Markus,
I think your explanation is very good concerning the 17C switch. Here is a brief explanation about timing in general. It is followed by my explanation for timing in the 70/71 280SL with manual transmission.
I'm very open to corrections and suggestions to improve the explanation. Please contribute. Once we have something close to finished I'll add it to the technical manaual.
TIMING AND VACUUM RETARD 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency. For the most part these advance and retard operations are accomplished by the distributor itself via the rpm sensitive centrifugal weights inside the body of the unit.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the auto cam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing". Vacuum retard has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum retard drops out entirely; it has no part in the "total timing" equation. When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark retard, and when the throttle plate opens, manifold vacuum drops, and the vacuum retard can returns to zero, returning the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum retard doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor auto cam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum retard, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum retard is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
At steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum retard is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway.
1970/71 280SL w/Manual Transmissions
In addition to the distributor the 17C degree switch and the warm running device (WRD) also come to mind. When the SL engine is cold the fuel injection system enriches the fuel mixture. When the engine is below 17C the cold temperature switch in the engine cylinder head provides a ground to pin #6 of the 8 pin relay located nears the battery. When a ground signal is present on pin #6 the two way valve is energized and vacuum pressure is applied to the distributor retarding the timing. The above paragraph tells us that rich mixtures burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency. As the engine warms the FIP leans out the fuel mixture and the 17C switch opens to remove vacuum retard on the distributor. The 100C switch is also designed to affect the timing via the two way valve. When the engine temperature exceeds 100C the switch closes and a ground is provided to pin # 5 on the 8 pin relay. This causes the two way valve to be energized and the timing retarded. This has the same effect on the engine as the 17C switch although for a different reason. With the timing retarded, fuel is burned more efficiently
??, idle is increased and engine temperature is reduced?
??. As the engine temperature cools below 100C vacuum retard is removed. As stated above timing retard via the two way valve will be initiated when the engine is very cold or warm. It will be removed when engine temperature is within normal operating range.
Note: In all cases vacuum retard is only available if the intake manifold pressure is high enough to control the vacuum can on the side of the distributor. As the throttle plate is opened, under acceleration, the intake manifold pressure is reduced and the two way valve will cease to retard distributor timing.
On the 1970/71 models vacuum retard is also used to reduce exhaust gas emissions. In addition to the temperature controls described above the vacuum retard function via the two way valve is initiated in the 1970/71 SLs by the 4 pin speed relay located near the battery. This speed relay works closely with the engine rpms and throttle/loading as set by the intake manifold pressure as follows:
At 2400rpms and above the speed relay energizes the two way valve. If the engine is operating above 2400 rpms and is not under a load condition the throttle plate will close, vacuum pressure is sufficient and will be applied to the distributor to retard the timing. If the engine is under a load condition the throttle plate is open and vacuum pressure will be reduced removing or reducing the timing retard function on the distributor. The purpose of the speed relay is to retard timing when the engine is above 2400 rpms and not under a load condition or coasting. When coasting and not under load distributor timing is retarded and fuel is burned more efficiently reducing emissions.
? At 2200 rpms and below the two way valve is not energized by the speed relay.
Jeff C.
1970 280SL 4-speed
