exhaust all cramped into a small rear engine bay made emission and noise control difficult which were the problems Porsche had with the 911. After deciding that the mid-engine layout did not allow enough room in the passenger compartment, a front-engine, rear-wheel drive layout was chosen. Porsche engineers wanted a large-displacement engine to power the 928, and prototype units were built with a 5-litre V8 engine rated at . Ferdinand Piëch proposed the use of a 4.6-litre 90 degree V10 engine with 88 mm bore spacing, a derivative of the Audi 5 cylinder engine (used to power the Lamborghini Gallardo), which in turn is based upon the Volkswagen EA827 unit. The Porsche board objected as this engine would give rise to a rumour of a new 911 with a front-mounted Volkswagen-based engine, but also it is theorized that the board wanted to maintain some distance from the Volkswagen group. The resulting all-alloy M28 engine has multiple unusual features. Its bore spacing is 122 mm, reflecting the use of thick, all-aluminium cylinder barrels with no steel liners. Water jackets were very large, which indicated that Porsche may have wanted to eventually take this engine racing. In order to maintain a low hood line, the engine was designed for airflow first, thus the spark plugs are located at the top of the head. The four-bolt bearings are sizeable and are fed oil via grooves in the bottom surface of the block. The bearings themselves are supported by a large one-piece structure that forms a lower block, and the cast aluminum oil pan is bolted onto this part. The oil and water pumps are driven by a timing belt; DOHC engines introduced in 1985 used a hybrid timing system; the timing belt operated only the exhaust camshafts, and the intake camshafts were operated via an internally-mounted simplex roller chain from the exhaust camshaft. This method reduced the complexity of the timing belt layout, and as a result, required fewer components, such as idler rollers and guides, and led to easier and less costly maintenance of the timing belt. This timing system was later used in the Porsche 944, and was also used by Audi and Volkswagen on their belt-driven DOHC engines. The first two running prototypes of Porsche's M28 V8 used one four-barrel carburetor for initial testing. Actual production cars employed the planned Bosch K-Jetronic fuel injection system. When increasing concern within the