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1) The small piston has both of the compression rings closer to the piston crown. These rings will be more prone to coking the oil and getting stuck. Synthetic oil becomes more critical. The ring lands closer to the crowns decreases emissions and increases efficiencies. This relocation of the rings also increases the resistance to heat flow from the ring lands to the skirts and then to the cylinder walls, and to the surfaces cooled by the piston cooling oil jets. So closer to the fire AND further from the ice.
2) The lighter piston will warm up faster and will be hot faster on boost runs - less thermal mass. If an application or service takes the piston temperatures near a point where the temperature would cause the aluminum to fail, a change such as this might not be a good idea. If an engine is pushed hard before the block is up to temperature, then piston expansion can reduce running clearances during this transient state. The ligher piston will have this effect increased. I don't know to what extent this creates any real problems/concerns. Might be more critical with all metal pistons as opposed to (these) high silicon pistons that have reduced thermal expansion.
3) The shorter piston skirts reduce the heat transfer path (not length) from the piston crown to the cylinder walls and also reduces the active area cooled by the piston cooling oil jets. The absence of skirts on the sides also reduces the heat transfer and increased temperatures. These reduced skirt contact areas will reduce friction losses to some extent. Again - if an application or service takes the piston temperatures near a point where the temperature would cause the aluminum to fail, a change such as this might not be a good idea.
4) The resistance of gas flow past the piston skirts is trivial compared to the resistance getting past the rings and can be ignored as a significant factor. Blowby gas volumes will be affected by other things. Most critially would be stuck piston rings from using oils that are not able to take the heat of the ring locations.
5) The closed pockets on the sides of the heavier piston might be traping some oil that could increase mechanical losses. Probably impossible to know.
1&3 add up to hotter running pistons and ring lands. This should increase MPGs but might not be desireable outcomes for engines with higher boost levels.
The reduced mass of the piston reduces inertial loads which can be vey high at high RPMs. Longer rods reduce piston lateral loads.
So how do these changes to heavier rods and pistons work with the existing balancing shafts and crank counter balances? I am not familiar with those issues.
, Sun, 8 May 2005 17:38:25
, Sat, 7 May 2005 13:32:05
, Fri, 6 May 2005 16:54:18
, Thu, 5 May 2005 22:45:12, Fri, 6 May 2005 11:36:30
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