Unusual amounts of knock at unusually low timing

Hello,

So I am running into little problem with knock on the 1990 Corvette I am working on. The motor is a fresh rebuild with the factory cam advanced 2 degrees, ported heads, tpis miniram, and all the bolt ons. I have been working with Tunerpro RT to try and get it dialed in and so far I have been very successful. I was initially getting knock when tuning WOT, but the problem was apparent by the fact that my injectors were maxed out and I couldn't get any better of an afr than mid 13's at WOT. I just dialed in a set of ZO6 28lb injectors and the car runs great at part throttle but I am getting heavy knock at WOT. I have my afr set to around 11.3 to start and I am getting knock at just 18 degrees? I am running a normal compression ratio, stock plugs, plenty of fuel on tap... you can hear heavy knocking. I'm really lost, let me know what you think

Dan

a few points, just to make you think:

- if you can hear knocking, and your timing isn't too far advanced, and it isn't lean.... logically something in your engine is broken. if something sounds like mechanical clatter, it usually is.

- you have to go REALLY lean or REALLY advanced to actually hear 'heavy knocking'

- if you were actually getting audiable pre-detonation, your spark plugs would be melting, so what do they look like...?

- lots of types of engine noise will set off a knock sensor, too

a few questions,

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I have my afr set to around 11.3 to start

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and is this on a wideband or just commanded AFR? if wideband, are you sure of accuracy?

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I am getting knock at just 18 degrees?

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actual timing or commanded? is your base timing set correctly? just 'cause your aldl data says 18 degrees, or your timing table, doesn't mean your physical timing is 18 degrees.

Hey Steveo thanks for the reply. Definitely looking forward to getting to the bottom of this as I am so close to having this car perfect!

- I hear rattling (knock) at WOT. This is confirmed by my ever increasing knock count on Tunerpro. The engine has about 30 miles on it, its brand new. I have Z06 Injectors now so plenty of injector and pump. Logically I agree, plenty of fuel, timing "seems" low, car should drive well, yet it rattles even jumping into it at 1500 RPMS. I don't feel like anything is "broken" at this point, car drives well locally and under light and part throttle.

- My wideband is showing an AFR when I go WOT of about 11.3

- Attached is a photo of my spark plugs. I decided to include a copy of my tune as well in case anyone whats to critique it (be nice please)

- I agree again that I may need a less sensitive knock sensor but I can also audibly hear the rattling, this makes me nervous.

1. The wideband is reading 11.3 at WOT, in accordance where I decided to set it to start it.

2. I'm not sure how to answer your second question. My aldl data says 18*, I thought I could take it at its word??? I will attach a log as well, but its not a very good log. I didn't do much driving before laying into it although it was up to operating temperature

Attachment 11619Attachment 11619
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those plugs don't look like they've experienced detonation. i think you have mechanical noise and false knock. exhaust leak, perhaps.

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I'm not sure how to answer your second question. My aldl data says 18*, I thought I could take it at its word???

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depends on what you're using for timing reference, distributor base? cam sensor? what ecm?

Yea, the plugs looking pretty good. I find it odd that I am even having this happen but I can't imagine what else it could be that makes such a similar noise at wot.

Its got a stock 1990 ECU, I'm using and $8D Bin. I can't remember off hand but I believe its a 7727? There is no cam sensor on these.

Dan

Can you elaborate on how I can verify my true timing? I want to make sure i'm on the right track here...

I'm sorry, I made a mistake, reviewing my log, I was actually receiving 26* of advance just prior to the knock beginning. In regard to my previous 2 posts; below is a screenshot of the knock occurring.

Please correct me if I am on the wrong track. Spark advance relative to TDC: This is the actual spark advance occurring within the engine (26*), Knock Retard: after the knock occurs 7* of timing is pulled from 26* bring my total down to 19*

Not sure how spark relative to reference pulse plays into this

Also, Please review the two charts I have up as well. You may noticed I removed about 7 degrees of timing from the stock map of SPARK ADVANCE vs RPM vs MAP, and then removed another 7 degrees from the Idle Spark Base Advance as well. I'm not 100% i'm sure how this works but I am assuming that my distributor is set to 6* btdc, plus what the ecm sees on the "Spark advance vs RPM vs MAP" table, plus the "initial base advance" table.

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i'd unplug the set timing connector and grab a timing light to make sure you're actually seeing 6*BTDC.

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Originally Posted by RobertISaar

i'd unplug the set timing connector and grab a timing light to make sure you're actually seeing 6*BTDC.

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I'm 99% certain but I will check again and verify just to be 100%. I reset the timing just before I began tuning. Ill post back my results tomorrow when its not raining lol!

Dan

Ok, just checked the timing. It's set exactly to 7 degrees, I can tweak it to 6. Will the difference of just one degree make a huge difference in the overall tune?

no.

again for you to actually hear pre-ignition you'd have to have grossly over-advanced the timing

I don't think you're chasing detonation here, i think its noise from something else.

The only thing I can fathom is that my ignition timing is so low, that when it pulls an additional seven degrees advance what I'm actually hearing is a popping from it being so retarded. Against my better judgement I just might pull the wire for the knock sensor and try running it. Maybe I'll top it off with some race fuel just to be on the safe side.

so far all you know is "there is a noise" and "your knock sensor filter doesn't discard the noise (probably because its loud)"

i really dont see where the ignition timing theory comes from

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Originally Posted by steveo

so far all you know is "there is a noise" and "your knock sensor filter doesn't discard the noise (probably because its loud)"

i really dont see where the ignition timing theory comes from

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I'm with you Steveo, but what concerns me is the fact that the noise only occurs at WOT and sounds exactly like knock lol. I am not totally even sure how to diagnose it to be honest. I could try getting the car up on a lift and do a quick pull but the noise could disappear or change without the weight of the car. It sounds like its coming from the engine, but its a fresh rebuild done by a professional shop. And its not like anything else I've heard. Ill keep messing with it and post my results. On the bright side, plugs look great, not indicative of knock at all. So close to getting this thing done; one step forward to steps back.

Dan

Guys,

Just had a free moment to get back on this project. Just wanted to provide a quick update. The knock is definitely false. Now I have to figure out what could be causing it. Disconnected the knock sensor and did a run with it. Its actually running worse, at WOT it immediately pulls 6 degrees every single time. I don't know if that's possibly a fail safe or if there is an issue but i am going to start checking wiring and such.

If anything comes to mind, please feel free to post.

Dan

what kind of hardware are you using to reprogram with?

I am using a moats autoprom APU1 and Tunerpro RT.

I had a spare Knock sensor so I swapped it. Still doing the same thing. Doesn't even matter if its connected, as soon as I blip the throttle it pulls at least 5 degrees. Looked into changing out the ESC module, however on the 1990 its part of the ecm?

Dan

a knock sensor code in most gm ecms does some amount of additional timing retard... disable the code probably fixes that. alternatively you can zero the maximum knock retard and leave the sensor?

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Originally Posted by steveo

a knock sensor code in most gm ecms does some amount of additional timing retard... disable the code probably fixes that. alternatively you can zero the maximum knock retard and leave the sensor?

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I didn't know to remove the knock sensor code, so I'll give that a try. I'm assuming it's under the flags menu? I can reduce the maximum knock retard. It's not ideal because If real knock occurs there will be little safety net, but under the circumstances I dont think I have much of a choice. I'll give this a try next week and report back.

Thanks guys!

although it's nice to have, you don't need a knock sensor if you have consistent fuel quality, combustion chamber temperature, and run reasonable timing advance that isn't too close to the limit.

the more you modify an engine, the more likely it is that the knock filter will no longer work properly, and that sensor goes in the garbage.. although some ecms have a bit more ability to tune knock retard out of a certain range, so if you have localized false knock you can still have knock detection in other ranges.

if you can zero the max timing retard, that allows you to keep the sensor for knock count logging without actually turning your car into a marshmellow every time it thinks there's knock.

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Originally Posted by steveo

although it's nice to have, you don't need a knock sensor if you have consistent fuel quality, combustion chamber temperature, and run reasonable timing advance that isn't too close to the limit.

the more you modify an engine, the more likely it is that the knock filter will no longer work properly, and that sensor goes in the garbage.. although some ecms have a bit more ability to tune knock retard out of a certain range, so if you have localized false knock you can still have knock detection in other ranges.

if you can zero the max timing retard, that allows you to keep the sensor for knock count logging without actually turning your car into a marshmellow every time it thinks there's knock.

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I agree with you, I think we are there. I know a lot of people will disagree with removing the knock sensing capabilities, but at this point I've tried to make it work and I'm out of options so the next best thing is a little richer fueling, no knock retard and Sunoco 93 octane gasoline. I would feel worse about this if it was forced induction but if the car was 10 years older anyway It would have the same SBC and no knock sensor or computer. Never broke any of my carb'd cars....

Thanks for all the advice. This car has definitely been a learning experience

Also your statement about marshmellow made me laugh... that's exactly it, it goes soft the minute you step on it!

thanks again,

Dan