Low RPM Knock Retard

[Gtking616]

Hello everyone.

I've been tuned by Lund on e85 for some time now, but I've been noticing an extremely odd issue lately. Extremely randomly and sparsely, when accelerating from a stop, I'll see some knock retard varying anywhere from minor .5 upwards to 6. 90% of the time it's pulling between 1 and 2 though. If I continue accelerating it slowly drops on its own and starts adding timing again. If I let off and immediately begin to accelerate again, it disappears and the car adds timing like usual.

I haven't been able to log it because it's so random and uncommon I'd essentially be permanently logging all day. I never see it at WOT, car is really happy when getting on it. It's just that really rare and random occurrence when accelerating from a full stop.

I've checked under the car, inspected the exhaust with a rubber mallet, didn't notice any issues. Car has stock headers, only has a cat back exhaust, and Fords TB and intake from their "power pack" for 18+ GTs. I did switch over to 91 for a bit a while back and didn't notice the issue but I don't think it was long enough be conclusive.

Anyone have any sort of idea what exactly is happening? Ideas on where I should look to fix it? I presume it has to be false knock. E85 in my area tests good, and I've filled up at different stations (even though it seems to be all the same supplier). I've had 3 oil analysis done and the car is healthy too, nearing 60k miles soon.

Any help is appreciated. Thanks for your time.

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[HKusp]

I think you're on the right track looking for something loose. Assuming you are on good quality E, there is no way you should be knocking.

 

[engineermike]

Some months ago, I spent WAY too much time working on what sounds like the same issue. At the time I had seen it on two cars. We thought it was related to the supercharger rotors being unloaded but found people with NA cars doing the same thing. We thought it was the GDI pump but found people with Gen2's doing the same thing. I think the conclusion is it's internal engine, something like piston slap or cam phasers or tensioners.
We were seeing 10-15 deg retard at times. The real issue was that if you then floored it, the timing was consequently starting super low and after what the PCM considered to be severe knock, the advance rate is super slow. The car would feel like a terd unless you back out and hit it again, which resets the knock retard at zero.

I used PCMTec to log all sorts of knock-related data and learned a lot about how it works. The PCM basically keeps a rolling average noise level then divides the current level by the average level. The knock threshold is typically when the current is about 2x the average, but that threshold is tunable in PCMTec. The intensity determines the amount of timing removed, and I believe it's defined as the current level minus ~2x the average, again all tunable. I had tried increasing the threshold from 2x to 3x or more, with no luck (some might call this "desensitizing" the sensors).

PCMTec allows logging of all the intermediate parameters. It was only then that I realized that I was never going to be able to elegantly tune out the noise. At that particular load and speed during transition (.6-.9 load, 1500-2000 rpm, but mainly during dynamic load changes), the current was like 10x the average. In other words, it was off the charts and removing timing wasn't helping it.

Ultimately, the only solution I could find at the time was to create a band-aid solution by reducing borderline timing by a few degrees and offsetting with more knock advance-allowed, and limiting knock retard at that load and speed to a couple of degrees. I later found where the newer Whipple and even the latest OEM tunes incorporate a similar solution.

 

[StangTime]

Can you hear the knock? I had the Performance Calibration tune on my car and ended up going back to stock. My car developed a disliking to the tune and started audibly knocking when driven as you have described. Still looking for a solution to it because the tune is great but the knock is unacceptable. I'd be surprised it's real knock given you're running E85.

 

[Gtking616]

Can you hear the knock? I had the Performance Calibration tune on my car and ended up going back to stock. My car developed a disliking to the tune and started audibly knocking when driven as you have described. Still looking for a solution to it because the tune is great but the knock is unacceptable. I'd be surprised it's real knock given you're running E85.

I don't think it's audible when it occurs from memory. Hard to tell apart from the 2k rattle that's ever present lol. I figure it's just an ever dreadful case of false knock and I have to find whats hitting what. I do wonder if maybe the Bullitt TB can be problematic and somehow cause noise/vibration/resonance that can be picked up by the knock sensors. I know tuners don't like varying from the stock TB, just a wild idea.

 

[1SLwLS1]

My Roush setup on 93 does the same thing. I like Mike's solution/problem-solving and I haven't been able to explore the Roush tune through HPTuners to see if they executed a similar solution in the stock Phase 2 tune, but I was told by Roush support that it is somewhat normal and to be expected with a 12:1 motor with a PD blower, even though that should be somewhat irrelevant on your e85 setup.

 

[engineermike]

@1SLwLS1 ive been working on optimization of a Roush tune for a couple of months now. If you’re embarking on self-tuning a Roush I can give you some pointers, though my files are all pcmtec at this point. I even got flex fuel working great using the stock pump and injectors.

 

[1SLwLS1]

@1SLwLS1 ive been working on optimization of a Roush tune for a couple of months now. If you’re embarking on self-tuning a Roush I can give you some pointers, though my files are all pcmtec at this point. I even got flex fuel working great using the stock pump and injectors.

Thank you, I will definitely reach out to you next year when the warranty expires. I have been researching E85 options, even though none of my commutes readily accommodate e85 stations, it seems like a great way to nullify the issues of the tiny brick while simultaneously increasing the power a bit with timing advantages.

 

[ZXMustang]

Yeah 100% false knock if you are on full E. Even like e70, the car would send a rod through an open window before you'd see knock. So you need to check everything. Mine was headers touching the AC lines on the passenger side. Since you are stock manifolds, it might be something else loose. Something random. Might need to pull the intake off and see if anything in or around the valley is loose.

 

[Gtking616]

Yeah 100% false knock if you are on full E. Even like e70, the car would send a rod through an open window before you'd see knock. So you need to check everything. Mine was headers touching the AC lines on the passenger side. Since you are stock manifolds, it might be something else loose. Something random. Might need to pull the intake off and see if anything in or around the valley is loose.

I've been dreading have to pull the intake manifold off but it's definitely on my check list once I verify it's not coming from elsewhere in the engine bay. It's just so strange that it's not more common if that were the case.

 

[illadvised]

I've been dreading have to pull the intake manifold off but it's definitely on my check list once I verify it's not coming from elsewhere in the engine bay. It's just so strange that it's not more common if that were the case.

Do you have a supercharger or anything in the way? I take my intake mani off pretty frequently and its not hard or time consuming to do. You could probably undo the bolts and lift just the front and check the valley with a flash light

I saw you said you switched to 91 for a while, but did you switch the tune as well? Do you have another tune you could switch to? I am not sure if that would even help your issue, but it is easy for me to switch. I have a few tunes and usually one of my first troubleshooting steps is to just put on a different tune and see if it changes anything

 

[dpAtlanta]

Careful now, due to political correctness, you may want to change the title of the thread to:

Low RPM Knock Special Needs

 

[Gtking616]

Do you have a supercharger or anything in the way? I take my intake mani off pretty frequently and its not hard or time consuming to do. You could probably undo the bolts and lift just the front and check the valley with a flash light

I saw you said you switched to 91 for a while, but did you switch the tune as well? Do you have another tune you could switch to? I am not sure if that would even help your issue, but it is easy for me to switch. I have a few tunes and usually one of my first troubleshooting steps is to just put on a different tune and see if it changes anything

I have 4 tunes for the car currently. Full E85, Flex tune, 91 tune. All three of these are from Lund. The last one is the Ford Performance 91 tune. I transitioned from E85 to flex to 91 for a bit then back over to E85.

Yeah I figured I don't have to pull the whole thing off just to check what I need to. It's just a giant headscatcher as to why it's so extremely infrequent.

 

[HKusp]

Something is loose/banging. Have you recently had any/done any work to the car before you noticed it happening?

 

[Gtking616]

Something is loose/banging. Have you recently had any/done any work to the car before you noticed it happening?

Car has been unchanged for quite a while now.. been two years since I moved over to E85. Only thing I can think of is I swapped the hood with a Cervinis GT500 hood. I'll double check it but it's definitely bolted on snug and lines up properly and latches properly. No other changes. There was a moment where I thought it could potentially be a vacuum leak, because I seem to hear some excessive "whooshing" when the hood is open almost as if there was a vacuum leak, but I'm uncertain.

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