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PostPosted: Wed Aug 09, 2006 12:22 am 
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1275cc
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mmm sweet i need my 080 flattys for my 998 so will call him friday

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mini parts for sale http://www.ausmini.com/forums/viewtopic.php?t=42247


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PostPosted: Wed Aug 09, 2006 12:31 am 
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1360cc
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i thought the main thing about pistons was their weight..

anyway whats the advantages of this vs a train which i could also afford.


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PostPosted: Wed Aug 09, 2006 9:14 am 
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1275cc
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Nice looking pistons and for my engine size yummy. Bet they're worth a fortune too, good thing I'm not in the market for any at the moment coz I would have to buy them and go broke in the process.


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PostPosted: Wed Aug 09, 2006 10:41 am 
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1275cc
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i think that the price is very reasonable from what i have heard from Graham.
I have a set lined up to go in my 998 project :D :D


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PostPosted: Wed Aug 09, 2006 11:25 am 
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848cc
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I just rang to get a price on the 998 flat top pistons.
$250 for a set.


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PostPosted: Wed Aug 09, 2006 2:10 pm 
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848cc
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Just spoke to my resident mini mechanical expert regarding going to flat tops rather than the slightly dished pistons he previously ordered. He said that it is a bad idea to go to flat top pistons as it ruins the 'gas swell' (actually on reflection and thinking about it maybe he said 'gas swirl'). He said something regarding the physics of the gasses in the combustion chambers expanding during ignition not working as well with flat top pistons.
I'm not a physicist so I don't completely understand how changing the shape of the top of the piston would change the behavior of the expanding gas but I've never heard of these guys being wrong before.

He suggested that if you want more compression you are better shaving more off the head.


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PostPosted: Wed Aug 09, 2006 3:26 pm 
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1275cc
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yes but when u shave more off the head, the head flexes and you blow head gaskets left right and centre.....

Graham is very concious of this sort of stuff. I would deff reccomend that
you call him and ask his opinion on this, im sure there is a reason why he
has dished them for the 1275s and kept them flat for the 998;s ... perhaps that
has something to do with it.


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PostPosted: Wed Aug 09, 2006 3:53 pm 
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1275cc
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Well rang the man and the .40 998cc flats are going for $250 a throw - very reasonable so I am picking thenm up on the 21st August when I am up there for work.

Might be able to scam that cup of tea I missed out on 18 months ago(and steal all the biscuits)

Hooroo

Rob Forsyth
Miniot!!

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PostPosted: Wed Aug 09, 2006 3:54 pm 
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1275cc
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Maybe... the combustion chamber design in a good 12G295 or worked 12G202 is fairly large (28cc or so i think). If you want a higher CR, you need a flat top piston. The loss in efficiency with a flat top is probably much less than by having a really extremely shaved head, (even if you don't consider reliability) or poorly shaped chamber.

I'll be getting some when the wallet has recovered (for the next project....)

GR is da man!


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PostPosted: Wed Aug 09, 2006 4:08 pm 
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Hhhmmm,,,nice--->, But!!! not that happy with such a "sharp(ish)" lip around the dish on the big bore ones, can you get him to change them to a nice radiused-rounded edge instead please?

:-)

But well done GR (&Kev) good pistons are hard to get & usually quite expensive to buy, these may well take the market by storm

when`s the 1100 ones comming or are the 998 ones able to be "lopped off" the top of them for use with 1100s???

cheers Kev & tell GR i`d like to see less of a sharp lip around the edge of the dish on the big bore ones in the next batch ok? :-)

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PostPosted: Wed Aug 09, 2006 4:20 pm 
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848cc
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Hmmm. I have a 12G295 head. All nice and shiny and new(machined). But I trust my guy with these things.
My guy did say he was having trouble sourcing 998 pistons as his usual supplier was out.
Good pistons are hard to find and hard pistons are good to find.
:wink:


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PostPosted: Wed Aug 09, 2006 4:35 pm 
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1275cc
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GR has made these flat tops thick enough to dig em out for a dish if need be :wink: If you don't understnad something call GR, he really is helpful and will sell you what you need, he will even tell you not to buy his stuff if it isn't what you need, he's that type of guy.
Matt, I would sugggest that a quick call to GR in regard to the sharp edge on the big bore numbers. He's probably don't for a reason. Best let you two techo types chat about it, then you will get a stright answer rather than going the 2nd hand info route.:wink: Again, i am sure the big bore's are easily reshaped with enough meat in them to do so.:wink:

The 1100 pistons will be done in the next run, he wanted to concentrate on the pistons that were sort after most, that being 998 and big bore numbers. This is of course from his experience as to what people want to build :wink:

Hope this helps guys.

Cheers
Aaron

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PostPosted: Wed Aug 09, 2006 4:49 pm 
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848cc
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From a physics point of view the dished pistons may be a little worse. The combustion force is spread evenly over the crown of the piston. If the piston is dished then some of this force acts on the slope of the dish - so it doesn't act directly downwards - and the sideways part of the force would be cancelled out by the equal and opposite force on the other side of the piston. (the force would just go into flexing the piston or something) - so it would be wasted

There are also the effects due to the way that the intake mix actually flows into and around the cylinder, I don't think the shape of the top of the pistons will affect this much as the intake stroke starts with the piston near the top of the cylinder and finishes with it near the bottom, so by the time it reaches the bottom the piston is nowhere near the intake valve/port and shouldn't affect it much. I guess it probably depends a fair bit on engine speed and cam timing too because these will affect the proximity of the piston to the valve when the gas is flowing and the mixing time.

Another effect is how the flame propogates through the combustion chamber upon firing. The flame will start at the spark plug and travel outwards to all other areas of the combustion chamber. This affects power, economy etc. The hemi engine is a very good design from this point of view as the flame spreads evenly throughout the combustion chamber and reaches all the edges at the same time. The A-series engine probably isn't very good because of the offset spark plug and strange combustion chamber shape. Dished pistons may improve this a little.


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PostPosted: Wed Aug 09, 2006 5:19 pm 
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848cc
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My imagination has painted a picture (bear with me here.) of the dished pistons advantage. As the piston draws down on the inlet stroke it would be sucking air fuel mix from the carbie. A dished piston combined with an inlet valve placement on the side of the combustion chamber would be more likely to start a controlled swirl (tornado style) inside the combustion chamber running from top to bottom. This swirl would act as a bit of a self perpetuating syphon to draw more air fuel in using less force. It would also aid more even air fuel mixture in the combustion chamber.
It works great in my head and has sold me on dished pistons for life. :lol: . I wonder if in reality it works anything like this?

Oh and wiki says.
Combustion chamber
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A combustion chamber is part of an engine in which fuel is burned. The leftover hot gases produced by this combustion tend to occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber. This pressure can be used to do work, for example, to move a piston on a crankshaft. The energy can be converted to various types of motion or to produce thrust when directed out of a nozzle as in a rocket or jet engine.

In a reciprocating engine, the moving pistons are flush with the top of the cylinder block at top dead centre, and the combustion chamber is therefore the recess in the cylinder head which contains the valves. Some engines use a dished piston and in this case the combustion chamber can be considered as partly within the cylinder. Various shapes of combustion chamber have been used, such as L-head (or flathead) for side-valve engines, "bathtub" and "hemispherical" for overhead valve engines and "pent-roof" for 16-valve engines. The shape of the chamber has a marked effect on power output, efficiency and harmful emissions; the designer's objectives are to burn all of the mixture as completely as possible while avoiding excessive temperatures (which create NOx). This is best achieved with a compact rather than elongated chamber. The intake valve/port is usually placed to give the mixture a pronounced "swirl" (the term is preferred to "turbulence" which implies uncontrolled movement) above the rising piston, improving mixing and combustion. Finally, the spark plug must be situated in a position from which the flame front can reach all parts of the chamber at the desired point, usually around 15 degrees after top dead centre. It is strongly desirable to avoid narrow crevices where stagnant "end gas" can become trapped, as this tends to detonate violently after the main charge, adding little useful work and potentially damaging the engine.

The term is also used to refer to an additional space between the firebox and boiler in a steam locomotive. This space is used to allow further combustion of the fuel, providing greater heat to the boiler.

[edit]
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PostPosted: Wed Aug 09, 2006 5:27 pm 
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848cc
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Wow, that works great in my head too :lol:

I don't know how much of a difference the dish would make to the swirl though, as it would probably tend to swirl anyway. I think i will have to have a bit of a read of my Vizard books and see if i can find out anything more about it.


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