Issue link: http://digital.nexsitepublishing.com/i/100966
Throttle Cables Cables for throttle actuation are used on the 914, the front engine cars and the early 986 Boxster, with the 993 the remaining mechanical linkage throttle system. Although there is always some concern over cable stretch, wear and breakage, the cable systems have proven to be quite reliable as long as they are routed correctly, securely attached at the ends and other critical points, and remain uncontaminated with dirt or debris. Most of the Porsche throttle cable housings are similar to the middle sketch in Figure 2 and have adjustable end points; i.e., the end of the housing can be moved by changing the position of the lock nut attachment on the threaded housing end fitting. This additional adjustment option is often helpful with the 914 idle, since it allows small changes in cable length to change the throttle plate idle position. The normal idle adjustment is made at the cable end attachment or with a bypass screw that controls air bypassing the throttle plate. Cams can be used with throttle cables, such as on the 944, to vary the linearity of the throttle opening with throttle pedal travel. If a cable attachment lever is an eccentric plate with varying radius, the relative change in throttle opening for a given change in cable position will vary as the lever radius of the cam changes at the point of contact with the cable. The original 944 throttle body has an eccentric cam on the throttle plate that provides a relatively small throttle opening for small foot pedal deflections, but then increases more rapidly as the pedal is further depressed. A number of aftermarket suppliers offer an alternate 944 throttle cam that is circular in shape and linear in opening action. This provides greater throttle opening for the initial pedal motion and a faster throttle response coming off idle than the stock setup. There is no real change in engine performance, of course, but some owners like the more responsive feel. The latest approach in the use of throttle cable controls is employed by the Boxster using ���deflection boxes��� to assist in routing the cable around 90 degree corners. An enclosed plastic box is fastened to the body at the corner and the cable is routed through it. The box contains an internal sector that allows the cable to move with a minimum of friction or effort and also protects the cable from dirt and debris that might accumulate. Maintenance on throttle cable installations is minimal except to periodically check for free operation, cable condition and tightness of the end attachments. In the 2000 model year, both the 996 and Boxster had their throttle cables replaced with electronic control systems in which an accelerator pedal module electronically controls the Motronic engine control and throttle valve. This was done to assist in providing electronic throttle control to the Porsche Stability Management (PSM) system that was initially an option and, starting in 2005, became standard equipment on all the models. Transmission Cables A major change from most Porsches was introduced in the 1998 model 986 Boxster and 1999 996 Carreras with the use of a cable shifter mechanism between the gear shift lever and the mid or rear www.pnwr.org November 2011 37