Re: Sterling 525P Pistons
rsalemi said:
The Sterling pistons are produced by Federal Mogul and were ordered from Northern Auto Parts - they are a replacement piston for the 4.0
They have a slightly shorter compression height and with the shorter 258 rod ended up .055 down in the unmachined block. The head was not milled; measures 58.6 CC (on the high end of spec) and with the .065 FM headgasket calculate out to 9.1 to 1 CR.
The quench is out of the supposed spec; but the engine doesn't seem to care.
The chamber is outside the questionable zone for poor quench. It should work good.
One misunderstanding about quench is that it's useful importance is more where not to build chamber clearance, rather than a magic number for what works. There is a narrow clearance range where the chamber promotes detonation: a range from ~0.065" to ~0.110".
Run less clearance, less the 0.065", and you prevent detonation by not having enough clearance volume in the perimeter to sustain combustion, and gain some power and emissions benefit from the charge turburlence of compressing all the charge to a centralized location in the chamber. The need for tight perimeter quench clearance becomes more critical, demands a tighter clearance, as the compression is raised higher. The step from the perimeter quench band needs to be distinct between it and the resulting chamber (sometimes hard to accomplish with an irregular piston dome).
Run more clearance, more than 0.110", and you prevent detonation by not allowing the charge volume to experience the localized high pressure regions that promote detonation. You may lose a little turburlence, and you lose the emissions benefit of the more tightly controlled charge density at ignition, but not much power in the rpm ranges we operate the 4.6L six. Real power loss is not that great for a 9:1 compression ratio engine.
Where you have to be careful with quench, is the 10:1+ engines with poor clearance volume (~0.070-0.100") and where the three dimensional chamber shape has irregular sharp edges to develop hot spots and detonation. This is where the perimeter quench clearance has to be idealized and compared to the irregular chamber/piston clearance. Some chamber shapes have no perimeter quench band and can be a detonation problem (some of the older AMC six chambers for example). Lucky for us, most modern engines use flat top or dished pistons that prevent this piston dome interference problem by design.
Large bore engines suffer the most power loss from designing on the wide side of the poor quench range because it is simply more difficult to efficiently light a large chamber with a single plug (the old Chrysler hemi's used dual plugs to overcome the drawback of the large hemi clearance volume). If you are building an open chambered late 60's or early 70's BBC or BBF into a high performance package, with irregular pop up pistons, be aware of the quench trade-off. The Chrysler hemi of the same age avoided the problem with a fairly smooth high compression piston dome (sometimes it's better to be good by chance, rather than over-engineer the design with misunderstood concepts).