Does anyone have any theories on why Audi does not use an aluminum block on either the 1.8 or 2.8 engines? Especially considering that many other manufacturers switched to aluminum years ago. I'm sure it would do wonders for weight distribution and handling.

I think the block is basically an ancient design, same as in far earlier Golfs and Rabbits... but check the archives, there was just some discussion of this.

Weirdly, the newer 5 valve heads are aluminum. You'd think they would make the block aluminum so that the block would expand and contract (as the engine warms and cools) at the same exact rate as the valve head. My 1986 Mazda 626 had an all-aluminum engine.

--Shane

...that's why many/most/if not all aluminum blocks have some kind of liner on the inside of the cylinder walls...the liner is usually made of...you guessed it, cast iron...over the course of 100,000 miles, iron engine blocks don't wear out as much as an aluminum block does...that's why they need liners...I would imagine Audi kept the old block because it is a proven/bulletproof design, and simpler to manufacture than an aluminum block with liners etc etc. Whatever the case, I don't think they'd save more than 30 lbs or so...and while that may be significant for weight distribution on a track, it doesn't really make much difference on the street...

Just my 2 cents...

-Ash 98.5 1.8TQMS (Where's my APR? Wait is killing me!)

I think that all engine blocks, whether they are made of cast iron or aluminum, have cylinder liners. This may have something to do with the wear properties of the cast material. In the case of iron blocks, the steel liner is placed into the block while the block is warm, then the block shrinks onto the liner. The liner is held in place at the bottom, while the cylinder head keeps it from sliding out under the immense cylinder pressure.
Iron blocks and aluminum heads are not a new combination. Small block Chevrolets and Fords have used this combination for generations (certainly since the sixties depending on the application), and the E36 BMW 325i used an iron block with aluminum heads. I think the reason is that head design changes quite frequently, look at the changes Audi has made in the last five years, but blocks tend to stay in production for years. A good example is the various types of Chevrolet small blocks, or the 3800 series Buick engines, or the Land Rover/Range Rover V8, or the Rolls Royce/Bentley V8 (yes, this 48 year old engine has just been reintroduced into the Seraph/Arnage body shell!).
HTH
Antony

Linerless aluminum blocks use an aluminum alloy with embedded silicon carbide (?) particles, which are then exposed in the cylinder bores by acid etching, leaving a piston ring sealing surface which wears longer than a cast iron block! But this is an expensive process, and most manufacturers use the cheaper iron sleeve design.
Chevy used cast iron heads on the small block V8 until 1986, introduced in the mid-year Corvette production.