03-23-2007, 12:03 PM #1
Pistons: Forged vs Semi Forged. What do you run?
I have been searching threw some old posts looking for a good discussion on pistons and couldnt find one, so I decieded to start one of my own.
I been wondering about the different advantages to running either forged or semiforged pistons. I know that forged pistons do not grow as much under high temps and also have cooler crown temps but have a bad tendancy to cold seize. Thus having to warm them up before every use.
The advantage to semi forged is that they dont cold seize as fast. Meaning that you dont have to warm them up. Now I know that standard semi-forged pistons are lighter than forged, but I will get to that in a second.
Forged pistons have a denser molecular structure which gives them the ability to grow less but they tend to reach their max diameter much quicker. So when the cylinders are cold the pistons have a chance to grow quicker than the cylinder walls thus creating cold seizure.
Semi forged pistons do not do this because their molecules are farther apart which makes the piston grow about the same rate as the cylinder. Although these pistons grow more at higher temps than the forged.
Now for the weight end of things. I know right out the box the forged pistons are heavyer. But the advantage of forged pistons is they can be windowed or "lightend" without loosing the strength of the piston. Windowing pistons is not a new concept and has been used for years in the motorcycle and atv circles. Basicly you are cutting windows in the lower portion of the piston connecting breathing passages for each of your intake ports. What this does is 3 things.
#1 creates more intake pressure because you have more fuel\air mixture being pulled from the windows in the piston.
#2 lowers crown temps because you have a large charge of fuel being pulled to the bottom of the piston.
#3 every ounce that you remove to make the window "lightens" the piston. Correctly done you can make forged pistons lighter than semi forged.
I have been wondering why I havent seen more of this type of modification on pwc. I currently am working on some prototype windowed pistons for my 1200. When I get it running I will let you know my results. Should be pretty interesting.
Anyway give me your oppion and what kind of pistons you are running.
03-23-2007, 12:49 PM #2
Not sure what brand names you are comparing but here is my .02
My experience with a forged piston for the 65U motor (Wiesco) was not that great. It was fine for racing applications because of its lighter weight but large piston to sleeve clearences had to be maintained due to thier habit of growing. This led to a short piston life from skirt collapsing. Also the dome would tend to cave in slightly after 20-30 hrs.
Cast pistons for the 65U (Pro-X or OEM) were fine for racing or rec use. The pistons did not grow nearly as much letting you run much tighter clearences and longer life. However they were slightly heavier. We did window the pistons to keep crown temps down in our racing conditions.
This was my experience with these pistons which seems to be almost opposite of your research. Just take it with a grain of salt because I am sure there are other brands with even different results. I personally would run Pro-X in your application with .0045 clearence. Use the OEM wrist pin bearings though.
03-23-2007, 02:09 PM #3
All the pistons I have worked with were either Weisco or OEM. Windowing a non forged piston will lead to piston colapsing due to the strength of the metel. Forged pistons dont grow as much as semi forged at higher compressions and temps...I know this from high performance car motors.
Only a forged piston can be windowed. The semi forged casting can not hold up to drilling holes in it. It makes it weak and causes it to collapse.
03-23-2007, 02:16 PM #4
I also think that mabey your clearance issues where from not letting the cylinders reach opptimum temperature.(possibly from too much flow threw the heads.) Which brings me to another point. Duel cooling heads can acctualy move too much water too fast not allowing for the water to transfer its heat.
This is why yamaha went from 1100 head gasket(which was more open). To the 1200 which had less flow. Because they wanted the water to pickup more of the heat. The only way to do this is by restricting the flow of water threw the head.
Just like any other part of the motor, you have to have the right setup in order to benifit from haveing pistons that wont grow as much.
I guess my point is that its not the piston that growing....more the cylinder thats not growing.
03-23-2007, 06:17 PM #5
You can never have too much inlet water; only the outlet side should be throttled. When the water outlet is restricted, water pressure is raised in the cylinders, raising the local boiling point. If low-pressure water hits an area that is above 212 deg. it boils and vapor locks that area. High Pressure will collapse this gas pocket and let cool water carry the heat away.
03-23-2007, 07:14 PM #6
My thoughts exactly.
Ok this is what I have been getting at. I have been reading where alot of forum members are making extra relief lines and going with bigger hoses. To me this cools the cylinder walls too much not allowing for proper cylinder expansion. More input but restricted output.
With forged pistons this higher cylinder temperature can be takin advantage of with a cooler piston.
Windowing the piston and angleing the windows towards the ports will create a charge of fuel under the crown cooling it better. This cooler crown will allow for higher rpms.
The angeled windows also create a boosted charge to the ports allowing for more fuleing at higher rpms. This also takes some weight off the pistons.
This is an old trick from back in the day just wanted to see if it would cross over to the pwc motors.
03-23-2007, 07:15 PM #7
You answered our question of how to raise cylinder temps to expand away from the pistons
03-23-2007, 07:32 PM #8
Forged Pistons requre more room as they do expand more than a cast piston. Also you want to let forged pistonst get to operating temp. before getting on it hard. Forged pistons are lighter and stornger than a cast piston. I do not know any info about a semi-forged? Never heard of such a monster.
03-23-2007, 07:52 PM #9
all cast pistons are semi forged. Meaning during the manufacturing process they dont keep it in the keel as long so the molecules are not as dense as forged pistons. Ask any top fuel dragster which pistons hes putting in his motor....forged or cast.
The reason they use forged pistons is because of the temper in the metel. Forged pistons by definiton have tighter packed molecules than cast pistons. For this reason they DO NOT grow as far as cast pistons. But they DO grow FASTER. This is a missconcepton about the differances in piston growth. The reason PWCs sieze these pistons easyer is because the piston grows into the wall faster. The open loop water cooling that these motors use do not build enough temperatures to properly allow the cylinder to expand.
Now the cast pistons do not expand as fast but given enough time at wot these pistons will stick and break.
How many of you have been riding along wot and the motor die. Only to come to a stop and then restart. Then descovering later that you have a small scar in the cylinder. Or one of your cylinders looses a little compression.
At wot the piston grows until it reaches the cylinder wall which is being cooled by the flowing water. This causes the minor seizure. When the motor dies the water stops flowing alowing the cylinder to expand and vola the piston frees itself.
What really turned me on to this was putting a water temp sensor inside the water jacket in the head. I noticed that even at wot the water never got over 100 deg. Being that these motors would operate better at closer to 160-170 deg. which would allow larger cylinder growth to accomidate the ever growing piston at higher rpms.
03-24-2007, 05:48 AM #10
Forged...windowed.....dual cooling in/restricted out.....water temp sensor in head/temp displayed on data tach.....this is on my boat....wifes: OEM or Pro-X.
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