Posted by Charlie Biler x on January 31, 2010 at 21:24:59 from (64.201.85.120):
In Reply to: Re: 4.750 piston posted by steve il on January 31, 2010 at 18:02:07:
steve il,
The new pistons are not a copy of the old classic designs. They will be much lighter because of the modern materials and the mill cut profile.
The old pistons were many times cut as a cylinder. They had to be much looser in the bore, when cool, because the older materials would expand greatly when they heated up. New design pistons are ground much like the shape of an I beam. They actually become round when they reach operating temperature.
Another problem is that the new pistons are made from a different aluminum alloy. They will be at least a third lighter than what you are currently running. Balance is a major factor in engine performance. They will also shed heat at about double the rate of the old round and Invar strut type pistons. With one cool piston and three hot pistons; it would be a bear to fuel balance an engine.
Another problem is weight versus RPM. Every time you halve the weight of the piston and rod combination; you can easily increase RPMs by a third, with no change in stress to the crankcase. This is true as long as the pistons are all balanced the same. With an unbalanced load you will build a harmonic disturbance and it would be like smacking the crank with a hammer every second revolution. To get the best performance, and bearing life, you should put the rods on a scale and make all of them the same weight as the lightest rod.
You can always swap out one piston in an engine. You just have to put them on a scale and make sure all the pistons are in balance. Engine manufacturers all balance the new pistons before they install them in an engine. Today they even balance the rods too. That little detail is one reason today's engines run the way they do. They can make the bearings smaller and blocks thinner because they have learned the lesson that balance gives free power with less stress.
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