914 D-Jet DistributorsThe 914 D-Jet used a number of distributors. The purpose of this page is to talk about the operation of the advance and retard systems used and what they’re supposed to do and why. Mechanical Advance - All Engine ConditionsBecause it takes time for the air-fuel mixture in a cylinder to combust, it is necessary to start the combustion process BEFORE the piston reaches top dead center (TDC). However, piston speed is not a constant, it increases as the engine speed increases, so it is necessary to start combustion earlier the faster the engine goes. This transition is handled by the mechanical advance system in the distributor.
The mechanical advance is a set of counterweights, cams, and springs below the points plate in the distributor. As the distributor turns faster, the counterweights move more outwards, and the cam acts on the distributor shaft, changing the relative position of the points plate to the shaft. In the 914 D-Jet distributors, this process begins at idle engine speed and continues up to 3000 rpm, where the maximum mechanical advance of 27 deg. BTDC is reached.
The manner in which this advance occurs as a function of engine speed is known as the “advance curve” of a distributor, and is determined on a test bed when an engine is developed, then the curve is implemented in the distributor by using specific weights, cams, and springs. A distributor can be “re-curved” by a skilled shop to provide a different advance curve, if required by engine modifications.
In the 914, the way the timing is set is that the engine is run at 3500 rpm with the vacuum hoses to the distributor removed, and through the access port in the fan housing, a timing light (connected to the spark plug wire for cylinder #1) is aimed at the timing notch in the housing. The distributor clamp at the base of the distributor is loosened, and the distributor is rotated until the red 27 degree mark is in the center of the timing notch. The distributor is then tightened and the timing is set. Note that if you are using the stock points and condenser, instead of a Pertronix, Crane, MSD, or some other electronic ignition system, you will first have to set the dwell as changes in dwell cause changes in timing. Dwell angle is automatically handled by electronic ignition systems.
Vacuum Retard - Idle ConditionFor reduced exhaust emissions, the timing at idle is retarded by several degress, lowering HC and Nox levels. The timing is retarded by using a vacuum cell attached to the side of the distributor that is connected to a port on the throttle body. The port is positioned below the throttle plate, where there is a strong vacuum when the plate is closed at idle. The extent of the vaccum retard is on the order of 8 to 12 degrees for 1973 2.0L engines, and 8.5 to 12.5 degrees for 1974 and newer 2.0L engines. The retard vacuum cell is a unit in combination with the advance vaccum cell described below, and the unit is designed so that if vacuum is present at both the retard and advance ports simultaneously, the vacuum advance dominates and the vacuum retard is inactive. Both the retard and advance cells use a single arm connected to the points plate, which moves the position of the points plate either to the retard or advance side, depending on which cell is active. Vacuum Advance - Part-load ConditionThe air-fuel mixture under part-load conditions is leaner (due to a leaner part-load mixture and additional mixing of the air-fuel mixture with residual combustion gasses in the cylinder) and burns more slowly and needs more time to combust, requiring additional timing advancement. Like the vacuum retard system, the vaccum advance system uses a vacuum cell connected to a port on the throttle body. In this case, the port is positioned right at the edge of the closed throttle plate, so that under idle conditons, there is no vaccum present. However, when the plate is opened a few degrees (like under part-load conditions), vacuum is exposed to the port and the vacuum cell is activated. The extent of vaccum advance is on the order of 6 to 8 degrees for the 1973 2.0L engine, and 5.5 to 7 degrees for the 1974 2.0L engine. Discussion and TroubleshootingThere are many references on the web that cover various aspects of issues with distrubtors, such as timing and dwell stability, this page will not focus on those topics. Instead, the discussion here is limited to the specifics of the 914 advance and retard systems.
Porsche and other auto manufacturers in the early 1970’s were confronted with increasingly restrictive emissions requirements on their cars from American and European federal regulators, to improve air quality. Over time, these new regulations proved to be enormously beneficial to countries around the world, not only in terms of lower air pollution, but lower health care costs, higher fuel efficiency, and additional benefits (e.g. such as the elimination of lead from motor fuels).
These new regulations proved to be challenging to meet. In the case of the 914, it became one of the first cars to use electronic fuel injection, which has many advantages in terms of emissions, perfomance, and efficiency over carbureted systems. Ignition timing is one area that the 914 also used to improve emissions. While vacuum advance had been used for some time with carbureted engines, the use of a novel vacuum retard system helped lower idling emissions significantly. However, this implementation came at a cost to idle performance and to the complexity of the control system and overall monetary cost of the system.
By retarding timing at idle, the 914 lowers HC and NOx, but it also lowers torque. This means that any additional idle loading on the engine, from mechanical loads such as an air conditioning compressor, or from electrical loads acting on the alternator, such as lighting, heating and cooling blowers, were not as well-tolerated, and caused more idle drop. On modern cars, this problem is solved by using active idle control, where when additonal loads cause idle drop, more air is directed around the throttle plate and the drop in idle is compensated. The 914’s D-Jetronic system lacked such active control, and as a result became very sensitive to idle loading.
As the model years of the 914 progressed, emission restrictions became more stringent. Later cars (75-76) had additional systems added to combat emissions, such as an air pump and exhaust gas recirculation (EGR). But one other change was made that is not noticed by many owners, which is that on the later cars, the vacuum advance port from the throttle body was attached to a hose that was capped off, and not connected to the vacuum advance cell of the distributor. Why? Because while vacuum advance improved engine throttle response under part-load conditions, it also increased HC and NOx emissions, limiting the ability to meet emissions requirements. Furthermore, since there was no change in the vaccum cell unit used on these later cars, the vacuum retard remained active long after the throttle was opened, because no vacuum was ever present in the advance cell. This had the additional effect of causing even worse off-idle response.
It is clear from this discussion that this is a complex system and that for proper operation, all components must be functioning properly. Below is a list of items that should be checked to make sure that the advance systems are working properly.
· Mechanical Advance: To make sure this system is operating properly, do the following. Remove the vacuum hoses connected to the distributor. Use a timing light with an adjustible advance/retard control to monitor the position of the timing mark. Check the advance of the distributor as a function of engine speed. Data below is for the two distributors used with the 2.0L engine, data for the 1.7L engine can be found in the Porsche 914 Technical Handbook. Note this assumes the engine is properly set to 27 degrees of advance at 3500 rpm. · Vacuum Advance and Retard: Both of these systems depend on a number of components operating properly. Below are some things to check.
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