> >Hi,
> >
> >finally I got some "official" information concerning the ABH:
> >
> >The internal product information for the introduction of the V8 4.2
> >(10/91) says:
> >
> >"The engine block is made of an aluminium alloy with 17% silicon
> >contingent. In a special etch process the hard silicium cores are
> >exposed, which form the piston contact surface. Usual piston liners are
> >therefore needless.
> >
> >The light alloy pistons have galvanically applied iron coatings with a
> >flimsy tin film on it. Benefit: Highest friction resistance."
> >
> >The press map of 12/1991 says the same thing.
> >
> >So, what is it?
> >
Alumsil - The piston coatings are identical to the Porsche and GM ( Vega) engine technologies of that era- that's why Audi used
similar techniques- it was well proven by the late '80's.
These engines need really GOOD quality rings to withstand the abrasive nature of the silicon that is left
in the bores. Also the break-in procedure is critical to seat the rings ( see the Porsche 928 shop manuals for one such procedure)
In the original designs (circa 1972-1980) iron coatings ( over alum pistons) was the preferred method to insure that the piston
skirts would also survive the rigors of the silicon bores.
One cause of high oil consumption in these engines can be due to poor rings and/or bad break-in procedures. Also heavy piston side loads
can really screw up the ring/bore surfaces.
Modern plasma coatings can also withstand the silicon bores but they are more costly than the iron coatings.
I've been racing and/or rebuilding alumsil engines for over 30 years ( ran 200 hp Vega and 450 hp 928 engines) so I'm more than a little
familiar with the technologies.
