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350SLC, 500SLC, 300TE, 190E2.3 Sportline
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On many discussion forums we have have read of the "free horsepower upgrade" available to those with M102 and M103 motors by way of removing or changing the value of the R16 ignition reference resistor. Many who perform this simple modification (generally removing R16) claim great results using the "seat-of-the-pants meter". It is amazing what we can make ourselves believe if we want to. Well, I have got some bad news for those who have simply removed R16. Read on.

It is generally accepted that maximum power and fuel economy will result from using the maximum amount of ignition advance possible without causing pre-ignition (knocking or pinging) for a fuel of given octane rating. The EZL ignition modules fitted to the M102 (1.8, 2.0 & 2.3) and M103 (2.6 and 3.0) motors follow different advance curves dependant upon a programming resistor known as the R16 reference resistor. Most threads suggest that a higher value resistor results in more advanced ignition timing and thereby deduce that removing the resistor completely will give maximum ignition advance. NOT TRUE. A higher value resistor can give more advance up to a point but an open circuit (resistor removed) is read as a fault condition and sets the most retarded ignition curve. Many falsely believe that R16 can just be a variable resitance that will dial up whatever ignition advance you wish. This is not the case. The ignition module has a finite number of ignition advance curves or maps that are selected by certain fixed resistor values (0, 220, 470, 750, 1300, 2400 ohms).

As an owner of both M102 and M103 powered cars I decided to test the effect of different values of R16. As a temporary arrangement, I removed the plug containing R16 and removed the resistor (220 ohms in my 300TE and 470 ohms in my 190E). I then connected a 2-core cable to the plug of sufficient length to reach into the cabin of the car where I had a box containing a rotary switch with all the above listed resistor values as well as an open circuit position. I then test drove each car (with 98 RON fuel) and noted the effect of different resistor values. Most obvious was how little effect different resistor values had. Only at higher engine speeds at full throttle on long hills was any change noticeable at all and only then by repeatedly switching from one resistor to another. How anyone could notice a change after stopping the car, getting out and removing or changing the resistor, getting back in and driving further, escapes me. Also interesting was that the best performance was not necessarily obtained at the highest resistance setting and certainly not when open circuited.

I then decided to verify my findings with a timing light. As suspected, greatest change occured at higher speeds (> 3000 rpm) and high load (simulated with vacuum hose disconnected from ignition module). R16 has no effect on ignition timing at idle. As also suspected, an open circuit R16 gave the same minimal advance as a short circuit. At the intermediate values different amounts of advance occured and this was dependant upon which motor. For my M103 six, 750 ohms actually gave the greatest advance. In my M102 four, 1300 and 2400 ohms both gave the same result so I went with 1300 ohms. The original lower values were presumably to suit our low octane (92 RON) regular unleaded fuel here in Australia. We also have 95 RON premium unleaded and more recently some suppliers have changed their premium to 98 RON (which I now generally use). In the 300TE (which we use for long journeys) I have fitted the R16 plug with a small switch which is normally set to the 750 ohm position (for premium fuel) with a 220 ohm position available for the situation where only regular fuel may be available (eg. when travelling in country areas).

My reason for posting this is to alert those who are contemplating or have performed this modification to take a measured approach with their car. Ideally a dynanometer would give the most valid readings but at the very least a timing light should be utilised. Without such equipment to verify the effect of any change of the value of R16, I would leave it untouched. I would certainly advise against removing it unless you can measure results different to those obtained with my cars. Possibly, different results may occur with different models delivered in different markets. It comes back to not modifying something unless you fully understand the implications of doing so.
 
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