31 Jul
2001
31 Jul
'01
4:57 a.m.
This is the same process used by Porsche and Mahle to cast the 924 and
944/968 engines. The problem was in the casting process. GM tried to
cool the blocks too fast which led to uneven distribution of silicon in
the aluminum cylinder wall. Porsche and Mahle used the same process,
but put heaters in the cores of the cylinder molds to slowly allow that
walls to cool. This allowed the silicon to evenly distribute in the
wall. I had the head off my 944 at 145k miles and the cylinder still
had hone marks with no visible wear or ridge at the top cylinder wall.
Now at over 300k miles, the compression is starting to drop. Probably
needs new rings.
joe wrote:
At 09:48 PM 7/30/01 -0700, Fred wrote:
GM saved a lot of money by making an aluminum
block with a friction
reducing coating (Teflon??). It worked well for a limited time.
Actually they didn't add a coating to the cylinder wall. What they did
was to make the engine out of high silicon aluminium, then they chemically
etched away some the aluminium in the cylinders to leave high-silicon
content for the pistons to ride on. This was GM attempt to get away from
having to add steel cylinder sleeves to an otherwise aluminium engine
block. (You CAN NOT run aluminium against aluminium.) Needless to say this
design was highly dependent on high quality oil, frequent oil changes and a
GOOD oiling system. Unfortunately the oil pump and oil system were a joke
and Americans aren't known for frequent car maintainance.
Joe