Question to start off with... What's "more advance"? A larger negative # for spark advance or a smaller one?

I have my car set to RON93. The "official" objective of that is to change the ignition timing to run on lower octane fuel (according to the MB Star Diagnosis Computer). The real reason I do it is I find it gives my car much MUCH more power right off idle when I touch the gas peddle, as well as more peddle response overall.

I'm trying to find out if that is truly advancing the timing or retarding it from how it would operate if I was running in "default" mode, and by how much.

I bought the Car Code PC scanning software and OBD-II connector. I've been doing a good deal of data acquisition. I'm finding the OBD "SPARKADV" value [$0E] to vary between -10deg to -40deg. There's no true consistent pattern to "when" it's at -10deg vs. -30 to -40deg beyond when it's at low RPM, it's at low advance -- and it does seem that if I step on it, thus increase the absolute manifold pressure and MAF's measured flow, timing goes towards -10 deg shortly, then advance towards -40.

At idle, it's between -10 and -13deg. At a steady 30 mph in 4th gear (about 1500 rpm), timing's a steady -27deg. Interestingly, at a stead 90 in 4th, it's the same amount of advance. It only advances towards -40deg under hi-load and hi-rpm. Though again, there are plenty of times where I let off the peddle after accelerating, and only -then- do I see the timing jump to -40 for a few seconds, then back to round -27deg.

...so my question to all those in the know is, is this "all good"? Is there an OBD-II value to tell me if my engine is experiencing detonation?

thanx much all,
steve

You would be getting a timing adaptation code if you were detonating.

Your values for timing seem appropriate except for the minus sign, but expressing timing is a matter of how you look at it. Timing is usually expressed as degree before top dead center BTDC. Since all cars advance timing the value is considered plus.

But, the actual value if expressed mathmatically would be expressed as a point before a time or t-x or just -x with x being the degree of timing.

One thing I might point out is that timing is individually controlled per cylinder to stop knocking. The actual cylinders are identified by general position (cam and crank sensors) and specifically by the knock sensors.

Also that motor has dual plugs and the relative timing of each plug is adjustable as there are two coils also per cylinder. There are various firing strategies and the plugs usually don't fire together. If the plugs can be identified as "A" and "B" they are usually fired this way: A then B, B then A, A then B, B then A etc etc with the point being to sweep the flame front back and forth and keep even temps to avoid detonation from hot spots (while keeping it as hot as possible - probably). The length of time from A to B also can change and it can be set to zero so that they fire together. I don't think there is anyway to verify any of this other than through on-board diagnostics.

So the value you are seeing is only a generalized expression of what is really happening. if you were to view this with the Star Tester you could get a reading of the knock control timing correction per cylinder. You could even alter the timing in 7deg increments till knock affects one cylinder thus allowing a degree of built in help for technicians.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanx very much for the informed reply. Interesting you say "if you were to view this with the Star Tester you could get a reading of the knock control timing correction per cylinder. You could even alter the timing in 7deg increments till knock affects one cylinder". I'd expect such a thing to be possible. But speak to any MB dealer/shop, and they'll tell you "it's all set in the computer ... nothing can be changed".