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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?Jeff2KPatriotBlue said:The 5.7 is pent-roof, iirc.
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.jerseyjoe said:Wonder if hemi heads would produce more power than the present pent design?
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 said: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.
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.AutoTechnician said: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.
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
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
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
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.
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.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