Flat top or dished piston in a 66/70cc china girl....

rogergendron1

rogergendron1

New Member
The topix here is the potential performance gain and ease pf measurment by using a flat top or dished piston rather than a domed one .

It is my experiance that when a domed piston is used in a high comp motor it has a tendancy to direct the combustion down around the dome and into the rings thus adding to blowby, where as a flat top or dished piston would contain the brunt of the charge right on top of the piston and create more downward force,and more compression and less blowby via containing the charge ontop instead of guiding down to the rings .

Though one could argue that directing the charge at the rings with a dome shape would only help the rings expand more and the blowby would be negligable. Also the dome shape could be said to help aid with squish band funtion by creating a tighter squeltch where the dome and squish angle meet where as a flat top piston leaves a wide squeltch angle or needs to be run with a horizontal squish .

One reason why i want to try running a flat top piston is so i can set the flat top piston to a perfect zero deck hight with the jug
 
With a flat top piston at perfect zero deck hight one could easily calculate head volume and displacement and get a more acurate static compression and dynamic comp ratio and readings.

Also the combustion process would take place on the pistons flat surface and there would be hardly any blowby compared to a domed piston

You could run a head with a smaller combustion chamber like one from a 50cc motor with a smaler bore say 40mm and the resulting 7mm over hange would be yoir horizontal squish and the rest of the chamber including the designed squish band would be your new combustion chamber , giving you a high comp motor with a flat top piston a zero deck hight and a horizontal squish ..

What do you think about this ? How well do you think a flat topped piston would work in our motors ? do you think it would need a completely redesigned head or eould it work with something already in production ?
 
It may work ok in conjunction with a hemi type combustion chamber, the lack of a squish band and the flat top would mean once you get a zero deck hight, any extra compression and/or detonation will need to be gained by the size and shape of the combustion chamber.
The hemi type chambers are great for detonation prevention or a 40 to 60% flat with a smaller hemi chamber in the middle with a very tight squish (under 1mm) may work with a little experimentation. An angled squish band with a matching angle on the piston top tends to make the most power due to detonation resistance but a dual plug hemi with less timing also makes good power because the chance of detonation is even less, having 2 separate flame fronts on opposite sides of the combustion chamber cause the fuel charge to burn sooner than a single plug in the center which mimicks a higher timing setting allowing for more compression with excellent detonation resistance. It's still a balancing act between compression and ignition timing and will take a little experimenting, but worth a try.

Blow by can be partially cured by drilling gas ports in the piston crown to behind the ring lands which puts some of the cylinder pressure right behind the ring, pushing it out even harder, but gas ports can also cause too much pressure behind the ring at peak cylinder pressures causing the rings and the bore to wear quicker, good for quickly seating the rings, but not so good for longevity which is why about the only time you'll see gas ports would be on a race engine that gets disassembled and rebuilt frequently.... But the more I think about the extra pressure pushing the ring out as the ring passes the exhaust port on it's way down, if there's still enough pressure behind the ring, this makes me concearned about ring snagging from the added pressure behind the ring... I'm gonna research this and see if it can even be done safely on a 2 stroke since I've never heard of it on anything but 4 strokers.
 
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I have a spare case now and i may try milling a piston a flat as safley possable and mabey add some weld into the underside so i can btjng it closer yo flat,

I wanna run that piston at zero witn a horizontal squish and a hemi or eliptical dome head , i may have to rechamber something or try a smaller bore head with a larger chamber

I wanna see how this works out
 
if i remember right i built a honda 250R with a short skirt flat top piston and a tiney dome head and it ripped like nothing else !!!!!
 
I know for a fact a dished piston contains the combustion process a lot more efficiently than any other type of piston.

but using a dished piston usually means that one would loose compression because of the dish. But it could be gained baxk and performance added even f there was a custom made combustion chamber that had a horizontal queltch that matched the ize of the oiter rim of the poston before the dish starts in the cwnter, then one would run a small sized combustion chamber with a strait plug head and hemi dome for around 10.5:1 comp static.

I believe this would squish the charge from the outer edge DOWN into the piston dish and smal combustion chamber.
It would hold that charge well and centralize it peefectly.

I think this type of designe could lead to a verry powerfull high comp motor as that hemi design and dished piston combo lead to a fast but powerful burn in the chamber. The first thing i would try though would be a flat piston with a rechambered head that has no squish and is basically one small eliptical chamber the size of the bore, then recut the keyway on the magneto and retard the timing or run a hd lightning cdi witb a built in timing retard curve.
 
the flat top or dished piston combined with either the hemi or eliptical combustion chamber should work just fine as long as you can keep the compression high and the timing just below detonation... You can always run it on E85 or add some methanol or toluene to your fuel to bump up the octane to get away with a little more timing or compression.

You might be able to find a flat top 47mm piston out of a chainsaw engine or an rc model engine, just gotta find out which ones have the same bore size... and there's always treatland, I've seen some rather flat pistons while browsing thru there that could probably even be milled a little.

Dished pistons do work great but finding one in this size may be a bit of a challenge... unless you know of any 4 strokers that have the same bore size and the pin to crown hight is fairly close to the same...
 
From what I understood about a dome shape piston is hot spots are more even and better managed perhaps? This would have to be with a proper done squish band ''after market head'' IMO on a H.T. motor. I was told that the way the air reacts with the squish band that the cooling effect from this was more optimal in this regard.

Still if the plugs chops look good mebbe nutton to worry about? Never heard any complaints here yet with some of the modificatoins going on concerning this yet?
 
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i think bairdco used a aftermarket piston in one of his racers before, cant remember if it was cast or forged. now piston coatings have come along way, but on a china?
 
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