79el camino said:
i am building a chevy 350. It is bored .040 over. I have purchaced 23 degree heads from trick flow with 64cc cumbustion chambers 2.02" intake valves and 1.64" exshast. i am useing a comp cams cam that has .494 lift and .234 duration.
i am asking for your opinion on what good streetable style piston i should use.
should i use something with a dome or flat top?
should i use floating wrist pins or press fit?
should i use a forged piston or a cast?
what is your favorite piston style?
In theory a flat top piston in a wedge chamber gives the best flame characteristics, that is fastest travel speed across the chamber with excellent squish and quench characteristics. Pop up domes get in the way of the flame front slowing it down which is compensated for with excessive ignition advance which then introduces problems with detonation and preignition combined with high fuel consumption and emissions. A circular dish piston reduces compression to that tolerable by the fuels used, these are typical of OEM low manufacturing cost solutions, these have poor squish and quench as too much of the piston crown is too far from the head's squish/quench deck to be effective. This results in a tendency to detonate and preignite combined with poor power and excessive fuel consumption. The D dish piston keeps the flat top's fast burn rate, eliminating the use of excessive advance and its problems; it brings the flat top's excellent squish and quench characteristics making for much greater detonation and preignition resistance, this is often referred to as mechanical octane. Like the flat top, it pushes the mixture into a pocket in front of the spark plug giving a more reliable light off and through burn for good power and lowest fuel consumption with that power. The dish is available in several volumes and with the deck clearance space, head gasket volume, and combustion chamber space is used to optimize the compression ratio that available fuels can tolerate.
A digression to squish and quench. All engines hemi, pent, or wedge use this in some design form. It is an area where the piston and head close very closely together. With hemi's and pent's it's located around the outside diameter of the bore pushing the mixture toward the middle. On a wedge chamber it is found on the side opposite the sparkplug and valve pocket, pushing the mixture toward the sparkplug. These parts, or features of parts, perform two functions; one is squish the other is quench. They are separated by time in the cycle of compression to power. Squish happens first on compression as the flat surface of the piston closes toward the matching surface of the head. This ejects the mixture toward the sparkplug with great force both stirring the fuel and air together and increasing the density of the mixture directly in front of the spark plug. This both improves the chance of the plug lighting a burn (reduces miss and late fires), and it speeds the burn so cylinder pressure is optimized for piston position to press on the crankshaft with the greatest force possible (best power and use of the energy you pay for). At what is called the "late burn" part of the cycle is where detonation is like to occur. The temperatures and pressures ahead of the flame front are getting very high to where the remaining mixture is entering the "diesel" zone where it's happy to just blow up. To counter act this tendency is now the quench function of the close fitting parts of the combustion chamber. This is now a zone with a lot of surface area to volume, so it acts as a heat sink, delaying the point where the temperatures and pressures become so great that the mixture explodes instead of burns. These days of restricted octane fuels has made this feature very important as you can no longer just throw more Tetra-Ethyl-Lead at the problem.
Piston construction has lead from simple aluminum alloy castings and other aluminum alloy forgings along with what is often called impact extrusion to the hyper-eutectic aluminum alloy castings. The old simple castings lacked strength and were subject to expansion and contraction issues that were managed with steel inserts. These required moderate clearances and wore quickly resulting in lost power, high fuel consumption and emissions. Forged pistons are very strong but expensive. They require a lot of clearance as they expand and contract a lot with temperature changes. This makes for poor ring seal till they are fully heated up. This is bad for fuel, life span and emissions. The poor cold to cool sealing of these pistons wears the piston and cylinder wall as the fuel in the blow-by washes the oil off these parts allowing metal to metal contact. The hyper pistons are a very strong casting, good into the range of 500 HP and they are very thermally stable. This means that the clearances can be reduced which stabilizes the piston. This keeps the rings sealed, preventing blowby from wiping the oil off the piston skirt and cylinder wall. The result is better fuel economy, longer life, more power, and lower emissions.
For a street engine, even a high performance one, press in pins are fine to use. Floating pins are used as a means of insuring there is articulation in that connection should an overheated piston grab the pin in a race engine. And a floating pin makes field replacement of the piston between rounds a simple thing should that need to be done. I can't recommend that OEM press fit rods be modified to floating bushings as this removes much needed material, thus strength, from these rods. However they can be honed for a floating pin without a bushing. For a street engine of moderate performance the steel pin can be run against the steel rod with the upper rod just drilled for an oil hole. In a high power street engine or a sanctioned race engine where stock rods must be retained, there are Casidium plated pins which will run under high load and heat against a bare steel bore without galling, but these things are a bit pricier than plain pins. You will note in your search of pistons that some hypers are for a floating pin, these however, can be run as a pressed pin, most shops can do these, or you can hone your existing rods and run 'em steel to steel if this engine is in the 400 hp zone or less and isn't being raced at least not long and hard. Casidium doesn't last forever, it isn't something you use on a street engine expecting not to see the pin again for a 100,000 miles. But on a street engine the steel on steel shouldn't become a problem with good lubrication.
Bogie