4.2 V8 Audi: Race Bred & Road Worthy

Gear case (3)Where to start with the V8? Well, 1990 of course. This motor started out as a 3.6 Liter, but only for 2 years. The upgrade to the 4.2 V8 started a legend. The 4.2 V8 Audi was an aluminum block and head motor. But not just any aluminum block. The block was cast with a high silica content, and the cylinder bores were scintered out to size, which literally dissolves the aluminum away from the harder compounds, leaving an extremely durable cylinder wall material. They just don’t wear, pretty much forever.

Picture this: When a cast iron block with aluminum pistons gets hot, the pistons expand more rapidly than the block, slowly decreasing the piston to block clearance. This increases friction, and if the piston temperature gets high, can start to “burn” the piston. In overheated cast iron motors, we see little globs of aluminum stuck to the cylinder walls, usually near the top where the piston side forces are highest. When an aluminum block with aluminum pistons gets hot, the block expands more than the pistons, increasing the clearance. This decreases friction, at the expense of consuming more oil. Which is exactly what happens. It is typical of a 4.2 V8 Audi motor to use oil at a rate of around one quart every 2000 to 3000 miles. More oil of course at high speeds. Good thing they hold 10 quarts of oil. The oil cooler was its own radiator, mounted underneath the coolant radiator.

The original factory recommended timing belt interval was 90,000 miles, which quickly changed to 60,000 miles. It seemed that a lot of these cars were sold right when the car got to 60K, as the timing belt interval corresponded to the second major service. If one notices the pattern now days, the manufacturers seem to cleverly stagger these events. There are a couple of cheap timing belts available, and some mechanics did not change any of the rollers during a timing belt job. The result seemed to be a number of these engines that broke timing belts before replacement was due.

The cylinder heads were oddly like a knockoff of the Volkswagen 2.0 16 valve heads, and flowed well. The 4.2 V8 had some headgasket failures over time, failing at the oil return passageways, and sometimes leaking large amounts of oil. The early 3.6 had never really shown the same problem. Valve cover gaskets leaked, and crank speed sensors died. Breather hoses for the PCV would rot over time. But the basic motor just never failed by itself. A dead 4.2 V8 always seemed to have a mechanic or the pilot to blame.

The 4.2 V8 in the 1996 Audi A8 had a number of changes. The two distributors were replace with individual coils. A variable geometry intake manifold with a different throttle body and breather system topped the motor. The oil cooler which had been a separate air cooled unit under the main radiator became a water cooled unit on the right lower side of the block. It was fed coolant through a short weak plastic pipe that failed in a number of cars. The cooler itself was known to fail, spewing oil into the coolant. Even more headgaskets leaked oil from the return passageways. The A8 had a two wheel drive version with a 3.7 engine, which we don’t cover here. Who buys a V8 with 2WD?

With the 2003 model, the engine changed to include 5 valves per cylinder, and the power output rose to 315 hp, which rose more every year to 335hp in 2005. Next was a chain driven version which went back to 4 valves per cylinder, which is still in production as of 2014. A high powered version drives the RS4, and it grew turbochargers for the RS6. This 4.2 power plant was the first engine in the famed Audi race car which continues to dominate LeMans, except now they use a Tdi diesel to win that race. The 4.2 V8 was also the first engine in the Audi R8 super car.

Gear case (3)