From: Wally Plumley [wplumley@bellsouth.net] Sent: Friday, February 01, 2002 9:27 AM To: 928 Subject: [928] Re: Sold! 83 928 At 08:58 AM 2/1/02, thurman wrote: > I have seen my engine described variously as: >"A big heavy lump of low tech iron" >"An aluminium block engine with sleeves" >"An engine using the Nikaseal (sic) process of bore etching" >"All aluminium but none of the above" > >Can someone please tell me how my block is made? And more importantly, >if, after getting a better car, can the old engine be removed and >rebored as part of a rebuild? The 928 block is made of Reynolds 490 high silicon (not silicone) aluminum alloy. It is precision cast, with controlled cooling for the bores, so as to bring the silicon to the surface. After boring and honing, the block is chemically hones to remove a very thin layer of aluminum from the surface of the bores, leaving a very hard, slightly porous layer of silicon for the pistons and rings to rub on. Since aluminum does not function well rubbing against this silicon surface, the aluminum pistons receive a very thin coating of iron as a wear layer. Net results: The cylinder bores are perhaps the longest-wearing ever put in a production automobile. It is virtually unheard-of for a 928 engine to wear out. Reboring the cylinders is possible, but requires special care and knowledge, and usage of a chemical honing treatment to establish the silicon layer. Replacement pistons must have an anti-galling wear layer, such as iron, applied. This coating is rare and expensive in aftermarket pistons. The block can be bored, and then have iron sleeves installed, but this eliminates much of the advantage of the Reynolds block. Wally Plumley 928 Specialists From: Jay Kempf [jkempf@tds.net] Sent: Friday, February 01, 2002 9:48 AM To: 928 Subject: [928] Re: Sold! 83 928 Thurman, Eric, Great question: None of the above. Your engine (block) is a very special high silicone content aluminum alloy. This same alloy (Reynolds 390) was the same that ended in disaster for the designers of the Chevy Vega. That's because they didn't do the research that the Porsche design team did. Porsche came up with a heat treatment method that involved a very long multi step cool down process for the block. This allows portions of the geometry to cool slower than others. This makes some of the molecules in the alloy sort of separate and migrate toward the different hot and cool zones. Specifically the cylinder walls were cooled slowly by introducing heat with core heaters while the block is still at or near its transform (zone between metal and liquid) temperature. For aluminum this is just under 1000°F. The silicone in the block (silicone is basically glass or a kind of hard ceramic) migrates toward the walls and separates somewhat from the aluminum making super hard and low coefficient of friction nodules near the surface. When machining the block a special process is used where a grinding paste on a felt wheel finishes the bores. This removes the soft aluminum to a very shallow depth leaving the silicone nodules to be in contact with the ring in the finished engine. Very cool stuff! Basically what this achieves is a bore that won't wear in normal use especially if you use synthetic lubricants and keep them clean. This is somewhat of a simplification and definitely not in scientific terms but is basically the story. There is a ton more details that make the geometry of the block not crack or warp with all of the internal stresses that this uneven cooling introduces but that doesn't matter. Just trust that the Porsche freeking geniuses that figured this out got it right. It is a major miracle and almost unprecedented in automotive engineering. Techniques like this and even more ridiculous ones are used all the time in aerospace applications like inside the hot zone of turbine engines but not in a reasonably priced sports car. BTW the bottom half of the block and the heads did not receive this attention. They are pretty standard post mold stress relieved castings. Wally, Jim B., how'd I do? Did I forget anything? Jay Kempf 79 US 5ish speed. -----Original Message----- From: 928intl.com [mailto:Mark@928intl.com] Sent: Friday, March 15, 2002 3:07 PM To: 928 Subject: [928] RE: 85-86 vs 87 and up block Is there a difference between the 85-86 32 valve (US) blocks and the 87 S4 and up block. Yes but you can still use a 1985-86 block for a 1987 and later with some minor mods. There are fuel injection bosses in the valley that need to be removed and the starter bracket is different. Thank in advance, Tom midlman@seanet.com 87 S4 AT (no stroker in the works just yet ;-)