I'm beginning to understand the disconnect now, and it begins with my intent when I first posted on this thread. I was trying to clarify that higher octane fuel can yield better performance because there is a theoretical best timing which the computer will attempt to approach. But due to inconsistencies in geometry, fuel/air distribution, or anything else that can alter the way a given cylinder responds relative to theoretical, a cylinder can and often will detonate prematurely or reach auto-ignition temperature at the timing/fuel/air settings the computer lists as ideal. Often only one cylinder spark knocks and they all get retarded. So the engine doesn't get to run at the theoretical map because it's getting retarded (opposite of advance) more often than would happen if higher octane fuel were used.

I did not originally say that timing was bounded by the map curve because that wasn't the point I was trying to make.

If it sounded like I was saying that timing was limitless,, then I miscommunicated my point. My point was, that fuel/air ratios AND timing can be FINITELY altered in both directions from the mapped codes by a predetermined amount which is also built into the computer. So NO, timing isn't advanced boundlessly from the mapped codes, but many ecm's have enough instrumentation such that they can tell for themselves what the ideal timing should be relative to the mapped codes. This is especially true of the Haltech E11 unit that I've been working with.

Even the first GM ecm's, the Computer Command Control units from late 70's-early 80's had limited "learn" capability where they actually deferred (not limitlessly) from the mapped fuel/air and timing codes to what had previously worked better for it. Some of the older control systems such as this had to be unplugged every so often such that they could build new memory banks of how the engine liked to run.

 

If I am not mistaken, the stock 4.9 is limited to 28 degrees total timing...it would take a serious lean condition/ problem to cause the engine to not be able to reach that point with low octane, no?

Again, just learning..

 

That is certainly the intent,, to map the proper advance for a given engine configuration. Most vehicles are mapped for optimum performance (i.e. the desired characteristics be it economy, high performance, etc.,, which is where performance chips come in). The 4.9 truck engines I would think would be mapped for a combination of good low end torque and decent economy. It doesn't necessarily take anything drastic to cause a cylinder to detonate or preignite. It could be the geometry of the head casting has a cylinder with slightly higher compression than the others, could be carbon deposit, could have a water jacket with a wall thinner than in other places causing a hot spot,,, could be that some cylinders are getting more fuel/air than the others,, or an exhause valve which is hotter for one reason or another, or any combination of things that can cause a cylinder,, if only temporarily to spark knock at ideal settings.
I don't want to pretend to be an expert on Ford engine management systems, because I'm not. I do have a lot of experience on some ecm's and their theory is generally the same. It could be that the ecm in question cannot advance beyond the mapped code. But I do know that that is not always the case.

 

So when you said this you didn't mean it? The computer does not advance the timing. It is programmed with a desired timing setting based on engine inputs. If it gets knock at that setting it retards the spark. You can try to dazzle us with your life story all you want; it doesn't change how the thing works.

 

Screw off. I was merely trying to assist a guy with a question about OCTANE where I thought I could help. Now you're here reaming my a$$ about something unrelated that I've already explained. So I over generalized.. You want a cookie?

 

I'll have chocolate chip. You should really think about what you're saying when you're just running off at the mouth to stroke your own ego.

 

This is the way I look at it, maybe it is too simple, I don't know. If one always runs crappy gas, and the timing is always retarded because it is always knocking, then in introduction of good gas may allow the computer to "unretard" the timing a bit. I like to think of "unretarding" the timing as advancing it.
Theoretical situation: if the timing is at -5 btdc and you "advanced" it -2 btdc, that is still retarded timing overall, that you still advanced.
Non-theoretical situation: When the post-hurricane-semi-gas-crisis hit, my local gas station ran out of 87 octane, so I put in 91. The truck seemed to run alot better. Since I am a total nerd, I took out my digital readout timing light out and mapped out my advance curves. One tank still had 87 in it and the other tank had 91/87 octane mix. The 91/87 tank produced about 3 degrees more "advance" than the other, but still about 25 degrees of total advance, less than the 28 degrees than the max if have heard about.
My final answer: The computer will advance the timing from a previously retarded setting if all other input/output parameters say it can.

 

Now thats interesting...

honestly, I am not sure what the max timing is set at, I know its around 28, I had 28 come to mind when I typed that last post...anyone know for sure?

this caught my attention, if max timing was the same, where did the improvements show up? any chance there could have been a different variable present, like different load~vacuum level (map), engine temp ~ect, intake temp~IAT(ACT), rpm, throttle position?

 

Oops, left that part out.... the 87 octane tank ran about 22 degrees of total advance and the 91/87 octane tank ran about 25 degrees of advance. Seems like a small amount and one would think hardly noticeable, but the "seat of the pants" dyno test showed improvement.

That said, When I tune my z car, I tune it to 35 degrees of total advance, regardless of initial advance, and it runs like a demon. I thought about investigating why the advance on my truck was a little low. It seems to run fine, and it ain't broke, so I am not going to fix it.

 

Would have been interesting to know if it was attributed solely to the octane increase...

maybe you'd like to experiment with a bottle of octane booster and a few controlled runs...

 

Your right, it would be interesting
I would have to use the same tank for all expirments because the two tanks do have a 4 psi pressure difference between the fuel pumps, hard to say if this has any effect on the timing though. The day I mapped the advance curves all I did was "flip the switch" between the tanks which would have been one variable I had that day

 

I usually run my truck at 12 degrees initial. I added 4 degrees to it on the dyno and it lost lots of power up top. I don't remember the exact numbers, but I believe it was down about 13 hp at peak and was down at least 6 or 7 anywhere above 3000 rpm. There was no knock on that run. I know from past experience that I can get the initial timing well over 20 degrees before it starts telling me to back it off.

 

Don't go there, O.B.'s are Just another Rip Off Scam!

Way over my head gentlemen, that's why I prefer the Pre '72 modes of transport on our hwys & bywys, or better yet my 1/4 Horse for Off Roading & some On Roading as well.
To Many Bloody 'Puters, Sensors, MAPS, etc. & all of it nothing but garbage to make someone else Richer.