[Jeff2KPatriotBlue]

The 5.7 is pent-roof, iirc.

 

[SUNBURNTsnype]

The 5.7 is pent-roof, iirc.

I know the modern "hemi's" aren't true hemi's due to the pent roof design, but they are still marketed as such. But I'm wondering if anyone from Chrysler has ever made either a "true" hemi v10 or a "modern" hemi (aka pent roof) v10, either as a proof of concept design for R & D, for a concept, or for any production vehicle?

 

[Sir Moses Wellington]

I doubt it, prototype is likely, but the thing is with displacement and emissions and the 5.7L barely making it and the 426 not making it because of those issues, I doubt they would even attempt such a thing. Why the interest?

 

[dana44]

Main reason the 426 Hemi doesn't meet smog requirements is the shape of the piston top. The 5.7 does it because of a flat piston top and the dual sparkplugs, the combustion chamber design of shrouding the sparkplugs to force the start of the burn to go straight across the piston to the other side of the piston where another sparkplug is doing the same time is not the most efficient way to burn clean, might as well have a sharp lip to curl the burn like a closed chamber head. Anyway, the answer to you question is not known, but I wouldn't put it past them.

 

[paullybob]

In the late stages of WWII, Chrysler did have a working and tested V16 HEMI engine (XI2200). It was slated and actually tested in a P-47 Thunderbolt (XP-47H) and was designed to produce a mind-blowing 2500 hp!! It actually increased airspeed of that particular plane from 439mph to 504mph. The war ended and so did the engine program.

http://www.enginehistory.org/Museums/chrysler.shtml

That thing did indeed have a HEMI, lol!!

 

[Sir Moses Wellington]

Yea, that V16 was an insane engine. But the thing is, at the 504 mph speed, it's getting near that other experimental aircraft they were trying out to see if they could make a supersonic propeller aircraft. The issue was the tips of the propeller arms would go above supersonic before the rest of the plane, and I forget the exact explanation but apparently the leading edge would create a sonic boom and that would create a massive amount of turbulence and the other arms wouldn't be able to catch enough air to continue the speed and would slow down, this would happen before the whole plane would be close to supersonic. Looking at everything, I think the top speed of a propeller aircraft is 616 or so, while you could probably hit super-sonic in dive (theoretically) with one of these planes, they don't count that in official records, it has to be done under the airplane's own power. Hence why the F-86 doesn't have that claim even though it was not uncommon for them to go over Mach 1 in a dive.

 

[jerseyjoe]

Wonder if hemi heads would produce more power than the present pent design?

 

[AutoTechnician]

Wonder if hemi heads would produce more power than the present pent design?

True hemispherical combustion chambers are not that great. They require pistons with huge domes to achieve high compression, which greatly decreases combustion quality and efficiency. Modern combustion chambers are much better.

The current trend is for narrow-angle 4-valve pent-roof designs - like the 3.6L V6. The pent-roof design makes for a small, efficient combustion chamber with good detonation control. Four valves allow for good flow while retaining good velocity at lower engine speeds.

 

[dana44]

This is where one gets into cost over ability. The pent design works well because one is talking volume of fuel/air being able to enter a chamber and exhaust exit the chamber. Valve size itself can only be so much with both cases, and then you get into overall parts and complexity. The Hemi has the advantage in this case because it only has two valves and one camshaft, the pent is four valves and one or two camshafts, the pent has the higher flow volume due to the four valves, so the efficiency of a rounded dome vice the angled dome of the pent design it almost becomes a wash as to which is more efficient. Personally, when you start looking at it, I think the Hemi chamber beats out the pent design by just a little bit when you start crunching the numbers. The Hemi flows in the neighborhood of 260-275cfm in a 5.7 and the pent design flows around 250-260cfm in a 2.4, In other words, the smaller engine flows more than the Hemi by just a tiny amount in stock form, but has twice the parts to make it work the same, so at this point it is actually the piston top design which reduces the efficiency of the pent design, and in the 5.7 (and the 6.1) it is the combustion chamber design which reduces the engine overall output. From there, start doing forced induction and you can then get into a power rating based on equal forced induction to determine which engine is more powerful overall. I know more than 1000hp was pulled out of the 2.4 as a turbo, not sure what the limit was for a 5.7 Hemi, but that number was well into 750hp and remaining together, and all I know from articles is that the amount of boost between the two engines was at a minimum half of what the 2.4 was given, so who knows for sure what the Hemi limit is.

True hemispherical combustion chambers are not that great. They require pistons with huge domes to achieve high compression, which greatly decreases combustion quality and efficiency. Modern combustion chambers are much better.

The current trend is for narrow-angle 4-valve pent-roof designs - like the 3.6L V6. The pent-roof design makes for a small, efficient combustion chamber with good detonation control. Four valves allow for good flow while retaining good velocity at lower engine speeds.

This would require some real testing, as in a real Hemi combustion chamber and a piston shape which would promote combustion burn instead of just compression. The major improvement of the pent design is that the cylinder is able to purge much better with the two exhaust valves over the single exhaust valve of the Hemi (piston design puts a bit of a kink in this for the Hemi), but for the life of me I haven't seen any articles or studies on the piston design to assist in the exhaust purge.

 

[AutoTechnician]

You're confusing flow with efficiency. There is much, much more to a head than just how much the ports can flow at .700" of valve lift. I also have no idea where you're going with this whole turbo which engine and see which one makes more power thing, or which one can hold the most power?

I'll reiterate: True hemispherical combustion chambers are not optimal. The domed pistons they require hurt combustion. The extremely large valves and ports that 2-valve engines require (2-valve engines in general, not just the Hemi) can hurt low-speed velocity (and therefore torque) without careful design consideration. VCT and Active manifolds can help the situation for 2-valve engines, but there is only so much you can do.

If a true Hemi chamber were better then a 4v pent-roof chamber, or even if the modern hemi chamber were superior - you'd see more engines using them. Also, FYI: The modern 5.7 Hemi is not a true hemi head. It's more like a 2-valve pent-roof or a "bathtub" shape.

 

[dana44]

Which is why I said the current design needs improvement in the domed piston design. The Hemi design is patented, which is part of the reason why you don't see more doing it, I am saying there is more potential in the design of the combustion chamber and the piston top design to get even more out of the design. The purpose of the forced air induction is to point out the potential of both designs, which is why the Hemi design dominates, and has for decades, straight line racing, and why NASCAR outlawed the design. The first design Evernham wanted to use when Dodge went back to NASCAR was the Hemi design, which was shot down before even being tested.

Like I also said, the extra moving parts (two extra valves, springs, retainers, rocker arms, etc.) is the trade-off for efficiency, all these things require hp to move and has to be taken into account as far as cost goes. Are the extra parts a trade-off in flow potential over the cost of the Hemi? I remember the Hemi was more expensive to build than the OHV/OHC engines in the past, now it is less expensive to build a 5.7 Hemi over a 4.7 OHC engine. Also, like I stated, the extra exhaust purge in the pent design is one major reason why the pent design is able to run clean, not the fact it has two intake and exhaust valves. One thing I was pointing out with the pent four valve head was that the two intake valves and two exhaust valves flow pretty close to the Hemi single intake and single exhaust valves, and cubic inch to cubic inch, the pent design outflows the Hemi, but given its overall cylinder size, it does not make as much power. At this point, add the forced induction it is hard to tell from stock or near stock form which one produces more power given the 2.4 has a stronger bottom end to handle the pressure, so it is an unfair comparison. The 5.7 does not have a cradle on the bottom end like the 2.4 (or 2.0), but if it did, there is a good chance it would be able to handle the same amount of boost, and actually outperform the 2.0/2.4 pent roof design in output.

 

[Mopar392]

You're confusing flow with efficiency. There is much, much more to a head than just how much the ports can flow at .700" of valve lift. I also have no idea where you're going with this whole turbo which engine and see which one makes more power thing, or which one can hold the most power?

I'll reiterate: True hemispherical combustion chambers are not optimal. The domed pistons they require hurt combustion. The extremely large valves and ports that 2-valve engines require (2-valve engines in general, not just the Hemi) can hurt low-speed velocity (and therefore torque) without careful design consideration. VCT and Active manifolds can help the situation for 2-valve engines, but there is only so much you can do.

If a true Hemi chamber were better then a 4v pent-roof chamber, or even if the modern hemi chamber were superior - you'd see more engines using them. Also, FYI: The modern 5.7 Hemi is not a true hemi head. It's more like a 2-valve pent-roof or a "bathtub" shape.

The modern HEMI or GEN-III is a Semi-HEMI. It has the shape of the general HEMI head shape but with the outer sides of the spark plugs being filled.

One large intake valve in the OHV engines is about %60-70 of the cfm you get from 2 intake valves design in the OHC engines will less complicity.
Generally low-rpm torque in the modern NA push-rod engines is greater than the NA OHC engines.

The modern HEMI went for extremely domes pistons to flat-top pistons because with domed pistons the flame had to travel longer and push extra weight resulting in, as you said, poor fuel burn and combustion, which was one of the reason the 426HEMI was discontinued because of poor emissions.
Though, a 5.7 hemi has a not very large dome-shape pistons. The 6.1 & 6.4 have a sorta flat top to it because the extra cubic inches increases the compression ratio.

The more you stay away from the dome-design piston and be as flat and still get a good CR ratio, the more efficient your engine is.

 

[AutoTechnician]

@Dana, hemispherical combustion chambers are not patented. They've been in use since at least the early 1910s. The first "domestic" use I can think of are the Ardun OHV conversion heads for the Ford Flathead that were released in 1947. The Honda Super Cub 50 has had a hemi head since 1958, and Honda's other motorcycles had used hemi-heads for a long time - until they were superseded by better, more modern head designs.

Pentroof chambers are clean and efficient because they're compact, have excellent quench and combustion control characteristics, extremely fast burns and little surface area for combustion heat to escape. True hemispherical heads are the exact opposite. The domed pistons slow combustion down and require tons of timing advance. The lack of quench coupled with the amount of timing required results in a very detonation-prone engine. The massive combustion chamber surface area and size results in a dirty, slow burn with tons of heat being absorbed into the cooling system. You cannot fix the inherent size and surface area of the hemi chamber with fancy pistons. It's already hard enough for the modern "semi-pent-roof-ish-hemi" to meet emissions standards, and that chamber is far more advanced than the old-school hemi used in racing.

@Mopar and Dana; flow is not everything. A 2-valve engine can easily flow as much as a 4, or even 5 valve engine of the same displacement, and make the same peak power. The advantage of multi-valve is better control of the air flow, especially at low engine speeds. Take a look at the Toyota 1UZ-FE V8 or even the 3UR-FE V8 in the new Tundra. Those are 32V DOHC V8s that are famous for having tons of low end torque. The 1UZ-FE makes 95% of its peak torque at 1000RPM, and the 3UR is similar.

There are pros and cons to everything though. I'm sure Chrysler, GM (and even Ford with their new 6.2L V8) have their reasons for using 2-valve designs.

 

[jerseyjoe]

My 2 cents, I like the current "Hemi" for ease of maintenance and less complexity. I just wish a small cube version were made, I believe there could be gains in rpm and fuel economy. I like the old 361 B motor for that reason and the 273 A motor. A friend had a 273 in one of the early vans and it sure got respect on the street back in the day and good fuel economy.

Anyhow I bet a V10 with old style heads would have a totally different sound, just like the old Hemis compared to other V8's

 

[dana44]

The pent design is no more efficient than the Hemi as far as dirtiness goes for smog, it is not any more efficient, it just has the ability to purge better, thus burn better, than the Hemi. I misspoke about the Hemi design itself, the hemispherical design is used by others, the last one was a Ford 4 cylinder or V6, can't remember, but was pretty close to the old A engine design, pretty flat and curved. If not mistaken, I don't remember seeing a single domed piston inside the 5.7 or 6.1, kind of flattening out the combustion chamber was the way to keep the compression up, along with those quench areas around the sparkplugs. I have told the story many times of the guys racing the 426 when it came back in the early 60s (1964 I believe), and the guys simply hated it because they kept breaking them as compared to the 392 Hemi. Turned out the engine was much more, not less, efficient in burning(burned faster, not slower) and by simply backing the timing back to a total of 36 degrees over the 38 degrees the 392 ran, fixed the problem. Of course, the 392 did have flat topped pistons, the domes inside the 426 are what makes the dirty, along with the bore size being 4.25inches. A proper shaped piston dome is actually a key that is being missed with the burn and cleanliness of the Hemi of today, I just don't think they really knew what they were thinking by adding the side quench areas and a dish in the piston to get the compression down, not up.

Torque and hp can be built or lost through the design of the camshaft itself. The pent design requires less lift to flow the same amount of air a 5.7 needs with the cross section of the valves being opened (lifts for the 4 cylinder engines crack .331-.360 inch at best), whereas the 5.7 is closer to .480 inch, and then you start talking duration and overlap to build the power, don't need to choke the engine down with active intakes, but it is the function of VVT (as you well know), to reduce the overlap, make the valves open and closer sooner in relation to BDC and TDC to make that bottom end power, then spread apart the opening and closing to not peter out and have an engine sucking for life when it hits 4500rpm, gives the ability to extend that to 7000rpm and above, but, this has absolutely nothing to do with the pent or Hemi design, and that was proven with the Viper engine with its VVT camshaft, all the videos on the track keep talking about the engine's ability to make more power the faster you take the rpm, and the videos show this to be true, and it is neither pent or Hemi.

 

[fargo59]

i thought i'd drop this link for further conversation on what a modern "hemi" is, and looks like. its also beneficial as there are more exact flow numbers.

http://www.carcraft.com/techarticles/ccrp_1210_inside_the_g3_hemi_cylinder_head/

i know the man who did the flow tests, and trust his figures.

as i see it, the modern hemi is a winning combination as it gives the pent roof air flow capacity of a multi valve OHC engine without the added expenses and packaging constraints. yes they have their own challenges, but i remember being told the hemi was cheaper to produce than the 4.7

emissions wise, the smaller, flatter chamber gave it quench, which is better for emissions control and flame propagation (ooo big word )

is it a magical technology that can be applied to any engine? no. its a rather simplified way of making 2 huge valves fit into a bore. how do you make a 2.05" and 1.55" valve fit into a 3.97" bore and not be butted against each other? simple, angle them towards each other.
hopefully im making sense.

 

[AutoTechnician]

Quote
The pent design is no more efficient than the Hemi as far as dirtiness goes for smog, it is not any more efficient, it just has the ability to purge better, thus burn better, than the Hemi. I misspoke about the Hemi design itself, the hemispherical design is used by others

Sorry, but you are not correct. Pent chambers can be much more efficient. The dirtyness of a true-hemi is caused by the large distance the flame front has to travel and the massive surface area of the chamber sucking tons of heat out of the combustion. It's a poor chamber design for a modern engine. Pent-roof chambers are more efficient and burn better because they're small and compact. This is from the narrow valve angle and that most of their surface area is valve, rather than head. That keeps more heat in the chamber, and less in the cooling system.

Quote
Turned out the engine was much more, not less, efficient in burning(burned faster, not slower) and by simply backing the timing back to a total of 36 degrees over the 38 degrees the 392 ran, fixed the problem.

It may have burned slightly faster than the 392, but they still burned slow. Modern engines burn fast enough that many of them only run 15-20* of timing advance at WOT.

Quote
I just don't think they really knew what they were thinking by adding the side quench areas and a dish in the piston to get the compression down, not up

They knew exactly what they were doing with the 3rd Generation hemi. Reducing the chamber size and adding the quench pads was the way to go. It helped solve some of the inherent flaws of the hemi chamber design while allowing for those two big fat valves that made the Hemi famous. The Eagle 5.7 and 6.4 have actually dropped the quench pads in favor of a closed-chamber design. It does not look like much, but you can bet it has thousands of hours of development and computational fluid simulations to perfect its shape.

Crazy domed pistons that a true hemi requires will never burn clean, cost a lot to manufacturer, and are very heavy.

 

[MoparJoe]

Dont't forget gas in the 60's had a lot of beutain which helped the burn but was a really bad idea for evaporation , you will never see a true hemi again, they are not fuel efficient for the crappy fuel we have today....

 

[dana44]

Overall, I figure they didn't use the Hemi design on the V10 because it would produce too much power to be streetable. Figure they now run well over the 500hp with a simple wedge designed head, imagine what a more efficient (as in power producing) design would be like. Figure even with aluminum it would add 100 lbs to the nose of the Viper, too, but would be closer to an additional 150hp but in an even larger package than now.