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post #1 of 17 (permalink) Old 12-07-2014, 11:58 AM Thread Starter
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A look at the Hellcat Hemi

For those that want a more indepth look at the new Hellcat Hemi check out this article by Mopar Muscle We Take A Hard Look At Dodge's Paradigm-Shifting 707hp Hellcat Hemi It provides a lot of good information on the Hemi's internal workings and many good photo's showing the differences and similarities between the new 6.2 and the old 6.4 Hemi.

Here are just a few excerpts from the article:


91 Percent New
When Dodge announced that the Hellcat was 91 percent new, a lot of questions went through our head. The word new is a hot button that we in the media (and PR guys at manufacturers) like to use to elicit interest, but to a hot rodder who likes to mix and match parts Frankenstein style, it can spell frustration. In this regard, the Dodge boys were guarded in their answers, and you'll be left to speculate on your own. Can the Hellcat crank fit in a 392 Apache block? Can you put a Hellcat manifold and blower assembly on a 6.4L, 6.1L, or 5.7L long-block? Should you use a Hellcat block for a *****in naturally aspirated buildup? What kind of interchangeability is there between the Hellcat's parts and previous Hemi variants? All of these are great questions, and nobody will know the answers for certain until somebody tries. Nevertheless, we have some facts that give important hints about mixing and matching Hellcat parts, especially with the 392 Apache variant.


One of the questions we asked the engineering team was, "what do you mean by 91 percent new?" The answer is: Relative to the 392 Apache Hemi, the cumulative dollar value of the parts that got design changes or material upgrades is equal to 91 percent of the engine's value. Parts that got changed that were of higher value, such as the block and cylinder heads, carry more influence on the equation than small, simple stuff like fasteners. Moreover, and perhaps more importantly, it turns out "new" doesn't necessarily mean incompatible with parts on an earlier Hemi variant. And while Dodge understandably won't confirm, for instance, that a Hellcat piston will fit in an Apache's bore, the evidence is that it could. That kind of speculation is our job, and we'll do our best to paint an accurate picture here.
To sum it up: The Hellcat is pure Hemi architecture, and you will immediately recognize it as such. With the large 4.09-inch bore of the 392 Apache, and the short 3.58-inch stroke of the 5.7L and 6.1L Hemi, there is much in common with prior Gen III Hemis. The engineering team added to the Hemi's already great design features like the hemispherical combustion chamber and the deep-skirted, cross-bolted block by leveraging them with evolutionary changes in materials and design. They did not change key dimensions like bore spacing, bore diameter, main and rod journal diameter, rod length, deck height, cylinder boltholes, cam location, cam journal diameter, or other critical areas. And for good reason—the Hellcat needs to package into the same space as other Hemi variants and be easily serviced, so the more that can stay the same in meaningful ways means less cost for the car, and we're all for that!
Short-Block
The engine block is the bedrock of the Hellcat. All 707 of the Hellcat's horsepower get transmitted through the block, which is made of cast gray iron. Outwardly, it looks very similar to the Apache block that underpins the 392, but there are two primary changes—three if you count the standard Hemi Orange powdercoat paint! During development, it was discovered that a significant change in the coolant circuit was needed in order to manage the Hellcat's increased thermal output. Specifically, the water jackets now continue all the way down to a level equal with the piston crown's bottom dead center. Second, the amount of material in the webbing between the bores and the main caps has been increased. Lastly, the piston oil squirters have been retargeted to cool a slightly different region of the piston, which itself is significantly different than the cast piston in the 392 Apache. These revised squirters also have a pressure-sensitive lube valve to shut off flow to them at low pressure; this is done to prioritize volume and pressure to the rest of the lube circuit.

The difference in displacement between the 6.4L Apache and the 6.2L Hellcat is all in the stroke (3.72 inches versus 3.58 inches), so the rotating assembly is different dimensionally. The Hellcat's shorter-throw forged steel crank is actually machined from the same forging as the longer stroke Apache crank, but because of increased journal overlap, it's stronger. Moreover, the surface of the Hellcat crankshaft is induction hardened, and all journals are undercut, fillet rolled, and micro-polished to very tight tolerances. On the front counterweight you will see a laser-etched QR code; the manufacturing information for each crankshaft is unique, and the code is tied to a specific date, time, and place of manufacture. A line of alphanumeric code is also laser etched near the QR code and denotes a sequence of specific-sized select-fit bearings that are to be installed in each journal. The result is a bearing interface with extremely precise tolerances that can handle enormous amounts of surface loading.

On the end of that crank is a Hellcat-specific damper designed to withstand an impressive 13,000 rpm. That damper is fitted to the Hellcat's crank snout with a diamond-like carbon-coated washer and damper bolt that is 2mm larger than the one in the Apache. That snout likewise has a strengthened fillet radius for additional fatigue resistance. All that beefcake adds up to an assembly that can easily weather years of side-loading from the Hellcat's 10-ribbed drivebelt.

The Hellcat's forged crank is carved from the same steel forging as the 392 Apache, with a few key differences. The stroke is ground to a shorter 3.58-inch throw (the same as the 5.7 and 6.1), and all bearing surfaces are induction hardened prior to being given a fillet roll. A micro-polish finish is then given to all journals.

The Hellcat's rods—which are the same length as the Apache's—are of a forged powdered steel construction, and are upgraded relative to the Apache rod with proprietary metallurgy. The rod's small end receives a full floating pin in a bronze bushing; it has an interesting detail in that it is the full width of the rod's cheek on the compression side of the small end, but a machined taper was given to reduce its mass at the top of the small end. The Hellcat's steel piston pins have a diamond-like carbon coating, and are some of the tightest-fitting, precision-machined pins we've seen. And coming full circle, keeping those pin clearances tight at full rated power when the heat is on is the job of the piston oil squirters, which play a huge role in the thermal management on the top side of the short-block. Make no mistake, this is what we normally encounter when we examine top-shelf race hardware—there is no skimping here!

Moving up the rod to the piston, we see big changes relative to Apache. A forged piston was the only type considered by the Hellcat's engineering team. To wit: At full rated power each piston sees the force of 22,000 pounds on every combustion cycle at full-rated power. With strength a priority, the Advanced and SRT Powertrain team went to work with some very interesting tools, the first of which is a computer model called finite element analysis (FEA). This allows engineers to design a virtual piston and place virtual mechanical and thermal loads on it to identify weaker areas. It saves hundreds of man-hours prototyping and testing parts, and moves a more proven product to market faster. The next powerful tool the team used for the piston design was thermal telemetry. Advanced and SRT Powertrain built a prototype engine with thermo-couples attached to the pistons; as the engine runs through its battery of tests, it's possible to see in real time the thermal stresses across the piston. This proved out the earlier FEA work, while allowing the team to further refine the piston design.

A cursory inspection of the Hellcat piston alongside a 392 Apache piston reveals much. Most obvious is the Hellcat's forged design versus the Apache's cast design. And while both have anti-scuff, oil-shedding coatings on their skirts, the shape of the skirts themselves are quite different. The Hellcat's is noticeably wider at its base and its support ribs intersect more critically with the pin boss nearer the center of the piston. Up top, a gradual reverse dome gives the Hellcat a 9.5:1 compression ratio, while the Apache's slight positive dome delivers a higher 10.9:1 compression ratio. On the profile, most noticeable is the .070-inch taller compression height of the Hellcat slug (Hellcat specs out 1.28 inches to the Apache's 1.21 inches). That places the extra .070 inch (a total of .09 inch crown height) right up top at the piston crown, which allows the Hellcat to better withstand the Hellcat's higher temperature and cylinder pressure. Along with the shorter-stroke crank, it's here that we see the genius of the displacement give-back relative to the Apache—those .2 liters of lost displacement have endowed the entire short-block with the strength needed to back a 100,000-mile powertrain warranty that even covers racing.


(Lot's more good information can be found in the full article for those that enjoy knowing the detailed workings of their motors)


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post #2 of 17 (permalink) Old 12-07-2014, 01:56 PM
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Conflicting information........A356 aluminum cylinder heads do not have hemispherical combustion chambers.........


10 Things You Need to Know About the 707-hp Dodge Hellcat V-8 ? News ? Car and Driver | Car and Driver Blog


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post #3 of 17 (permalink) Old 12-07-2014, 03:14 PM
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Conflicting information........A356 aluminum cylinder heads do not have hemispherical combustion chambers.........
I thought it was kind of an open secret that the Hemi motor hasn't been a true hemi for quite some time? It's just a marketing term nowadays isn't it?
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post #4 of 17 (permalink) Old 12-07-2014, 03:20 PM
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I think the 5.7,6.1 and 6.4 are Semi-Hemi not perfectly hemispherical.....6.2 isn't a Hemi at all...
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post #5 of 17 (permalink) Old 12-07-2014, 03:21 PM
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it's a semi-hemi

2012 SRT automatic, PB 12.0490 with DR, then 11.56 with 5.5psi magnuson Sold
2017 hellcat A8 stock, 10.86 with DR's
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post #6 of 17 (permalink) Old 12-07-2014, 03:56 PM
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I thought it was kind of an open secret that the Hemi motor hasn't been a true hemi for quite some time? It's just a marketing term nowadays isn't it?


That's my recollection. The sec. gen. hemi had the chamber canted a bit from the cyl bores to make the engine narrower but I believe the chambers were still hemispherical.
btw the word/name hemi has come to mean more than the chamber shape. It is a great part of history sorta like the word charger.
Also their is a good reason we don't use the term "Fiat Challenger"

Last edited by WS23RT; 12-07-2014 at 04:03 PM.
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post #7 of 17 (permalink) Old 12-07-2014, 04:28 PM
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That's my recollection. The sec. gen. hemi had the chamber canted a bit from the cyl bores to make the engine narrower but I believe the chambers were still hemispherical.
btw the word/name hemi has come to mean more than the chamber shape. It is a great part of history sorta like the word charger.
Also their is a good reason we don't use the term "Fiat Challenger"
Wrong and Wrong
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post #8 of 17 (permalink) Old 12-07-2014, 04:54 PM
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Wrong and Wrong

Ok Ok I'm wrong and wrong. I'm not new to this wrong thing.
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post #9 of 17 (permalink) Old 12-07-2014, 05:03 PM
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Ok Ok I'm wrong and wrong. I'm not new to this wrong thing.
J/K I thought you were referencing my `13 Charger.....Which I agree needs some work and Dodge needs to get a grip....
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post #10 of 17 (permalink) Old 12-07-2014, 05:17 PM
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J/K I thought you were referencing my `13 Charger.....Which I agree needs some work and Dodge needs to get a grip....

Didn't know you had a 13 Charger. (I have a 09 challenger srt)

For me a real hemi is sec. gen. 426 cause that is what made my socks go up and down when they came out. For others the first gen. --392-- fill the bill. (a true hemi?)
BTW if the hellcat didn't say hemi on the engine I might just buy one anyway. But if it said fiat power or the like maybe not.

Last edited by WS23RT; 12-07-2014 at 05:20 PM.
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