From: showngo@telusplanet.net Sent: Sunday, February 04, 2001 7:51 AM To: 928 Subject: [928] Re: Considering a 928s4 (LONG) - long again Dave S. wrote: > Nope. '84-'86 "Euro" (16V) had 97.0 mm bore x 78.9 stroke, '85-'91 100.0 x > 78.9, and the GTS's '92/'92-'95 100.0 x 85.9 Some GTS's apparently had oil > consumption problems as pointed out below by Walt. All Euro "S" 16-valve engines from _mid/late '80_ through to the introduction of the 32-valve engines had a 97mm bore. All other dimensions/specifications above are correct. My understanding of the "oil consumption problem" on the GTS engines relates to the introduction of forged pistons on these engines. This was the first time the Porsche V8 used forged pistons. All previous Porsche V8's had used cast pistons. Because of different expansion characteristics between the two piston types, "cold" piston-to-bore clearances are not the same. Due to the different grain structure and increased grain density of forged aluminum vs. cast aluminum, the forged pistons "grow" slightly more than cast ones when both have achieved operating temperature. What this means is that the engine with forged pistons will require a slightly larger piston-to-bore clearance when cold. This makes it a little harder for the piston rings to seal, hence increasing the possibility of oil consumption. High compression engines using forged pistons (like the GTS) are often more critical of proper operation during the break-in period when new. I was unaware of any "porous block" condition, but my standard disclaimer applies.... I don't know everything about anything, and some of what I think I know is not necessarily correct. ;^) Brad Orr From: Marc Thomas [mmthomas@ix.netcom.com] Sent: Wednesday, February 27, 2002 10:54 PM To: 928 Subject: [928] FW: GTS Oil Consumption - repost from 12/11/99 ---------- From: "Marc Thomas" Date: Sat, 11 Dec 1999 10:03:12 -0800 To: 928@rennlist.org Subject: GTS Oil Consumption I have talked to many GTS owners and Porsche about the oil consumption of GTS'. The fact of the matter is that the oil consumption is usually in specification, and the technical reasons for the consumption are valid. The GTS pistons have unique oil scrapping system (combination of ring tension, type of ring and drain back capacity) designed to leave more than the usual amount of oil on the wall during each stroke, this is to improve piston to wall lubrication, so more oil ends up in the combustion chamber to be burnt. There is also a change in the material of the piston. Keep in mind that it is the first forged piston used in a 928! There are also specifications on cylinder to piston clearances, which fall within a specified tolerance, and may be responsible for variances in oil consumption within models. Since I have measured many GTS pistons, I have found that the tolerances do vary slightly even within any one group. The design of the piston is very different than S4 or earlier pistons, both in design and material. Why they made this design change is simple, Porsche wanted to improve the durability of the GTS under extreme driving conditions. It is the same reason that they added a oil cooling system to the manual transmissions! Constant improvement! But it did have a "down side". In my view, a quart of oil is a lot less expensive than an engine rebuild? Of course, I can revise your pistons and eliminate the oil consumption..... but it cost more than a quart of oil! I have seen GTS with 2000 mile per quarts, and I have seen them with 450 miles per quart. Porsche made a decision on a design and the jury is still out. So, If the oil consumption is out of the spec, then there is generally another reason why...breathers, guides, etc. Always check the plugs as indicators of internal engine condition...also use compression and leak down testing. Drive hard and drive safe, and have fun! (and keep a quart of oil in the back) -- Marc M. Thomas DEVEK 650-592-5287 phone 650-610-0557 fax http://www.devek.net From: Jim Bailey [jim@928intl.com] Sent: Tuesday, March 20, 2001 4:31 PM To: 928 Subject: [928] Re: oil consumption on a GTS The pistons for the GTS are shorter so they do not hit the counter weights on the crankshaft when they are all the way down . In the redesign they moved the rings on the piston . When you increase the stroke (how far up and down the piston moves) the cylinder head limits how far up it can go . Additionally side loading on the piston increases as the piston gets closer to the crankshaft the connecting rods work at a greater angle which is detrimental to oil control and wear . The longer movement of the piston and the larger diameter circle that the big end of the connecting rod and crankshaft scribes the closer it comes to the oil pan which increase the amount of oil being beaten into a froth . So the added windage and leaking of some compression past the rings causes more oil vapors / fumes to be drawn into the intake and burned . The 2.72 ring and pinion ratio for the GTS increased the RPM at normal driving speeds further increasing oil burning and engine wear form a 2.2 ratio to a 2.72 . At cruise speed the engine turns 23 % faster burning more fuel , oil and wearing out the engine faster .