Read this if you have done the "free horsepower upgrade"

[Greg in Oz]

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.

[el presidente]

Thanks for posting. Excellent research and presentation

[drbrandini]

Greg,

Nice work, I'm glad we have good detectives here such as yourself!

[2phast]

Adjust the ignition timing on vehicles has been proven in many circles to net a increase in performance. Normally in conjunction with other mods, this is done via chip tuning and requires the vehicle owner to run a higher grade of gasoline (92-93 octance in the US)

All these tests prove is that the modification does not work for you. It is really not a valid test nor is is really applicable to US market vehicles. We don't even know the condition of your test vehicles, for all we know you could be running on 5 cylinders (3 in the case of your M102). I am sure this is not the case but you can see where I am going with this.

Also, if your implying that MY "seat of the pants meter" is physcological, then you have really made some poor assumptions. After dynoing many cars over a 10 year span, I believe "my seat of the pants meter" is pretty accurate, not dyno accurate by any means, but accurate enough to tell that something positive is going on.

Your efforts are admirable though, much more through than mine on this topic, but then again, I tried it, found a noticable improvement and shared my findings.

I did not discover this nor was I the originator of this modification. That credit belongs to the dedicated members of mercedeshop.com, The SCCA racers and MB mechanics that verifed this modification.

[Greg in Oz]

I originally changed the value of R16 on our 300TE some years ago, long before I ever read about it on any forum. I decided to try it simply through studying the workshop manual for the M103 and realising that the setup we got in Australia was for our low octane regular fuel. I was prepared to use higher octane fuel if by changing R16 it meant greater power and/or better economy could be achieved. It was at that time I realised that progressively higher resistor values did not necessarily give greater ignition advance, the reason I ended up using 750 ohms for the 300TE. It was only last weekend that I decided to try the same experiment on my more recently acquired 190E 2.3 5-speed Sportline to find that for it either 1.3k ohms or 2.4k ohms gave the best result. On either car, an open circuit gave the least amount of ignition advance. I also seem to remember reading in either the M102 or M103 engine manual that an open circuit condition is read as a fault condition which give the most retarded ignition curve.

It was after reading numerous threads on several discussion forums that I decided to describe my findings with my cars. I did mention in my original post that these results were obtained with my cars, both Oz deliveries from 1990. Different models and years from different markets may give different results. Please, nobody take this as suggesting that they don't know what they are doing. I simply am suggesting that it might be worth verifying the results of doing this mod to your cars with a timing light at the very least. I wouldn't want to think that any of us are driving our great cars that we enjoy so much in less than the best state of tune. It is important to me as I am sure it is to all of you. I can say with pride that I keep my cars in top condition mechanically and cosmetically, verified by top results in all types of club events from motor sport to economy runs to concours.

Thanks for reading and for the encouraging comments.

[Badinfo]

This is very interesting. Could someone figure things out if they have a light for US spec m103s? I would be interested to know if 750ohms gives us the greatest advance (or lowest retard). Because I mean, Greg said he checked the timing on his Oz deliveries, and found 750 got the most advance (or least retard). A timing light sure beats my seat o the pants meter in determining advance/retard curves... Guidance requested...

[Greg in Oz]

Originally posted by Badinfo
This is very interesting. Could someone figure things out if they have a light for US spec m103s? I would be interested to know if 750ohms gives us the greatest advance (or lowest retard). Because I mean, Greg said he checked the timing on his Oz deliveries, and found 750 got the most advance (or least retard). A timing light sure beats my seat o the pants meter in determining advance/retard curves... Guidance requested...

I too would be very interested for someone to check their US spec car with a timing light. Better still would be to see some dyno figures.

I would have expected that the ignition modules to have been the same for all markets with simple changes to R16 to suit the differing markets. Maybe not? It just seems odd that MB would fit both a different module and different R16. From memory, the US spec M103 used 750 ohms whereas mine originally used 220 ohms. I think the Euro spec with the multi-position selector (fitted with all resistor values) used 750 ohms for the higher octane fuel setting (marked S) and 220 ohms for the lower octane setting (marked N). My owner's manual covers this selector and only suggests using either of these positions (not others).

[Mike300ce]

Greg in OZ i am also from Sydney and I tried the test and pulled out what i am 90% sure was the resistor and as you say it didn't prove to give me a power increase in fact the car seemed to lag if anything, my car runs better on premium with a little bottle of octane booster(only run twice to clean injectors), and with the resistor in.

I also have some other questions and would like to have an at length talk with you. My car is currently getting a few little dings ad scratches cleaned up but when they are repaired I would like to catch up, and maybe tag along to one of the day events of the MB club in SYD.

[Greg in Oz]

Originally posted by Mike300ce
Greg in OZ i am also from Sydney and I tried the test and pulled out what i am 90% sure was the resistor and as you say it didn't prove to give me a power increase in fact the car seemed to lag if anything, my car runs better on premium with a little bottle of octane booster(only run twice to clean injectors), and with the resistor in.

I also have some other questions and would like to have an at length talk with you. My car is currently getting a few little dings ad scratches cleaned up but when they are repaired I would like to catch up, and maybe tag along to one of the day events of the MB club in SYD.

Mike,
It would be great for you to come along and join us on club events. We would love to see your 300CE and discuss various aspects of our cars. We have one general meeting each month (except January) and average a couple of events or activities each month. Please join us at any of these. Contact me should you need more details.

Greg

[Mike300ce]

cool i will do thanks for the invite greg.
I was also wondering which of the resistors gave you the best ignition advance.

[Greg in Oz]

Originally posted by Mike300ce
I was also wondering which of the resistors gave you the best ignition advance.

For my M103, 750 ohms. You might also like to follow the thread here
http://www.peachparts.com/shopforum/showthread.php?postid=422677#post422677

[jasondew]

interesting. with all the talk about the r16 i did a little test today. i have recently installed an A/F meter and have used it to tune the mixture a little. these 4 runs were made with the car up to operating temperature, on the same road in the same directions. what i did was made a run down and back on a local road with the r16 in, then one with it out, then in again, then out again. my A/F meter indicated more rich at WOT with the r16 out. now connecting and removing the r16 isn't likely to do anything to the engine's fuel delivery. likely, it does affect the conditions in the combustion chamber, leading to a more or less complete burn. so, what does it mean that with the r16 plugged in i'm getting a more lean exhaust, indicating a more complete burn with the same amount of fuel? anyone? i have an idea but someone out there has got to know more than i do about the subject.

thanks,

[reynolds]

This is great. I played with my resistor several times off and on and couldn't really conclude anything. I didn't want to spoil everyone's fun by saying that I didn't think it made that much difference one way or another. I have a 1991 190E 2.6, came with a 750 ohm resistor. I have shorted it, open circuited it, and rigged intermediate values, and really could never tell for sure if anything ever changed.

I hope someone will perform the timing light test on a US spec car. If there is a magic resistance that gives a noticeable change, I would definitly give it one more try.

Wayne

[JSlabotsky]

Well I don't think I've seen a mesage board yet where guys didn't get into a heated discussion over timing advance. A few degrees timing advance makes so little difference that you'll never convince me that a person can feel the difference. For that matter, I'd almost go so far as to say that if it DOES make a dramatic difference, then probably the "before" setting was out of whack to begin with.

I've played with timing advance on several of my cars. I had a 1992 Mazda Miata with a Jackson Racing supercharger for a couple of years. They recommend retarding the base timing 4 degrees to avoid detonation, unless you run a knock control device. Everyone on the message boards swore up and down that there was a huge difference in the low-end power between 6 and 10 degrees advance at idle. Since I had an MSD timing controller, I decided to find out for myself. Couldn't tell the difference. Granted, my ass-dyno isn't as finely tuned as some, so maybe I'm nuts.

I'm a little new to the benz thing, but common sense would seem to indicate that an open circuit would be read as a fault, and consequently would revert to the safest possible setting. Now, I could see a case for replacing the resistor with a straight jumper - that would create zero resistance while maintaining a completed circuit. But this is all just a guess, so I probably should shut my yap....

[joe p]

Personal opnion? I feel like it works but, there are other small gains there as well. IE, ditch the reactor downpipes found on many 103's, play with 100 to 750 OHM resistors in the coolant temp return signal line and the resistance of the "timing spout". Afterall, adding 5 degrees of advance lead is not going to help much on a fully corked up engine.



Joe