Ignition retard is a system design on the actual distributor, whereas vacuum retard is a control design.
Why vaccum retard:
High vacuum is available in the intake manifold at idle so this system takes advantage of this natural condition. The vacuum port or hole is directly behind the throttle plate and is designed to send full manifold vacuum to the distributor. As the throttle is opened the plate passes over the hole and as the throttle opens further, little or no vaccum is present at the port. This assures that the distributor vacuum cell will be moved only at idle or when the throttle is closed. WOT ( wide open throttle ) movements from an idle position will not affect this system because the advance plate always moves to full vacuum advance when the the vacuum drops to zero.
WOT will create low manifold pressure which can have a negative affect on vacuum advance systems. It will have no affect on vacuum retard systems because low or no vacuum is what this system calls for to function properly. In essence, VR takes advantage of a normal condition where vacuum falls off quickly with WOT applications because this system responds to no or low vaccum signals.
Both mechanical and vacuum systems work at the same time on the distributors used on our cars. Vacuum controlled advance always works first regardless of the control system. It takes very little vacuum to make vacuum advance or retard work - as little as 7 to 8 inches of vacuum.
The terms '' vacuum advance '' and '' vacuum retard '' can be a bit confusing. In either case, the advance plate moves towards advance depending on when the signal is applied. Vacuum in advance systems is not present at idle and as you throttle up the increasing vacuum pulls the advance plate forward. With vacuum retard, full vacuum is present at idle and lowers as you throttle up; a spring inside of the vacuum cell on the distributor is released and the advance plate moves forward in the same manner as vacuum advance, only the control method is different.
Vacuum retard offers several advantages in terms of efficient operation. Should the vacuum cell or a vacuum line develop a small leak, which is very common, the high vacuum at idle will generally more than over compensate for any losses. Less than 6 - 7 inches of vacuum is all that's required for normal function, and a 113 engine can produce 18'' or more, so it usually works well providing there are no mechanical malfunctions.
This system of timing regulation is more in keeping with high performance applications which is what our engines were designed for. Either system can be made to work properly but fuel injected engines respond better to vacuum retard due to the design of the intake plenum and runners. Without at choke at the throttle valve to create high vacuum as the throttle valve opens, vacuum retard became the logical evolution to ignition control.
Prior to the fuel injected 230SL, 300SL engines had no vacuum ignition controls at all and were strictly mechanical advance. While early 230SL's used a vacuum advance system similar to those used on carburetted cars, it was soon realised that ignition controls functioned best using vacuum retard systems, so modifications were introduced during 1965 and remained in use until the end of production.
