62Ranchero200":2icbvuum said:Hi Xctasy,
Your first remarks were a bit over my head, so I'm going to have to digest them for a while.
I think when you say cold cranking compression, you're referring to dynamic compression, and to the fact that a cam with more duration and overlap will reduce the dynamic compression. So I could run a 10:1 static compression ratio, but a considerably larger than stock cam (more duration and overlap), such as say one of the Clay Smith 274 cams, and the dynamic compression would not be so high as to cause knocking, even though the static compression is rather high for a street engine running on pump gas.
Now you've told me something that I haven't even heard a hint of before ... that "forged pistons reduce knock, and have about a 100 degree less combustion temperature under load." Can you explain why that is so? If so, a set of forged pistons might aid in controlling knock, and of course increase durability as you mention.
Thank you,
Bob
Forged pistons allways run cooler for any given combustion chamber or working indicated mean pressure than cast alloy pistons. For this reason, they hurt emissions on engines, and aren't CARB approved unless the engine came out with them in the first place.
Its to do with metelurgy ( grain structure and eutectics). The modern forged piston is dimensionally stable, strong, and able to operate as well as a cast piston. There are different types of cast pistons, high sand semi forged pistons that a technically alloy castings, which can work as well as forged pistons, and these are often used instead of forged pistons in some of Fords turbo engines, but generally, a forged piston will allow more compression, more boost, and more duration before detonation sets in. Ford engineers do elaborate tests and through that work, they win cost verses performance edges. Like in 1992, they tested the standout 5.0 HO EFI, and were able to do away with that years brilliant TRW Forged piston, as it was costing the Cleveland plant a motsa to build that 225 hp engine. The next year, the forged pustons were gone, and durability was just as good. The reason was Ford did the testing to see where detonation set in, and having a hotter temperature wasn't a concern. Since that time, no need for forged pistons on the GT40 engines. So the work was a cost saving for Ford. But they also make botch ups, even with the very best engineers in the world. The brilliant cast alloy Zollener shallow deck pistons and the problem Ford had with plateau honed cylinders killed a lot of 4.6 and 5.4 SOHC engines in 1997, and Ford then had to spend millions reworking there production tollerances. So a cast piston is a cheap option, which can cause problems.
The 1979-1981 carb turbo 2300 had the first low power use of TRW Forged pistons, but they didn't help the Mustang Turbo stop dropping rods or rattling its way to oblivion with just 127, 131 or 140 hp of load. Its problem was mixture distribution and running a 9.5:1 compression ratio. The forged pistons the 2300 ran for that car probably helped Ford climb out of the extent of warranty claimes made on that disasterous engine.
The working combustion temperature and pressure in a same Compression ratio combustion chamber is raised as the top dead centre parking point is moved towards a perfect O deck (where a piston is ture with the deck), and gets worse as it goes down to that awfull 123 thou depth found on all 250 US engines, and on some 240 and 300 I6's as well.
Leedging is therefore the exposed heated annular part at the top of the piston to cylinder wall. Lower the piston parks, more thats exposed, more detonation potential. Squench is a combination of Squish and Quench and is different when the piston is raised or the hesd is decked. So Ledging, squish and quench vary with different tollerances of piston to block deck height, and between the common 25 and 45 thou gaskets. The lads at Dearborn and Geelong Australia had very differEnt deck clearances on there 250 derivatives, and used composite gaskets in Australia, and steel gaskets in the US. From 1971 to June 1976, the 155 hp gross L code Aussie engine had a ballistic 9.2:1 compression ratio, 97 RON/MON leaded fuel, and was a very strong engine. Every year from 1968 to 1981, the US 250 lost power from that intial 155 gross, and ended up with 99 hp net just because it had to run pump grade 87 or the occasional splash of 83. The engine was tuned down to avoid combustion temperatures and had to use the same head and gasket as the 200. That's why the 103 to 123 ledge, and why the we are discussion knock resistance. Anything that improves the stock knock resitance is good.
I presonally would never, ever deck a cast iron Ford block unless it was a 460, but lots of people here have done it ,and people here find that is the best step to getting performance and knock resistance back. I personally have seen blocks crack at the front of 250 log head or X-flow combos over here in NZ when they are decked, and US blocks are much thinner castings than our Antipodean engines. So take a risk and go to it, but I'd say a nice Jeep 258 piston answers all your problems. If you can cope with a 70 thou overbore, then the ledging, squish and quench is optimized in one hit.