 5.7 Liter 345ci - Hello Hemi

Chrysler's new Hemi may well be the definitive small-block of the new century.

By David Vizard

In the small-block popularity race Mopar offerings (whatever they may have been at the time) were always a distant third to Chevy and Ford. This was in spite of building some good engines but, whatever their advantages, they were subtle and went largely unappreciated. Add to this chassis' that were outpaced by Ford and GM and it's not hard to see why the Dodge/Chrysler small-blocks where never a big hit. A little over a year ago the new Dodge/Chrysler small-block Hemi hit the scene. It was launched on a wave of charismatic nostalgia (don't you just love their TV ads?) hearkening back to the time of the big-block Hemi's dominance in the '60s and '70s. Chrysler pulled it off then by throwing in a little advanced technology at a time when big inches plus some run-of-the-mill refinement usually got the job done.

The world has changed significantly since the big-block Hemi days, and today a little advanced technology won't come even close to making the grade. These days it takes considerable high-tech refinement to meet market needs. Assuming the intent is to produce the engine-of-choice for the performance enthusiast market (that's us), Chrysler's competition may look like the '55-'99 era small-block Chevy, but in reality, GM has produced their own replacement for the traditional small-block Chevy. It's the LS6. Make no mistake, this is a world-class engine and is streets ahead of Ford's 4.6/5.4 modular engines. It seemed the LS6 was set to become the traditional small-block Chevy replacement but the picture could change dramatically with the introduction of this new Hemi.

Assuming the 5.7 Hemi is at least on a par with the LS6 in every other respect, then its advantage over the LS6 is that it has a valve and port configuration that will, size for size, always out flow the LS6's parallel valve design. This though, won't count for much unless the rest of the engine is also top notch. This being the case, before we get serious about determining the power potential of the new Chrysler Hemi head, we at least need to make some determination of the potential of the rest of the engine.

A call to Chrysler proved they were more than willing to do a show and tell about their new creation. As a result, we got to spend a highly informative day up there looking at hardware and talking to some of their top engineers. Here is just a fraction of what we found out from what proved to be a progressive-thinking group of engineers.

Bottom End
A section through Chrysler's cross-bolted-mains Hemi block shows it to be a precision casting. This means that for its 5.7-liter (345ci) displacement, it is lighter than a typical comparable displacement engine, even though it has a deck height 225 thousandths taller than a small-block Chevy's 9.025 inches. A weight saving policy consistent with the needs of a high-performance engine appears to have been enacted throughout the design of this engine. As near as can be determined at this stage, a fully-dressed unit is well over 100lbs. lighter than a traditional small-block Chevy. The down side of the weight saving due to casting precision is that big over-bores are out. Although experience may paint a different picture, this block, for a real performance application, should probably not be bored more than .030 over. Having said that, the factory appears to have built in room to grow this block to significantly more cubes. Our guess for the future is that 400 inches is practical and the magic 426 inches might just be possible.

In keeping with the current trend for smaller bores and longer strokes for emission reasons, this engine sports 3.917-inch bores on 4.46-inch centerline spacing. With a stroke of 3.578 inches this yields 345 cubes or 5654cc. A 4.46 bore spacing (60 thousandths bigger than a small-block Chevy) could, if at a later date the bores were siamesed, accommodate bores as big as 4.200 inches.

Cognizant of the fact that longer strokes mean greater inertia loads, Chrysler's engineers made a conscious effort to design a lightweight heavy-duty piston. The finalized cast eutectic (just less than 12percent silicon) alloy slipper-style piston has more in common with a race piston than what is normally perceived as a heavy-duty truck piston. At 413 grams, it certainly is light but that's less than half the story. It has what must be some industry firsts for high-volume production engine pistons. The most noticeable is the use of application-specific coatings. The hard anodize/coating used on the top ring land acts as a combined thermal barrier /anti-micro weld mix. This has allowed the placement of the top ring just 3mm from the top of the piston. This move both enhances power and cuts emissions. The rings, at 1.5mm (1/16) wide are also significantly thinner than is seen in most production V-8s. Right under the top ring is another race piston feature--a reservoir groove. Its use helps reduce the pressure that can build up between the top and second ring which, if not held in check, will unseat the top ring from the bottom of the groove. Another obvious feature is the coated skirt. The coating not only allows a one-size-fits-all production-line bore piston, but also permits a closer fit for better ring seal and reduced piston noise. Add to this a short, lightweight, high-set wrist pin and it becomes apparent this is no ordinary production truck piston.

Like the piston, the powder metallurgy connecting rod has been designed to minimize its weight consistent with retaining the required strength. The powder metallurgy process, first used for high-performance applications by Porsche, allows for a more precision part than regular forging. This has allowed the deletion of the commonly seen balance pad, which saves a bunch of weight right off the bat. Add to this a cap bolt instead of a through-bolt, plus some thoughtful design, and the result is a 6.24-inch rod that is significantly stronger but marginally lighter than a stock small-block Chevy's much shorter 5.7-inch rod. This long rod also produces a more favorable 1.744:1 rod/stroke ratio rather than the 1.638:1 a 350 small-block Chevy has.

Moving on down to the crank case, we find a crank that appears to cater strongly to minimizing main bearing loads by having the inner counterweights larger than on a small-block Chevy where they are typically too small. This has not resulted in an unduly heavy crank because the counter weights balance out a smaller mass due to the lighter weight of the piston/rod assembly. The crank, with its radius leading edge counterweights, is definitely performance oriented and to further augment this fact, there is a windage tray directly under it. The important job of damping out crank torsional vibrations is handled by a lightweight damper that serves double duty as a pulley for the serpentine belt.

Valve Train
To minimize the pushrod length required to operate the rockers in a taller Hemi-style head, the cam was located high in the block, thus eliminating the traditional V8 lifter valley. The cam itself is a built-up piece that has much larger than normal journals and lobes. This results in less side loading on a roller-style lifter. To keep the weight down, the cam is hollow. As expected for an engine that would have good high-rpm breathing, appropriate attention was paid to the valve train. Developed as a system package, the stock valvetrain is good to over 6200 rpm but that is as far as the factory has tested it. The key to the success of this hydraulic roller valvetrain is in the design of the valve spring and rocker assembly. Firstly, the valve spring is of the beehive type. This is a much more effective spring than a regular parallel-wound spring. This means it can be far lighter for a given rpm requirement. This results in less lifter collapse and the subsequent loss of valve lift that it causes. The valves are operated through a very compact design of investment cast steel rocker. Using finite element analysis at the design stage, the rocker looks to be just about devoid of any excess material that might be along for a free ride. This has produced an incredibly light part that has about half the inertial mass of a typical aluminum rocker. The form, and more importantly, the size of the rocker was optimized for a valve lift in the 500 thousandths range. As such, this compact design is not so good for ultra high-valve lifts. Chrysler's engineers estimate about 580 thousandths is probably as much as it can accommodate. Fortunately, this is not a big issue for a Hemi-style head because the valves reach higher flow values sooner within the lift envelope than do parallel-valve heads.

Induction System and ECM
The induction system has, after its aeronautical counterpart, been christened a fly-by-wire system. The fly-by-wire system, which found its way onto F1 cars in the mid '80s, has the throttle butterfly opened not by the action of the driver's foot on the pedal but by the computer. In such a system the "throttle" pedal is simply a means for the driver to communicate torque demand to the engine. By going this seemingly complex route many functions have actually been simplified. For instance, no separate idle control or cruise control system is needed, as the electronic throttle will, via the computer, automatically handle such chores. Also, engine over speed can be handled by the throttle closing down rather than the dropping of sparks or cutting fuel.

The intake manifold itself met some pretty steep design criteria. Not the least was that it must not cause an excessive restriction to airflow. This requirement had to be met while keeping noise to a minimum and packaging the whole deal as compactly as possible. The resulting intake looks like a work of art for the end user and a nightmare for the tool and die maker. The system has a tuned length from the filter to the plenum and from here each port runner to valve is a tuned length. Computer modeling and dyno testing has allowed the ports, plenum and filter-to-plenum runner length and diameter/volume to be sized near optimally for a wide power band consistent with street performance needs. The end product proved capable of delivering cooler air than an aluminum manifold while only restricting the intake port 10 percent at full valve lift. That is about the same as a good aftermarket performance intake.

Unlike an LS6, which has a mass air system that allows the computer to adjust the fuel flow for any given airflow so that the mixture is always right (theoretically at least), the Hemi has a speed density system. This means that any increase in airflow into the engine goes unseen by the rest of the system. If a modification is done that increases mass airflow, such as a cold-air package, the computer has to be reprogrammed accordingly. Fortunately, two companies, Squier Inc and Street & Performance are working on this now and already have what is needed for transplants of the Hemi into other vehicles.

Ignition System
Without actually running bench tests it is difficult to evaluate an ignition system. Having said that, it would be difficult to believe that the engineers at Chrysler went to the trouble of putting two plugs into each cylinder and would then fall down on the job of firing them effectively. The system, with a coil for each cylinder, looks, on paper at least, as if it can do the job.

Conclusions So Far
Unless there is something that has been under-/over-estimated or overlooked, it seems that this new Hemi has evolved into something as high-tech as the LS6 engine. If so, this means (from the street performance enthusiast point of view) it is hot right out of the crate (see output curves in Fig 1) and chock full of major power potential. Its crowning feature is going to be the cylinder head, so let's take a serious look at this all-important piece of hardware now.

Cylinder Head
The advantage of a hemi design of combustion chamber is that the valves (and most importantly the intake valve) are always moving away from the shrouding effect of the cylinder walls (Fig 2) as they lift off the seats. However, the new hemi is not actually a true hemi as per its 426-inch predecessor. The hemi style of combustion chamber was put to good use during WWII when the output of supercharged aero engines could basically decide the fate of nations. For a two-valve combustion chamber, the hemi layout not only allows the largest valves to be accommodated but also to have the highest flow efficiency. The downside of a true Hemi configuration is that it does not respond well to a high compression ratio that inevitably requires a combustion-inhibiting, high-domed piston. For an engine with a typical bore/stroke ratio, this means it works great with a supercharger and CRs less than 8.5:1, but not as a normally-aspirated unit with 10:1 or more. To get around this problem the new Hemi has the sides of the true hemi form filled in. With the two spark plugs it is equipped with, this allows all the advantages of a true hemi, including blower capability, along with the ability, if required, to successful utilize high compression ratios.

Flow Capability
We can see that Chrysler's engineers were targeting the best two-valve head possible. There are two important questions that need to be asked here: How well did they succeed for the head in stock form and, since no aftermarket heads are available, what is its porting potential? The graph, Fig 3, gives the answers here and you are going to like them. First, the intake port. The stock port with its 2-inch valve flowed a whopping 270 cfm at only .600-inch lift. It hit the peak flow figures, which are produced at .700-inch lift on a stock LS6, at only about 375 thousandths lift. This is good news but there is a lot more. Peak figures are not the whole story. Good mid-range figures are also important. The new Hemi did extremely well here. At 250 thousandths lift, the stock head was nearer a $10,000 Winston (Nextel) Cup head than it was to even a good modified parallel-valve head.

A check on the intake port velocity (Fig 4) showed the intake to be a super high-speed port with valve-to-port areas very similar to what is seen in Formula One. Velocity probing showed 90 percent of the port flows at a velocity greater than 90 percent of maximum. This is far better than a typical 23-degree performance head for a small-block Chevy or, for that matter, the LS6.

The exhaust port showed the same high-function trend by hitting 161 cfm at 600 thousandths lift through its 1.55-inch valve. It also had a far better than average port velocity and velocity distribution (Fig 5).

We spent a day and a half on the flow bench in an effort to find out what this head does or does not like in the way of port mods. We are sure there is much still to come, especially with some bigger valves, but we did find what it took to produce, at 600 thousandths lift, some 302 cfm on the intake and 195 cfm on the exhaust. As the nearby photos show the work to achieve this proved simple. In essence, the porting involves little more than just tidying up what Chrysler's engineers provided in the first place (great job guys).

Final Conclusions
The bottom line here we think is that even in today's world, the Hemi, a year after its introduction, has got more going for it as a hot rod motor than perhaps the small-block Chevy did in 1956. Even with the limitations on valve lift brought about by the stock rocker, the cylinder head has more than enough flow potential to clear the 600-hp barrier and probably do it (relatively speaking) with ease. So will it be a success? This depends on the factory as much as anything. First, they must put it in vehicles that inspire hot rodders to do something with them. The Hemi truck is a great start here as the sport truck market is big. Introducing a viable pony car to compete with the Mustang would probably clinch the deal in that department. But it will still need a little more than just the right vehicles. The factory will have to support this engine in aftermarket parts. This appears to be happening but as far as we are concerned, it can't happen fast enough. We're avid Chevy fans and have owned many Chevy vehicles over the years, but this 5.7 Hemi has totally convinced us our next truck just has to be a Hemi-powered Dodge.