Is the new Pentastar V6 a HEMI or semi HEMI engine? I've read one person talked about the engine having the hemi-tick which most believe is the lifters, but that made me wonder if the engine is a HEMI.
You can't orient and actuate 4-valves on a hemispherical contour w/o a drastically complex valvetrain. At best, it is simply a 4-valve head with a rounded pentroof. I would love to see pix of one, though.
The original HEMI was revolutionary because of the shape of the head, but like the last post said, the new Pentastar is some penta-roof variation--very common in modern multi-valve engines. Today, HEMI is mostly a marketing term, but in my opinion it also means traditional pushrods and 2 valves/cylinder. Check out the engine info on allpar.com for interesting reads.
The pictures indicate a classic pentroof design...2 angled, flat-faces such as a shallow roof of a house. If there was any kind of hemispherical contour in there, it would need to be a rounded contour, not only encompassing the 2 faces of the "roof", but also a rounded contour in the perpendicular direction to that. That's the part that would break each of four valves into its own angle to properly seat into a hemisphere contour, which would drastically complicate how the overhead cams could actuate 4 independent angles per cylinder.
...or if you can imagine, instead of 2-faces like the roof of a house, you break it into 4 quarter faces like a pyramid. Then put a valve on each face. Then round out all of the sharp corner boundaries. Then you would have a pentroof-ish design that is awfully close to a hemi. Why don't they do that?...because the valvetrain would be horrendously complex to accommodate the particular angle of each valve on its own quarter face. That's why they don't do it. That's why a typical multi-valve engine is a simple, classic 2-face pentroof. It lines up radially to a single or dual camshaft configuration, easy peazy.
If it were as simple as utilizing a central spark plug location, then any wedge head ohv could qualify as a "hemi" by simply relocating the spark plug with access through the valve cover. It would be a needless complication, and it would probably ruin the carefully orchestrated ignition behavior that a genuine wedge head exploits to complement its native geometry. Is that worth it just to call it a hemi? I don't think so, and I don't think anybody would be convinced to indulge such a stretch in what a hemi is.
It's fairly well accepted (though arguable) that a classic 4-valve cyl pentroof design breathes better than any 2-valve, even if it is a hemi, so I don't know what is such the strong motivation to figure Dodge's v6 is a "hemi". It's as modern as any other modern multi-valve engine on the road. Dwelling on if it is a "hemi" is dwelling on something behind you on the evolutionary tree of engines, anyway.
Think of it this way- your multi-valve engine approaches the breathing issue with brute valve area with multiple elements packed into a defined space, while the 2-valve hemi approaches breathing from focusing on achieving the most graceful (least obstructed) flow that a single relatively large valve can muster. If everything is working as expected, the flow should be comparable between the 2 approaches.
As soon as you have 2-valves simultaneously breathing along a hemi contour, it ruins the simplicity and elegance of the flow dynamics that makes a single large valve doing the same thing work so special. It's like the difference between a 2 lane hwy where everybody runs haphazardly, jumping lanes erratically, with collisions bouncing about until the traffic behind pushes through with brute force vs. a wide single lane hwy where everybody flies through in perfect unison, orderly spacing, no lane changing, only forward direction. Naturally, the 2 lane has the potential to move more cars, if everybody drove exactly as expected in an orderly manner, and ultimately adding multiple lanes will ultimately achieve more flow even despite each person doing their "own thing". The wide single lane that proceeds in a rigid orderly manner has a certain magic to it (something that we cannot really observe in real traffic behavior, of course). Anyways, the point I was making is that as soon as you put multiple flowing valves in a hemi shape, it ceases to flow like a hemi would, anyway. It simply flows like a multi-valve configuration (for better or worse from the chaotic and complex flow dynamics). The hemi contour ceases to be any benefit if it is there or not.
I am not sure if hemi is even useful anymore now that we have much better understanding of fluid dynamics. Don't forget, you want to intake lots of air at WOT for power, otherwise it isn't too important, but that is more of a manifold / valve issue IMO. What you really need inside the combustion chamber is a shape that is going to encourage swirl / tumble etc to mix the gasoline well and to try and speed up the combustion rate by throwing the fire all around the place...
I know hemi is "hard to clean up for emissions" which typically means it is not too efficient for power either, but once you already have four valves, you hardly have any head anyway, it is all valves and sparkplug
theoretically a complete hemispherical combustion chamber (incl valve faces) would have an advantage against flame quenching and heat loss though because of surface area...
I'm interested in head design so if I'm in the potato field let me know, we don't have enough techy discussions here we need some more
No, it just means that it can breathe in a larger charge, but doesn't burn it as completely, because there is more to burn in the first place. Naturally, that problem becomes worse if you have to work with extreme rpm (less time to burn everything that did get in) and larger combustion chambers (which would follow in line with large displacement designs). That said, things (emissions concerns) are probably more sane on these modern hemi's than they were in the past now that we are dealing with bores for a 6-ish L displacement engine than gargantuan 426 ci monsters with domed pistons (further exacerbating the path-length aspect). Who knows, maybe a 5.0 L hemi would be a cake-walk for modern emissions...
The swirl and tumble is simply the "other" way to skin the same cat. Specifically, with wedge heads, it's a little more restrictive, hence breathing in less, but the swirl and tumble that comes with the design promotes a more complete burn.
The 4-valve combustion chamber is more "open" like the hemi design, so you don't quite get the same tumble and swirl that promotes complete combustion. The 2 valves that flow simultaneously fight each other, so you end up getting less overall flow than expected for the valve window area that 2 valves in simple, independent flow should give. So the tumble and swirl of the 2 fighting valves happens at a time that inhibits potential breathing, rather than at a time that promotes more complete combustion (as in a classic 2-valve, wedge-head). The benefit from multi-valve seems to come more from opening the door to higher rpm operation and supporting high flow at those higher rpms. Naturally, you complement such an engine with a cam that favors the higher rpm, and viola, you end up with hp benefits. It's not really for multi-valve breathing better across the board, rather being able to explore higher rpm's while maintaining reasonable breathing efficiency at those rpm's. Power production in the low and mid rpm is usually not stellar (despite the raw breathing potential), not only because the cam is really not favoring that range, but these are typically smaller displacement engines, to boot.
Hemi's and wedges develop output just fine in the relatively lower rpm range that they typically run, because there was never a problem with insufficient valve flow at those lower rpm, in the first place. The hemi's breath deep, but may not burn it all, while wedges breath less deep but better achieve a full burn. It's just the same ole give and take that you encounter in all kinds of engineering. Displacement for displacement, multi-valves breath about the same. The only way they come out on top is to spin them higher where the ohv valvetrains have trouble following, and multi-valves enjoy a bit more headroom before restriction becomes a significant penalty.
I remember the engineers at Chrysler when they developed the Viper engine saying that 4" was the max bore they could handle with a single plug without going nuts in HC emissions, conveniently 4" was the bore of a 360 V8 and a Viper engine