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Exact Purpose of Hemispherical Heads?

  1. Feb 21, 2013 #1
    These days you hear often about cars having "Hemis". Correct me if I'm wrong, but the hemispherical head allows for a slight increase in volumetric efficiency of the cylinders, but is there any other advantages to it?

    Thanks for you help!
  2. jcsd
  3. Feb 21, 2013 #2


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    It makes for a snappy marketing pitch.
    Apart that, few people could tell what advantages it might bring. I'm not sure there is actually any volumetric efficiency improvement, although there has been lots of work to optimize the mixing of the fuel with the air by fine tuning the valve positions and the shape of the combustion chamber. I believe that engines have been run with glass ports to allow laser beams to monitor the fuel/air mixing and combustion.
  4. Feb 21, 2013 #3


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    There are both advantages and disavantages to using hemispherical heads. Just check the Wiki page, for examples of both. A Google search brings much more discussion of them.

  5. Feb 22, 2013 #4

    jim hardy

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    If you ever fly through Hartford Connecticut, take a couple hours to see the aviation museum across the field from the terminal.

    One of the exhibits is an inverted V-8 aircraft engine with hemispherical heads, made by Chrysler in 1930's.
    It flew 400 mph in 1939.
    The hemispherical chamber allows big valves for a free breathing engine.
    In 1950's Chrysler advertised "aircraft technology" in their passenger car engines and these are the legendary "hemi's" that you still hear about today.

    It didnt take hot-rodders long to figure out they could be made quite powerful. This one likely began its life in a 1957 New Yorker.
    Last edited: Feb 22, 2013
  6. Feb 22, 2013 #5

    Ranger Mike

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    Hemi heads have been the engine builds choice for years. Chrysler and Ford Hemis rule in drag racing. A circle is the optimum shape for vector resolution
    (reacts equally to forces from all directions). Taking this one step further, when you introduce fuel/air mixture in a Hemi sphere combustion chamber and light it, you have uniform burning of the mix and force is applied in the most practical manner. Compare this to the typical wedge combustion chamber design which does not have best flame travel and force is staggered as the mix expands and applies force to the piston surface. Additional advantages are huge valves may be used, and depending upon the design Dual overhead cam design being the best, you can really pack in the mixture.

    Disadvantage is the EPA did not like the emmissions and was not the best fuel economy engine out there,,it would pass everything thing on the road but a gas station...
    My opinion
  7. Feb 25, 2013 #6
    I'm an aircraft mechanic, and when I went through my training, they didn't really touch hemispherical heads.

    So, in essence, it's just to allow greater flow into the cylinder by allowing larger valve heads to have more room?
  8. Feb 25, 2013 #7
    This is why aircraft reciprocating engines have a top and bottom spark plug to create a uniform burn, and evenly propagating flame front.
  9. Feb 26, 2013 #8

    Ranger Mike

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    i believe you have two spark plugs for safety sake. If one fouls out the engine still runs.
  10. Feb 26, 2013 #9
    Safety is one part, but if you foul out a single plug in one cylinder, on an engine that has single plugs, you still are running on the other cylinders. The main purpose of the dual plugs is the even flame propagation. It helps eliminate hot spots and detonation in combination with the tetraethyl lead that is added to the 100 octane fuel.
  11. Feb 26, 2013 #10


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    Hemis have actually fallen somewhat out of favor in modern, high-performance engines, since you can get better combustion with some other, somewhat odder looking shapes. Wikipedia has a good picture of what a more modern head design looks like (http://upload.wikimedia.org/wikipedia/commons/d/d6/Active_Chamber.jpg).
  12. Feb 26, 2013 #11
    Got it. On a side note, what are your opinions on Wenkel (rotary) engines?
  13. Feb 26, 2013 #12


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    Have they solved the seal issue? The tips of the rotor move at a constant high speed creating a very difficult materials problem. I believe that is why you do not see them in new cars today.
  14. Feb 26, 2013 #13
    I don't know. But didn't the Mazda Rx7 have one?
  15. Feb 26, 2013 #14

    jim hardy

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    I read that the shape of the Wankel's combustion chamber condemns it to low efficiency. It is long and skinny with high surface to volume ratio.
    That results in an unfavorable fraction of heat from combustion, which should go into expanding the working fluid , instead just lost by conduction into the walls.

    I owned a 1973 Mazda RX2, bought it brand new Sept '02.. I figured such a tiny car would get tremendous gas mileage - some of you may recall that was when cost of gas skyrocketed to almost 50 cents per gallon. Piston economy cars of the day approached 30 mpg.
    Well, i never saw over 18mpg on the little rotary beast.
    So i quipped: "I'd save money to put the Mazda in the trunk of my Chrysler and keep on driving it."

    My opinion of the Wankel - an interesting idea but not a good one.
    To its credit - the RX2 was quite powerful and gave me 100k miles of reliable service.
    It was a small, expensive gas guzzler.

    I'll take Detroit Iron any day.
    1951 Chrysler Hemi car engine:

    Photo credit to these kind folks: http://evolutsia.com/content/view/614/44/ [Broken]

    Observe how the angled valves allow them to be oversize compared to cylinder bore.
    Now, imagine a domed piston that raises the compression ratio up to around 13::1 ...

    This paragraph is NOT a commercial endorsement, just sharing a good experience:
    If you're ever in Ocala Florida plan on a day in Don Garlits' '' Museum of Drag Racing. " It's right on I95.
    I liked the machinery.
    My greater half Fair Anne, who is an artist, was so entranced by the paint jobs I had to drag her away at closing time !
    photo courtesy Don Garlits
    http://garlits.com/images2009/SwampRats/swamp%20rat-1.jpg [Broken]

    http://garlits.com/museum.htm [Broken]

    old jim
    Last edited by a moderator: May 6, 2017
  16. Feb 26, 2013 #15
    Yes. The hemi head design allows a much better port to valve design and a permits a central spark plug placement in the cylinder. This permits a very straight port to valve design This design has dominated NHRA top fuel drag racing for the last 50 years.
  17. Feb 27, 2013 #16

    Ranger Mike

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    Jim Hardy..excellent!!
  18. Feb 27, 2013 #17

    jim hardy

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    Thank you R..M...

    i made an error above post though -- Garlits museum is on highway I-75 not I-95.
    Too late to edit it today..

    translation - streamlined passageways around the valve.
    Since the air-fuel mix needn't make an abrupt turn, the engine "breathes" freely.
    You can see this on right side of photo.
    It helps get a full charge into the cylinder at high power high speed operation.

    old jim
  19. Feb 27, 2013 #18

    Ranger Mike

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    trivia-Back when the " new hemi" was introduced in 1964 it was a real dog. Drag racers were very upset tha tthe engine did not perform as well as the venerable 392 CID hemi..i think it was Pete robinson or Don Garlits that wa so pi--ed off he diecide to blow the s-B up and cranked about 60 degree lead ( advance ) into it. The engine really took off..seems the factory boys were way conservative on the timing advance and this was one very high RPM engine and needed a lot of advance..once this was discovered ..nothing could touch the hemi...
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