Engine blocks and cylinder heads are often provided in a "raw" condition with only approximate dimensions and rough machining. Each team will continue to machine and modify the part to their own standards. NHRA pro stock engines are restricted to a maximum single-camshaft, 90-degree V8. Several bodies have different rules. "Mountain Motors", run by the PDRA (eighth-mile) and at selected NHRA events in 2019, do not have a 500-cubic inch rule, with some engines exceeding , to upwards of . The NHRA limits engine displacement in Mountain Motor categories to . The Australian National Drag Racing Association and IHRA have a maximum displacement engine limit. Depending on sanctioning body and class, engines may either be four-barrel carburetors or throttle body electronic fuel injection and must be a naturally aspirated intake system. Those that use two four-barrel carburetors may allow them to be "split" (i.e. sawn in half) so that each of the halves can be more accurately positioned over the slightly staggered intake runners. The intake manifold and heads are open to modification. The most effective intake manifold configuration has continued to be the "tunnel ram" for nearly 40 years. The carburetors are raised above the engine; the length and configuration of the intake passages ("runners") is critical to horsepower output. The tall intake manifolds necessitate the large hood scoop that is a signature of the pro stock class. (The hood scoop is illegal in the NHRA because of EFI) The NHRA formula (starting in 2016) requires, and the PDRA extreme pro stock permits, cars to use electronically controlled throttle body fuel injection systems. In the NHRA, an electronic control unit (ECU) will be implemented on the EFI systems, including a 10,500 RPM limit, with modern engines approaching 12,000 RPM. The rules that exclude forced induction of any sort, plus allowing head modifications, have resulted in pro stock heads being the most sophisticated in any drag racing category, with valve lifts in the 1 inch region. Modern pro stock engines generally produce around 2.5 hp/in3 (114 kW/L), and make upwards of 1,500 hp while being naturally aspirated. A complete NHRA pro stock engine can cost upwards of $100,000. Drivetrain Pro stock clutches utilize multiple discs. These must be serviced after every run to maintain critical tolerances that can mean the difference between a good run or severe tire shake. Since 1973, the most popular transmission was the Lenco planetary design,