E85, Boost and Ignition Timing

[DeliciousTuning]

Originally Posted by Gen

I see a bit of this myself, typically on the freeway. Not sure if it's the same thing being discussed here, but usually around -0.7 on partial throttle if I ride the point the turbo begins to spool. Really the only time I ever see it.

We are discussing a little different. What we have been noticing is under quick blips of the throttle, say even sitting in neutral holding the revs at say 2K RPM and then give it a quick blip. Usually on a boosted application it will have a knock correction occur. We have seen a range from -1 to -5 depending on the application, tune etc...

At -2 and above I was a little concerned about it and found a way through EcuTeK RaceROM to remove the problem completely, but the question for those with -1 is it really much of a problem anymore. As the stock car potentially sees more problems then this. That is where I am trying to find out a little more info.

Cheers,
William Knose

[jamesm]

what i see is more along the lines of what gen is describing. mine doesn't happen when blipping the throttle in neutral. i'm also able to induce it the way gen speaks of.

[arghx7]

We're all familiar with knock retard in the sense that we understand the ECU pulls timing based on the sensor signal. Allow me to provide a little background on false knock detection from the perspective of the knock sensor frequency signal itself. I can't speak for the FT86 exactly, but I can speak from my experience with knock signal processing on other applications.

When a knock control system is being developed, the engine is typically run at steady state speed and load points in a lab on an engine dyno. A production sensor or lab grade sensor is installed in the intended locations. The engine is then intentionally knocked (often one cylinder at a time) usually just by advancing timing. Then somebody makes some judgment call about the level of knock based on audible sound and cylinder pressure trace on an oscilloscope. It's somewhat subjective, especially for light knock.

So a knock sensor isn't initially used to detect knock... first you knock the engine, then you set up the knock sensor so it can actually detect when it's occuring and its severity. That's the opposite of the way we're all used to thinking: we think, "The knock sensor will tell me how bad the knock is, and the ECU will retard timing in proportion to its severity." But before the knock sensor can do that, the knock sensor had to be "trained" through signal processing settings.

Getting back to early development in a lab: While the engine is knocking the lab data acquisition system averages the sensor signal over some number of combustion cycles. Then it can be post-processed using something like a Matlab tool. You end up with a plot resembling this one I pulled off google images:



What you have here is basically frequency on the X axis, and energy at that frequency on the Y axis.

Whoever is in charge of the knock control system makes a judgment call as to which frequency the ECU will consider "real" knock for a given cylinder. There's a bunch of signal processing done on top of that in the ECU (gain, filter, etc), background noise learning, etc, and out of all that the ECU will judge knock and pull timing. Then it has to be tweaked in the car.

But it all starts with somebody's decision as to what frequency is going to be considered knock, on a stock engine with stock parts. When you change the arrangement of the engine bay by modding, or even drive in a certain way, you can excite certain frequencies on the engine. That frequency can be in the range that the signal processer considers to be knock, whether it actually is knock or not. If the software is tuned to be sensitive enough it could result in timing retard.

What I'm saying is, false knock can definitely occur. Some guy with some deadline to meet had to make a series of judgment calls in the calibration of the knock sensor signal processing, choosing the knock sensor locations itself, and the sensitivity of the spark retard. He didn't have turbo kits in mind when he did it. And a bunch of the knock control stuff was probably copied and pasted from the stock tune of some other engine. As a side note, I personally believe that the knock at high rpms issue (the one associated with a TSB that has been linked to the injector seals issue) was caused by some guy copying and pasting another table from some other engine.

Or adding boost is really making it knock under those conditions. Or, in some unpredictable cases, it's low speed preignition. It's hard to say really. Light knock/borderline knock is a tricky thing. But think about all the signal processing stuff next time you have oddball knock retard coming up.

[Sportsguy83]

Originally Posted by Gen

I see a bit of this myself, typically on the freeway. Not sure if it's the same thing being discussed here, but usually around -0.7 on partial throttle if I ride the point the turbo begins to spool. Really the only time I ever see it.

Might be an issue of when and where the transition from closed to open loop happens?

[jamesm]

Originally Posted by Sportsguy83

Might be an issue of when and where the transition from closed to open loop happens?

i've done lots of playing with closed loop delays, reshaped the transition area of the fuel map, etc. so what i know now is this:

- i can't hear it, even with electronic det cans.
- nothing i've done has (verifiably) changed it's behavior at all.
- fuel doesn't matter, it behave's exactly the same on e85 as on pump 93.
- manifold pressure isn't a factor.
- my car rattles (mostly the flywheel i believe)

i'm inclined to believe it's false knock, but is it just so repeatable and consistent.

[pixel67]

Very interesting... I have been running predominantly e85 for a few months now on my flex it without issue but have started running more and more 93. Under 93 audible tip in knock is definitely an issue for me. I haven't captured logs yet to see what is going on with knock correction...

[sw20kosh]

Originally Posted by DeliciousTuning

We are discussing a little different. What we have been noticing is under quick blips of the throttle, say even sitting in neutral holding the revs at say 2K RPM and then give it a quick blip. Usually on a boosted application it will have a knock correction occur. We have seen a range from -1 to -5 depending on the application, tune etc...

At -2 and above I was a little concerned about it and found a way through EcuTeK RaceROM to remove the problem completely, but the question for those with -1 is it really much of a problem anymore. As the stock car potentially sees more problems then this. That is where I am trying to find out a little more info.

Cheers,
William Knose

When you say "I have fixed it with RACEROM" what exactly does that mean? Do you mean you have toned down the sensitivity of the knock control system? Or have you used it to truely stop the detonation?

But...If it is truely fixable by RACEROM why is it still a problem to think about or consider? I guess I don't understand what is so different about the RACEROM custom maps.

[DeliciousTuning]

Originally Posted by sw20kosh

When you say "I have fixed it with RACEROM" what exactly does that mean? Do you mean you have toned down the sensitivity of the knock control system? Or have you used it to truely stop the detonation?

But...If it is truely fixable by RACEROM why is it still a problem to think about or consider? I guess I don't understand what is so different about the RACEROM custom maps.

Let me work up some info to share. Once you see it, you will be able to understand why it is so powerful versus not having it. It allows for a LOT more control beyond the stock ECU.

[SkullWorks]

@jamesm Have you looked at what your cams are doing as far as advancing or retarding in the same time frame? The cam actuators in this motor are quite noisy. during rapid angle changes you can hear them readily, perhaps this is being picked up by the knock sensor.

When we were running on the AEM box we controlled cam timing with different strategy than stock and getting hard on the throttle would result in plenty of noise from rapid cam movement, since our maps varied mostly based on load. I can also hear the cams more readily on Shiv's E85 map than with the stock tune

Once you identify the noise the cams make it is pretty distinct, I think it may be just sharp enough with any revised cam timing to trick the filters. I would assume there is a RPM VS noise function in the code somewhere, if the engine makes different harmonics than were experienced on the test bench it may not know what to make of the new sounds

[jamesm]

Originally Posted by SkullWorks

@jamesm Have you looked at what your cams are doing as far as advancing or retarding in the same time frame? The cam actuators in this motor are quite noisy. during rapid angle changes you can hear them readily, perhaps this is being picked up by the knock sensor.

When we were running on the AEM box we controlled cam timing with different strategy than stock and getting hard on the throttle would result in plenty of noise from rapid cam movement, since our maps varied mostly based on load. I can also hear the cams more readily on Shiv's E85 map than with the stock tune

Once you identify the noise the cams make it is pretty distinct, I think it may be just sharp enough with any revised cam timing to trick the filters. I would assume there is a RPM VS noise function in the code somewhere, if the engine makes different harmonics than were experienced on the test bench it may not know what to make of the new sounds

i haven't thought to look into that. i'll log it and see if there is a correlation.