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Old 05-20-2003, 08:53 PM
Greg in Oz Greg in Oz is offline
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Join Date: Jan 2003
Location: Sydney, Australia
Posts: 627
Measured improvement or imaginary?

A few years ago (before I discovered these forums) I experimented with the ignition timing on our '90 300TE (3.0 M103) by way of changing resistor values. In fact, I made up a multi-position switch box with every value resistor listed in the workshop manual.

In Australia, our car only came with a 220 ohm resistor from memory (despite the owners manual showing the multi-position selector as fitted in Europe). This resistor was presumably to suit our relatively low octane regular unleaded fuel (92 RON). I was prepared to make exclusive use of higher (95 or 98 RON) octane fuel if by changing the resistor value improved power output and possibly fuel economy. Initially I tried no resistor (supposedly for maximum advance) and could feel no change. I then tried different resistor values and got varying results. This made me decide to use a measured approach by actually checking the ignition timing. The first observation I made was that the resistor only affects the full load timing (ie. minimum vacuum). At part throttle it makes no difference what resistor is used. The second and more startling observation was that only two resistance values caused anything other than maximum retard, the original 220 ohms and 750 ohms from memory. Higher values or no resistor caused maximum retard and a theoretical loss of performance. Fueling with 92 RON confirmed that only the 750 ohm resistor gave enough advance to cause any pinging.

Are we often imagining an improvement because we believe there should be one without actually measuring it? Don't assume that a higher value or no resistor will give greater advance, measure it. Also, anyone who claims a change at light loads or small throttle openings is imagining it. The difference between the original resistor and the only other value that gave a change in the right direction on our car is very slight and only at full throttle.

I did however, eventually decide on a setup with a small 2-position switch and just two resistors. Normally it is set to the higher value and we run 98 RON fuel. Should only 92 RON be available (eg. when travelling in the country) we have the option to switch to the lower value resistor for less ignition advance.

I have yet to perform any tests on my recently purchased 190E 2.3 to see what measured changes can be made to it by way of the ignition programming resistor.

It would be interesting to see what results anyone has obtained from this modification by way of dynamometer testing.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.
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