Jeep Engines - Oljeep

152 MOPAR PERFORMANCE PARTSPistons and RingsIPart NumberCast Pistons and Rings for Jeep 4.0L EnginesBore StrokeIP48769623.905" 3.41" 6.125" 9.2: 1PISTONSThe 4.0L engine uses high strength aluminum alloy pistonswith an anodized top ring groove and crown. Piston skirtsare coated with a solid lubricant (Molykote) to reducefriction and provide scuff resistance. A pressed fit piston pinis used to attach the piston and connecting rod. The pistoncombustion cavity is a single quench design. For morepiston (and piston ring) specifications, refer to 4.0L EngineSpecifications, 'Engine Assembly' section of this chapter.In summary, the secret with pistons is that they belightweight and have smooth, round tops if they are domed.The secret with piston rings is to get them to seal to thepiston and the bore at W.O.T. (wide open throttle).Piston Compression Ratio RatingsPistons are usually described by their bore or overbore sizeand their rated compression ratio. This compression ratiorating seems to get people confused. Generally there are8.0:1, 9.0:1, 10.5:1, ll.S:l, and even 12.S:l pistons. Thereare dished and domed pistons. Generally domed pistonshave a high compression ratio rating and dished pistonshave a low compression ratio rating. However, the pistondoesn't have a compression ratio by itself. It has to beinstalled into an engine.In an effort to help clarify this situation and help racersknow what they are basically getting, most pistonmainufacturers rate their pistons by compression ratio. Todo this, they assume that the engine is at blueprint ornontinal specifications. For example, Mopar PerformanceParts has a line of 9.2:1 pistons. If your engine had anominal compression ratio of 8.2:l from the factory andyou installed these pistons, then you would have a 9.2:lcompression ratio.The, confusion comes in because most engines don't havethe actual nominal compression ratio. It might be 8.2:l. Itmig,ht be 8.0: 1, or possibly 7.8: 1. If your engine really hasa 7.8:l compression ratio and you install 9.2:1 pistons,you'll actually have an 8.8: 1 compression ratio. The pistonis only one of many factors that contribute to the exactcompression ratio. There is cylinder head volume (cc) andgasket thickness. There is also the deck height which takesinto account connecting rod length changes, strokevariations, and block height machining tolerances. (Valvenotches, chamfers, disWdome volumes, and pin height areassumed to be part of the piston.)So when you change the piston design and move the pindown to increase the compression ratio (or add a dome),you are only changing the piston. Let's assume that thedeck tolerances are ,020" to .030" below the cylinder head,and the cc range is from 68 to 72. The piston itself can'tchange variations in these other parts. On any given engine,the difference is between the rated compression ratio for thepistons and the engine's actual nominal compression ratio.For example, if the engine's actual nominal compressionratio is 8.2:1, then 9.2:l pistons increase the compressionratio one point (9.2 - 8.2 = 1). Therefore, if the engine'sactual nominal compression ratio is 7.8: 1, then the resultingcompression ratio after installing 9.2:1 pistons is 8.8:l. Theonly way to determine if your engine has an 8.2: 1 or 7.8: 1compression ratio is to measure it. This requires you to cc theheadls, measure the deck heights with a micrometer or similardevi'ce, and check the compressed thickness of the gaskets.All pistons share this problem. If you don't make the requiredmeasurements, you won't ever know you're exact compressionratio. And there are still other variations. The only way toremove all these variations is to blueprint the engine.