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forged vs cast

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So, are you blaming a detonation-induced failure on the piston? One that, as you said, could have been avoided?

FYI, forged pistons will survive longer while being subjected to detonation than a cast piston, due to the alloy's better heat-transfer rates through the crown and ring lands than high-silicon castings.

If you work much in the automotive side, you will see one interesting aspect with hypereutectic pistons. Manufacturers like Keith Black, who specialize in hypereutectic (high silicon) pistons spec larger ring gaps and they are typically nearly twice as large as the ring gaps seen on standard cast or forged pistons. The reason for this is because the hypereutectic (cast high silicon) pistons transfer heat of the crown faster and therefore the top compression ring runs hotter so it requires a larger gap to prevent butting. When I built my last small Chevy, I ran the top ring at .030 and OEM spec was .016. 2nd ring was .016 and oil ring was whatever it ended up being, around .016. That motor probably burned 4 quarts of oil within its entire 80,000 mile life before I sold it.

Forged pistons are more tolerant to detonation not only because they are stronger but because they run much looser clearances. When I built my small Chevy, clearances for forged pistons usually start at .0035 for street motors and run upwards of .0045 for motors with power adders such as boost or nitrous. With the hypereutectic pistons, I set up at .0015 for a street motor. KB recommended .0025-.0035 depending on the power adder used.

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If you guys want some information about pistons. Keith Black has some really no-nonsense, easy to understand, and illustrated articles about piston failures and design. Check it out.

http://www.kb-silvolite.com/article.php

Remember, your modern Japanese dirt bike uses a cast hypereutectic or high silicon alloy piston.

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If you work much in the automotive side, you will see one interesting aspect with hypereutectic pistons. Manufacturers like Keith Black, who specialize in hypereutectic (high silicon) pistons spec larger ring gaps and they are typically nearly twice as large as the ring gaps seen on standard cast or forged pistons. The reason for this is because the hypereutectic (cast high silicon) pistons transfer heat of the crown faster and therefore the top compression ring runs hotter so it requires a larger gap to prevent butting. When I built my last small Chevy, I ran the top ring at .030 and OEM spec was .016. 2nd ring was .016 and oil ring was whatever it ended up being, around .016. That motor probably burned 4 quarts of oil within its entire 80,000 mile life before I sold it.

Forged pistons are more tolerant to detonation not only because they are stronger but because they run much looser clearances. When I built my small Chevy, clearances for forged pistons usually start at .0035 for street motors and run upwards of .0045 for motors with power adders such as boost or nitrous. With the hypereutectic pistons, I set up at .0015 for a street motor. KB recommended .0025-.0035 depending on the power adder used.

Heat transfer is as much in the piston design as it is in the alloy.

And as I've already stated on more than one occasion, forged piston do not always run looser tolerances. The Wiseco Racer's Choice for my bike runs much tighter tolerances than the OEM cast piston.

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The higher crown cooling rates have little to do with the alloy. KB pistons run the ring pack closer to the crown so the ring pack runs at a higher temperature and transfers heat to the cylinder walls faster.

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The higher crown cooling rates have little to do with the alloy.
The reason for this is because the hypereutectic (cast high silicon) pistons transfer heat of the crown faster
Well make up your mind.

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Well make up your mind.
It's not just me, then.

I take exception to the notion that the rings ever have much of anything to do with heat transport away from the crown. First, it makes little difference from the standpoint of heat moving from the center of the crown to the much greater mass around the ring grooves as to where the ring is located. Secondly, the major path that heat takes by conduction to move away from the crown is to the skirt, and high skirt clearance is a major cause of piston heating problems as a result.

In either of these paths, there are a number of thermal barriers. One between the skirt and cylinder wall, and two more at each ring (one between the piston and ring, the other between ring and bore). The primary source of cooling on a piston crown is convection to the incoming fuel/air charge, including evaporative cooling by the fuel itself.

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It's not just me, then.

I take exception to the notion that the rings ever have much of anything to do with heat transport away from the crown. First, it makes little difference from the standpoint of heat moving from the center of the crown to the much greater mass around the ring grooves as to where the ring is located. Secondly, the major path that heat takes by conduction to move away from the crown is to the skirt, and high skirt clearance is a major cause of piston heating problems as a result.

In either of these paths, there are a number of thermal barriers. One between the skirt and cylinder wall, and two more at each ring (one between the piston and ring, the other between ring and bore). The primary source of cooling on a piston crown is convection to the incoming fuel/air charge, including evaporative cooling by the fuel itself.

Don't forget the oil cooling jets in most modern high performance 4-Stroke engines.

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Those do exist, but not generally. They are, in fact, a fairly recent addition, and most of the engines in this industry that do have them had them added within the last 4-5 years. They do add cooling, but they are still far less a factor than the intake charge, as oil makes a mediocre coolant.

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Not to be mean - but I think your dead wrong grey...

Modern progression has learned the GREAT importance to using oil as a primary cooling medium...and a little oil goes a LONG way...it is also why synthetics have really shined in extreme high end applications. You can use less oil to get better cooling without the power losses attributed to frictional losses when excess oil is used.

EDIT: less oil meaning you can squirt a valve spring with a finite amount of synthetic oil that will not "flash" off where a conventional oil would - which would then normally require more oil to get the job done and keep the oil from superheating and flashing off.

It's why I genuinely believe in a performance synthetic oil for our four strokes especially when aggressive cam and spring set ups are used - especially on the hondas with such a small oil reserve.

I would bet that heat in the oil is also why you see huge oil coolers on the PC kawi's - for added volume and probably some neat tricks inside with oiling set ups.

Nascar has been doing some serious research in this area - and a lot of cool data has been published if you look hard for it.

I also believe that the rings do provide a major cooling avenue for the piston but agree that the skirt also provides a major function here. However WITHOUT oil - the skirt would provide very LITTLE cooling function because the contact area is much too small to be effective. Oil essentially provides a direct contact patch between the skirt and the cylinder and makes up for the imperfections in fit between the two.

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Oil has poor cooling properties. That's why it's commonly used as a quench medium for tool steels, it cools them a lot more slowly than water would. The fact of the matter is, pistons cool through the rings. That's why pistons in Nikasil cylinders run cooler. There is less material between the piston ring and the cooling water jacket. My comment about hypereutectic pistons running cooler wasn't supposed to say hypereutectic pistons run cooler. It was supposed to say a cetain design of hypereutectic piston that runs the ring pack closer to the crown runs cooler. Don't misinterpret what I said about oil having poor cooling properties. Undercrown oil cooling is necessary to achieve the power outputs of modern engines. Cooling rates through the ring pack simply aren't enough to match today's high specific outputs so oil cooling in addition to cooling through the rings is a must, especially with engines running only two pistons rings.

Diesel engines typically have many compression rings, often 4 or more. This is not done to seal in the higher compression and combustion pressures. They run more rings to dissipate the higher crown heat seen by compression ignition engines. Pistons cool primarily through the rings.

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Pistons cool primarily through the rings.

Agreed. The rings cool the piston by transfering heat from the crown to the bore, via the piston rings. Then the heat of the cylinder is disipated from the engine cooling system, or via lots of cooling fins (on an air cooled engine).

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Diesel engines do in fact use more compression rings primarily for the purpose of maintaining the extremely high compression pressures required for their operation, and not for cooling. It's also common for larger and/or heavier duty diesels to use a second oil ring set along the bottom of the skirt.

I also did not state that there is no cooling through the rings, only that there are limitations as to the conductive potential along that path. The limited size of the contact area, and the intermittent nature of the contact between the upper and lower surfaces of the rings with the lands caused by piston reciprocation reduce the efficiency of that channel as a conductor.

Further, I did not say oil would not cool a piston, only that it does so rather poorly, synthetic or otherwise. Any examination of the heat drawn off of an oil supply by a radiant heat exchanger compared to a water radiator of similar size will show that, and it's an established fact. The problem is that oil absorbs heat slowly and conducts heat poorly within itself compared to water or air.

Likewise, there is a limit to how rapidly the skirt can transfer heat, most especially in a high performance four-stroke with a slipper skirt piston.

The normal temperature of gasoline engine exhaust runs around 1,200 °F, and peak combustion temperatures run considerably higher. This is also approximately the melting point of most aluminum alloys and it is only the constant influx of intake air that prevents the piston from deforming and failing. Convective cooling is the PRIMARY source of cooling for any piston crown. Not the only source, but the most significant and important. That is what I said.

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Further, I did not say oil would not cool a piston, only that it does so rather poorly, synthetic or otherwise. Any examination of the heat drawn off of an oil supply by a radiant heat exchanger compared to a water radiator of similar size will show that, and it's an established fact. The problem is that oil absorbs heat slowly and conducts heat poorly within itself compared to water or air.

One thing that needs to be remembered about piston crown cooling with oil jets is that, even though oil normally does not transfer heat as well as water, the difference in temperature (Delta T) between engine oil and the piston crown is really high. And, if you remember your basic thermodynamics, heat transfer rate is proportional to "Delta T". The oil coming off of the bottom of the piston crown must be REALLY hot, which is probably one of the reasons why "harrperf" likes using synthetic on motors like this.

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I totally agree on the use of synthetics for a number of reasons, one of the utmost of which is their superior tolerance of high temperatures. No argument there either.

Nor can I disagree with your point regarding the temperature differential between the cooling oil and the piston crown. But once again, I have to point out that I am not saying that there is no cooling effect from the oil jet under the piston, or even that it isn't significant, only that it is not the main source of crown cooling.

Since you brought up the Delta T issue, though look at the oil temperature of around 200 ℉ against perhaps a 5-600 ℉ crown vs. the combined effect of 75 ℉ intake air and a gulp of evaporating gasoline.

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I totally agree on the use of synthetics for a number of reasons, one of the utmost of which is their superior tolerance of high temperatures. No argument there either.

Nor can I disagree with your point regarding the temperature differential between the cooling oil and the piston crown. But once again, I have to point out that I am not saying that there is no cooling effect from the oil jet under the piston, or even that it isn't significant, only that it is not the main source of crown cooling.

Since you brought up the Delta T issue, though look at the oil temperature of around 200 ℉ against perhaps a 5-600 ℉ crown vs. the combined effect of 75 ℉ intake air and a gulp of evaporating gasoline.

I would agree with you that oil jets would not be the primary cooling method for pistons, but a fairly significant one none-the-less. It was a big feature of the original air/oil cooled GSXR-750s.

One thing that the cool intake charge has going against it is the insulating effect of the burned fuel on top of the piston. Good old Kevin Cameron (my hero) says that, if it weren't for that insulating layer, aluminum pistons wouldn't be able to withstand the heat of combustion. It must also somewhat insulate the piston crown from the the intake charge. :smirk:

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Kudos to Kevin Cameron. His books should be required reading for anyone who attempts to apply a wrench to a motorcycle.

Oh- you all should quit arguing. All of your points are valid to one degree or another.

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