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  1. #1
    RC Turnbuckle Jr. Wolfslash16's Avatar
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    Question Hmmm... I wonder if "HEMIS"...

    Hey guys, I'm just thinking out loud here, but hang with me, this might be hard to under stand, LOL. Most engines use a wedge shaped combustion camber like this one. Older cars used combustion chambers shaped like this one. Hemi's (Hemisperical) are used in Dodge trucks and other muscle cars like the Dodge Challenger. This combustion shape is used in formula 1 racing cars and such things like that. This is what most Hemi pistons look like. (This kind of piston is what is used in the current Dodge ram 5.7 liter hemi's.)

    Ok, looking a the hemi engine, wouldn't it be more effecint to have a piston shaped "in" like a bowl? Because when I look at it, wouldn't that mean that the gases would pushed to the side of the combustion chamber, making some of the fuel/air vapor leak past the piston rings? So if the piston was bowl shaped, it would puch the gaes in to the middle of the piston, so wouldn't that give you more power? And on that note, do you like it could be possible to make a "hemi shaped" combustion chamber on a Nitro rc car? If so, would it work or would it be a waste of time? Sorry for the long post, I've been thinking about this for months and I'm just thinking out loud here, I just want to know what you guys think!
    Last edited by Wolfslash16; 07-16-2012 at 11:29 AM. Reason: Hemi was spelled wrong 3 times LOL
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  2. #2
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    Never thought of reversing a hemi piston in theory it sounds right but you will really never know until someone makes it. I'm not sure about nitro I use electric but I would think it would make it better Since it works for bigger cars
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  3. #3
    RC Turnbuckle Jr. Jimmie Neutron's Avatar
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    Domed pistons create less area for volume, therefore increasing compression. The more compression you have, the more power you have.
    That is why bowl type pistons are not desired for high performance engines... plus, do you think that the bowl actually holds anything when it is flying up and down at 6,000+ revolutions per minute? Also, if your piston rings are leaking bad enough for large amounts of fuel to get by... you have way more issues than just loosing some fuel.

    2 cycle engines would not benefit from the "hemi" design... they might both be engines, but the similarities on function end there.

    http://www.animatedengines.com/twostroke.html

    http://www.animatedengines.com/otto.html
    Last edited by Jimmie Neutron; 07-16-2012 at 01:58 PM.
    Whatever it is I just said... I could be wrong.

  4. #4
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    Yes I agree, with a dished piston you add space to the combustion chamber. I believe Ford experimented and produced an engine with a 'dished' piston, check on that though.
    Interesting thought though!
    Matthew 7:7

  5. #5
    RC Turnbuckle Jr. Jimmie Neutron's Avatar
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    Some/most manufacturers do/did have different pistons for the same block...
    for example the pistons in a 4.6 F150 are different than the pistons in a 4.6 Mustang

    The Mustang pistons create less volume... so more power.
    Whatever it is I just said... I could be wrong.

  6. #6
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    Quote Originally Posted by Jimmie Neutron View Post
    Some/most manufacturers do/did have different pistons for the same block...
    for example the pistons in a 4.6 F150 are different than the pistons in a 4.6 Mustang

    The Mustang pistons create less volume... so more power.
    But the two engines use the same block?
    Matthew 7:7

  7. #7
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    This is my kind of thread! I was building a 318ci last year and it had dished pistons. I don't like them because you can't build as much power. All of the BBCs I build, the valves are VERY close to the piston (a few thousandths of an inch) and you just can't get that close with dished pistons. Also, on the exhaust stroke, you have that extra pocket in the piston full of burnt air and fuel and you don't get complete exhaust evacuation.

  8. #8
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    I've got the hemi in my 05 ram, and dodge did it right!!!



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  9. #9
    RC Turnbuckle Jr. Jimmie Neutron's Avatar
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    Quote Originally Posted by 1979chevy. View Post
    But the two engines use the same block?
    Depends on the casting... at first they were not all modular, but now they are.
    Whatever it is I just said... I could be wrong.

  10. #10
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    Quote Originally Posted by Jimmie Neutron View Post
    Depends on the casting... at first they were not all modular, but now they are.
    Wow, that's interesting.
    Matthew 7:7

  11. #11
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    Diesel engines use a flat head surface with the combustion chamber in the piston. The dish is bowl shaped with a point in the center. The point is to "stir" the air that is forced in to encourage flame front propigation when the fuel is injected. Unfortunately, you can't get enough valve area with an acceptable compression ratio for high speed, normally aspirated use, and pump gas can't handle the heat that comes with very high boost pressure. RC two-strokes use a hemispherical combustion chamber, but that's only because it's the only shape that makes sense- they are unable to take advantage of the benefits like the ability to have huge valves without shrouding, for example. The real problem with a true hemi is the lack of quench area and the huge dome on the piston that gets in the way of the flame front, which is why all modern "hemis" actually have large quench areas. The quench kills two birds with one stone- it reduces combustion chamber volume, reducing the need for a huge dome, and increases turbulence, reducing emissions and improving efficiency.

  12. #12
    RC Turnbuckle Jr. Wolfslash16's Avatar
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    Some/most manufacturers do/did have different pistons for the same block...
    for example the pistons in a 4.6 F150 are different than the pistons in a 4.6 Mustang

    The Mustang pistons create less volume... so more power.
    So let me get his straight... less volume, more power? Wouldn't that mean less fuel/air mix, thus creating less power? I'm just not quite getting what you mean by "less volume more power" What I meant when I said "dished" in pistons, kind of like a revered hemi piston, is to have the topo of the combustion chamber bowled in too. So this ) would be the top of the combustion chamber and this ) would be the top of the piston. Like this. ) ) Does that make any sense?
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  13. #13
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    Actually the less space, the higher pressure the fuel/air mix will reach. Therefore more power.
    When you said reverse do you mean having a dished piston but having a cylinder heads that had 'reversed cone' shaped combustion chambers? If that makes any sense lol.
    Matthew 7:7

  14. #14
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    Oh ok gotcha, that makes sense, seems like it would work fine, very interesting idea.
    Matthew 7:7

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    Then your valves get all crazy and start facing toward each other and bad things happen...

  16. #16
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    Quote Originally Posted by TraxxasTuesdays View Post
    Then your valves get all crazy and start facing toward each other and bad things happen...
    Haha yeah it'd take a whole new head for sure.
    Matthew 7:7

  17. #17
    RC Turnbuckle Jr. Wolfslash16's Avatar
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    Actually the less space, the higher pressure the fuel/air mix will reach. Therefore more power.
    When you said reverse do you mean having a dished piston but having a cylinder heads that had 'reversed cone' shaped combustion chambers? If that makes any sense lol.
    Yep. That's just what a meant!
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  18. #18
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    Quote Originally Posted by Wolfslash16 View Post
    Yep. That's just what a meant!
    Sounds like a really neat concept, I dig this kind of stuff man
    Matthew 7:7

  19. #19
    RC Turnbuckle Jr. Wolfslash16's Avatar
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    Sounds like a really neat concept, I dig this kind of stuff man
    Ya, it's things like these kind of "thought' experments that make real cars (and nitro rc's) SO much fun!
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  20. #20
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    Yeah, and this is how new stuff is invented.
    Matthew 7:7

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