how do hyper pistons work?

[modok]

Yeah, good searching Bill!
Did you ever think about how hyper pistons are interference fit at temp?

It's funny they don't look very springy, but they must be.
Maybe i should put one in the spring tester and see how much force it takes to squeeze it .005?

[Momus]

Air cooled 2 strokes are notorious for overheating pistons breaking down the oil film and then seizing on the skirts.

With these engines the cylinder is always cooler than the piston and as the piston expands the rate of heat conduction through the oil molecules increases and the diametral expansion is in effect self regulated.

These 2 smoke piston also run the ring band about 016" to 020" smaller ID than the skirt to stop the greater expansion of this relatively massive zone.

[Truckedup]

What engine? I race vintage British Triumphs and do build some street engines, iron cylinders, around 3 inch bore...Normal cast piston, not low expansion, .0045 minimum. Modern forged pistons, .0055 minimum...Anything less will result in a seizure if the engine detonates..

yeah that's exactly what I'm thinking, .002 is too tight, even if it IS some kind of lower expansion piston.


If you really want to know,, It's a Vw cylinder, adapted to a corvair, that fits a fiat 500 experimental airplane.
Don't believe me? I didn't think you would, it's that KIND of thing.

I gotta see a photo of it.... what head type ? A wedge or "bathtub" with inline valves? The vintage Triumphs have a 1930's deep hemi chamber that is detonation prone on pump fuel..And no cooling fan of course....Detonation at high power loading can cause near instant piston seizure...On a bike the rear wheel can lock before the rider pulls in the clutch and the bike may go down...I assume a piston seizure while flying is equally unpleasant...

[DrillDawg]

"How Much Silicon Do You Need?

Modern vehicle pistons are made from aluminum alloy, which means that the piston contains aluminum along with small amounts of nickel, copper, magnesium, and/or manganese.

Additionally, most modern pistons contain a large amount of silicon. Silicon is added to the aluminum because the resulting alloy is more resistant to wear and expansion than an alloy that doesn't contain silicon.

Resistance to expansion is key, as the modern engines are designed with very tight piston and cylinder wall tolerances. If the expansion and contraction of a piston can be minimized by adding silicon, tolerances can be very high. High tolerances improve performance and efficiency, which in turn increases fuel economy (a very big concern for modern engine designers).

The specific amount of silicon added to the aluminum ranges from 9-18%. At percentages below 12%, whatever silicon that is added to the aluminum dissolves into the solution. Once you reach 12% (or thereabouts), the aluminum alloy become saturated with silicon. This specific point is called the saturation point, and any silicon added after the saturation point will not dissolve in the finally aluminum alloy. Instead, this excess silicon will form a hard precipitate that remains separate from the aluminum.

An aluminum alloy that is satured with silicon is know as "eutectic." When the alloy contains silicon at a percentage that is less than saturated, it's called "hypoeutectic." When the alloy contains more silicon that then saturation limit, it's called "hypereutectic."

The characteristics of pistons in each of these categories are very distinct. Hypereutectic alloys are stronger, resist scuffing and seizure, and reduce groove wear and cracking of the crown at extremely high temperatures. They're also very resistant to expansion, because the high percentage of silicon essentially "insulates" the piston from the effects of heat.

Hypereutectic designs also allow for decreased distance between ring grooves, which improves the "seal" between the rings and the cylinder wall and improves efficiency. Finally, because hypereutectic pistons don't expand or contract, they're ideal for modern engines with tight clearance requirements. Generally speaking, modern engines use pistons made from a hypereutectic aluminum alloy.

If there is a downsides to hypereutectic pistons, it's that they're brittle compared to forged pistons. Therefore, forged pistons are more forgiving of extreme conditions (like those found in a race car), and they give you a greater margin of error when dealing with timing problems, as detonation is less likely to destroy a forged piston than a hypereutectic cast piston.

Still, when it comes right down to it, the operational differences between forged pistons and quality hypereutectic cast pistons are smaller than ever. Unless you're running an engine in a race, or under extreme loads caused by high boost turbocharging, a hypereutectic cast piston is likely going to perform just fine for your specific use."




I guess the amount of undissolved silicon is the key.

[Steve.k]

I have run all cast pistons in two stroke snowmobile and we did try forged at one point with not much luck. The two expansion rates do not jive together. The hyper pistons expand even less than cast so i would say they would need to be tighter. I've run the kb hypers in quite a few engines set to their spec with very good service. Even after several years on these pistons they do not change shape. I would think in a air cooled engine they would be very good. I recently had a block that was set at about 4.5 thou and was thinking on using. The spec was 2-2.5 so i called kb what they thought of extra clearance. He told me no to use!If the bore had been 3 0ver fine. But the exta was to much. He figure 1-1.5 over their spec was on the edge of to much. Tighter the better.

[Steve.k]

I have run all cast pistons in two stroke snowmobile and we did try forged at one point with not much luck. The two expansion rates do not jive together. The hyper pistons expand even less than cast so i would say they would need to be tighter. I've run the kb hypers in quite a few engines set to their spec with very good service. Even after several years on these pistons they do not change shape. I would think in a air cooled engine they would be very good. I recently had a block that was set at about 4.5 thou and was thinking on using. The spec was 2-2.5 so i called kb what they thought of extra clearance. He told me no to use!If the bore had been 3 0ver fine. But the exta was to much. He figure 1-1.5 over their spec was on the edge of to much. Tighter the better. We use Dave Trygstad big bore kits in our snowmobiles now and he will use nothing but custom cast pistons in Rotax two strokes. He said they have tried various types but cast is the only thing that lives when spinning 8500 all day.

[Frankshaft]

I have run all cast pistons in two stroke snowmobile and we did try forged at one point with not much luck. The two expansion rates do not jive together. The hyper pistons expand even less than cast so i would say they would need to be tighter. I've run the kb hypers in quite a few engines set to their spec with very good service. Even after several years on these pistons they do not change shape. I would think in a air cooled engine they would be very good. I recently had a block that was set at about 4.5 thou and was thinking on using. The spec was 2-2.5 so i called kb what they thought of extra clearance. He told me no to use!If the bore had been 3 0ver fine. But the exta was to much. He figure 1-1.5 over their spec was on the edge of to much. Tighter the better. We use Dave Trygstad big bore kits in our snowmobiles now and he will use nothing but custom cast pistons in Rotax two strokes. He said they have tried various types but cast is the only thing that lives when spinning 8500 all day.

Funny you should mention snowmobiles. The debate about cast and forged is on going. Some use forged, and have no issues, if this that or the other. But, the forged are more sensitive to cold seizing. The cast don't really do that. I am in the process of updating and going thru the engine in my 2013 Rush Pro R 800, they have a history of having piston issues. I have been on the fence for awhile. Factory upgraded h.o. pistons, or an aftermarket version, which are hypereutectic cast, or a wiseco or wossner forged. A place called Rk Tek is what I would run if I use forged, but I still see people seizing them on occasion. The cast don't. I don't want to be 120 miles into no where, and have a piston seizure, wouldn't be fun. I am aware of Trygstad too, and his take on cast vs forged, then you have Kelsey of Rk Tek, and he uses nothing but forged and says there is no issue as long as everything is right, you don't lean it out, you don't over heat it, you warm it up properly, etc etc. The Cfi Polaris 800's are more finicky it seems. But plenty of guys run the Rk Tek pistons, which are made custom to his specs, either wossner or wiseco, depending on availability. Is that who makes Trygstads pistons? KB?

[Truckedup]

Ducati has been experimenting with steel pistons...But nothing like the old Chevy Stovebolt 6's....And some Diesels engines may use them or soon use steel pistons.

[DrillDawg]

I think that most people don't realize that hyper piston are usually made with thicker crowns with the top ring placed higher than regular cast pistons, this places\stores the heat in the crown and is reason for a wider ring gap. If the rings pass the heat to the cyl wall sooner the skirts dont see as much heat.

[Steve.k]

I have run all cast pistons in two stroke snowmobile and we did try forged at one point with not much luck. The two expansion rates do not jive together. The hyper pistons expand even less than cast so i would say they would need to be tighter. I've run the kb hypers in quite a few engines set to their spec with very good service. Even after several years on these pistons they do not change shape. I would think in a air cooled engine they would be very good. I recently had a block that was set at about 4.5 thou and was thinking on using. The spec was 2-2.5 so i called kb what they thought of extra clearance. He told me no to use!If the bore had been 3 0ver fine. But the exta was to much. He figure 1-1.5 over their spec was on the edge of to much. Tighter the better. We use Dave Trygstad big bore kits in our snowmobiles now and he will use nothing but custom cast pistons in Rotax two strokes. He said they have tried various types but cast is the only thing that lives when spinning 8500 all day.

Funny you should mention snowmobiles. The debate about cast and forged is on going. Some use forged, and have no issues, if this that or the other. But, the forged are more sensitive to cold seizing. The cast don't really do that. I am in the process of updating and going thru the engine in my 2013 Rush Pro R 800, they have a history of having piston issues. I have been on the fence for awhile. Factory upgraded h.o. pistons, or an aftermarket version, which are hypereutectic cast, or a wiseco or wossner forged. A place called Rk Tek is what I would run if I use forged, but I still see people seizing them on occasion. The cast don't. I don't want to be 120 miles into no where, and have a piston seizure, wouldn't be fun. I am aware of Trygstad too, and his take on cast vs forged, then you have Kelsey of Rk Tek, and he uses nothing but forged and says there is no issue as long as everything is right, you don't lean it out, you don't over heat it, you warm it up properly, etc etc. The Cfi Polaris 800's are more finicky it seems. But plenty of guys run the Rk Tek pistons, which are made custom to his specs, either wossner or wiseco, depending on availability. Is that who makes Trygstads pistons? KB?

Im not sure who he uses. All I know is he uses cast as we had this discussion over forged.His earlier ones came from Elko I believe who build rotax pistons.Your right its usually a cold seizing with sleds as guys stop to bs then take off and dont re warm engine enough. If you know the deal and warm up likely no troubles. But sometimes its a short stop and then when hit snow again it throws enough on cooler to give a quick shot of cooler water. Piston temp climbing and cylinder cooling to fast.

[MadBill]

2-strokes look to be a natural for skirt coatings, particularly Line2line's abradable material. Yea or nay?

[Steve.k]

Yes most of the two stroke mfgs are coating the pistons. Every little thing to control deto and friction. We did a tour of rotax in gunschirchen austria and the engineers said once the engines climbed over 700cc detonation was very difficult to manage. If it were not for the electronics today we would be in a whole other situation.

[Truckedup]

Around here Dynotek in Batavia NY has dyno testing dedicated to sleds and bikes...Lots of big HP racing stuff..I asked Jim Czekala ,who owns the shop, about cast or forged in 2 strokes...He says about 50/50 from what he hears.....I assume sleds making 500 hp with power boosters probably use forged.

[Steve.k]

The four stroke boosted sleds are forged yes. Pretty much every two stroke cast. Other than the odd sled that runs wiseco forged.Some of the grass draggers and ice draggers forged. But most two strokes i see cast.We have run 100 hp nos kits on 600 triples with cast.

[modok]

That all makes good sense now that I'm getting used to the idea.

Look, and you guys all know this, I bet.
What happens when a piston overheats?
You see FOUR CORNER scuffing.