Re: Knock. A question for Graham or Marc

[XfireZ51]

Usually the discussion of Knock comes up when focusing on WOT tuning. However, after modifying a motor, I’m very much engaged in driveability tuning so part throttle, decel, idle is where I spend a lot of my time. Getting this area right is a prereq to moving onto WOT as far as I am concerned. And part of that tuning process is optimizing the spark advance for responsiveness and economy.
It would be helpful to know what the “knock signature” of the LT-5 is in a stock configuration. Did the stock calibration eliminate all knock under normal operating conditions, or if I datalog a completely stock motor, would I still record some knock happening? And if so, where?
Granted w a modified motor we are likely exacerbating the incidence of knock, (especially when using a SM FW), but it would be interesting to compare to what areas the stock motor, w stock cal, demonstrates an inherent tendency to knock.
Is the modified motor now exhibiting knock in areas where the stock motor does not?

[Marc Haibeck]

Hi Dominic,

For a typical stock engine and stock calibration.

If a run is made from first gear through to third gear to 70 mph. About 50% of the time there will be a tip in knock of about -3 degrees when the throttle is opened at 2000 rpm or so. Then there could be a -3 to -5 degree knocks in the area of peak torque at about 5000 rpm in one or two of the gears.

If a second run is made soon after that there is a 50% chance of a tip in knock. The other knocks are typically lower in intensity.

On a third run, tip in knock probability is about the same. The other knocks may be be as low as -1 degree or none as the combustion chambers are cleaned out by the heat.

Typically a healthy engine will not knock when over 5500 rpm or so.

My observations are with 93 octane fuel. A 500 hp modified engine will behave similarly. I never use over 11.5:1 compression on a modified engine that will be running on pump fuel.

I hope that Graham gives us his insight.

[grahambehan]

Good Morning,
It is an interesting question which the simple answer to is yes a stock LT5 will show some Knock retard in low MAP partial throttle operation, also so on some transient throttle/load conditions, but should not when at full load conditions.
So this begs a few of questions
Why?
The knock sensor and control system are from the 80’s.
The LT5 is a relatively mechanically noisy engine.
Spark transition from light load advance to higher load is, in today’s terms, relatively slow thereby giving to mush spark under some transient throttle conditions.
With a single knock sensor some cylinders combustion events are “seen” better than others leading to compromises to the criteria of “what constitutes a detonation event in a well seen cylinder to a less seen cylinder.
Is it real knock ?
Simple answer yes and no
There may be detonation events through throttle/load transitions
Since cylinders combustion events vary in the amount they vibrate the block at the knock sensor position and all cylinders require monitoring to the best of the systems abilities, a normal combustion event is a cylinder that is “seen” better by the sensor, in terms of block excitation, may be interpreted as detonation.
Is it harmful ?
In most cases no
There are instances, in terms of temp and load where the spark retard is limited, e.g. on L98 cals of the day knock retard authority was set at 0 up to 50 kpa vac.

Why does it change ?
The LT5 calibrations have a rudimentary form of Low octane spark modifiers which are primarily based on coolant temp with modifiers for load and rpm.
Any change in the internal structure of the engine can change the resonant frequency of the system, thereby changing what is interpreted as a detonation event.
Piston and cam changes can effect this signature too.
Hope this helps
Graham

[XfireZ51]

As always thx Marc for ur observations. Yes hoping Graham will chime in. Are ur comments regarding the knock characteristics u have seen during WOT or does it also pertain to part throttle, ie daily driving operation?

Oops, Graham responded just as I posted this. Super.

[XfireZ51]

Graham,

A very illuminating explanation. If I could, I’d like to explore this a bit further w u and Marc. My question relates to part throttle, as I have found performance to be as u describe under WOT conditions.

1. If the stock LT5 does present knock at certain low MAP/low RPM areas of the Spark table, are there specific points where that was universally true? Or did it depend on the individual motor?

2. As u state, modifications to the motor/drivetrain may introduce other noise that could be interpreted by the KS as knock even tho it may not be. It would seem to me that the only way to verify whether knock or not is by visually inspecting the plugs. If this were to reveal that it was “false knock”, what would u suggest could be done to either minimize or eliminate it, if anything?

3. The reason I ask #2 is that if it is “false knock”, the retard that is being employed I would think is still affecting the motor in a way that would be detrimental to performance during daily driving situations. As an example, I see some random knock occurring even in steady state cruising situations at somewhere between 1800-2400rpm and 45-70kPa. It seems to occur with very slight variations of MAP. Also some on/off throttle transitions.
My motor is modified with porting and cams, but I wonder if knock in areas such as I describe is something inherent in the LT5 architecture.
Your answer Graham certainly suggests that there are inherent inadequacies to
the knock detection employed for the LT5 and it is a noisy motor. No doubt made noisier w the modifications.

Thanks again for any additional clarifications u can provide.

[grahambehan]

In answer to 1, I cannot give a definitive yes, since this was not or has not been checked at tolerance extremes. I would say that due to conditions I put forth in my first response, the tendency would be yes.
As for 2 I would suggest running the engine on a high octane, non knocking gasoline, say 109, then see if it has the same characteristics.
As for things that can be changed, knock authority, temp parameters, load parameters, timing, fuel, knock attack rates and knock recovery rates.
In so far as to effects, in these instances the engine is not working at peak efficiency, but is the impact measurable or detrimental under normal driving conditions?

Regards

Graham

[XfireZ51]

Again, great thanks Graham. Would be very helpful and possibly illuminating if we could get into a well maintained ZR that is totally stock for some datalogging.

[grahambehan]

No problem, I am always too happy to help, if I can

Graham

[XfireZ51]

No problem, I am always too happy to help, if I can

Graham

Never a doubt that u can. 👍🍷

[jss06c6]

Gents,

I have a pristine 1993 that I'm about through with. 100% stock, including the MEMCAL provided to the owner by Marc. I will datalog this car and provide the file to you Dominic. Having to replace the Brake Master Cylinder, which should be complete tomorrow. Will have 93 octane fuel in it.

As always, the insights from Marc and Graham and yourself Dominic, are extremely valuable.

Steve

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[XfireZ51]

Gents,

I have a pristine 1993 that I'm about through with. 100% stock, including the MEMCAL provided to the owner by Marc. I will datalog this car and provide the file to you Dominic. Having to replace the Brake Master Cylinder, which should be complete tomorrow. Will have 93 octane fuel in it.

As always, the insights from Marc and Graham and yourself Dominic, are extremely valuable.

Steve

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Awesome Steve. That would very cool. I would envision a drive of about 1hr which would be inclusive of city stop and go, incline/decline(as much as possible in TX), decel, and steady cruise through about 80-85mph(without breaking the law of course [-X;)). Moderate acceleration.

[XfireZ51]

Steve's offer brings up another point of clarification.
Does the fact that the 93-95 block have 4 bolt mains alter the knock characteristics in any way?

[grahambehan]

Wow, now your testing my memory, let's see, the original sensor was around 6khz, we tested 6.3 khz for 93, but the pre-ignition test showed no real difference so my recollection is the sensor is the same, but I will check when I get home.

Graham

[XfireZ51]

Wow, now your testing my memory, let's see, the original sensor was around 6khz, we tested 6.3 khz for 93, but the pre-ignition test showed no real difference so my recollection is the sensor is the same, but I will check when I get home.

Graham

Graham,

Maybe this may assist you. These are the param values from stock bins for the 92 v 93 when it comes to Knock controls. Not much difference except for the values for Max Retard in PE mode. Interesting.

[grahambehan]

Thanks, that would suggest my memory of events that took place in 1991 is acurate.

[jss06c6]

Stand by. Hopefully I will get the '93 on the road tomorrow night or Thursday morning..

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[XfireZ51]

All,

Well thanks to Steve at Crossed Flags, he was able to provide a datalog of a totally stock 93. The log showed no sign of knock present during a driving session combining city and highway driving, both w and w/o AC on. Perhaps Steve may want to comment further, but at least based on this one sample, it appears that there isn’t any characteristic knock signature that would appear across the spectrum of LT-5s, regardless of state of modification.
If ur recording knock, whether false or not, it needs to be eliminated or minimized because it does affect the performance of ur motor. Even if its false, it would be a mistake to ignore it.

[Ccmano]

Now that’s interesting. No knock? A number of years ago Mark Haibeck documented (in a presentation still on his website today) what he called “typical” knock for both “tip in” and “shift” related knock. I copied the attached charts from that presentation below. I still think we’re missing something here. I find a car with zero knock highly unlikely. Mine was like that when I first started tuning. When I changed out the knock sensor and we suddenly had knock back in the picture.
H
:cheers:

https://farm66.staticflickr.com/65535/48846936973_b6e5220191_c.jpg (https://flic.kr/p/2hqrjyv)

https://farm66.staticflickr.com/65535/48846936963_edcff8bfa3_c.jpg (https://flic.kr/p/2hqrjyk)

[Ccmano]

Come to think of it, if we assume for the moment that shift knock is a real thing on a stock car, which means gear clash is being picked up by the knock sensor, it begs the question, are Fidanza single mass Flywheel equipped cars more likely to experience false knock? The transmission noise of a single mass equipped car is well documented up to about ~2k rpm. Is that noise, under various load and speed parameters, generating false knock throughout the rpm range as we shift and move on and off throttle without the dampening affects of Dual Mass Flywheel? Food for thought.
H
:cheers:

[XfireZ51]

Hans,

Did u "ohm out" the original KS? How about the new one? And when u installed the new one, use teflon tape? How tight did u make it?
I, like u, was suspicious of Steve's results and asked Steve if he had validated the sensor. He was able to show that it would record knock so his result should be valid.

[XfireZ51]

Come to think of it, if we assume for the moment that shift knock is a real thing on a stock car, which means gear clash is being picked up by the knock sensor, it begs the question, are Fidanza single mass Flywheel equipped cars more likely to experience false knock? The transmission noise of a single mass equipped car is well documented up to about ~2k rpm. Is that noise, under various load and speed parameters, generating false knock throughout the rpm range as we shift and move on and off throttle without the dampening affects of Dual Mass Flywheel? Food for thought.
H
:cheers:

Yep, think that's the question I was asking before. The problem is, false or not, it affects the performance of the motor. And that brings up the issue of the correct trans fluid.

[jss06c6]

Guys, I recorded data through engine shutdown. At shutdown, our motors clatter quite a bit. The knock sensor on the '93 definitely picked up the shutdown noise. It is 100% stock with Dual Mass FW setup..

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[XfireZ51]

Steve,

Do u normally pickup any noise at startup?

[jss06c6]

Not as much.. it's the rattle at shutdown that I pickup mostly. I'll rap the motor on Monday so I can see the response.

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[Hib Halverson]

I can't remember, did the LT5 cal use "burst knock retard"?

Also, I can't speak to false knock because I never had trouble with that, but when I had Barney there was a time when I did a lot of on-road and chassis dyno testing while logging KR. I found that the only way I could run "KR-free" in most conditions was to be burning 94.5-95-oct fuel. I'd get that by mixing pump gas with Rockett Brand 100-oct. racing gasoline.

[XfireZ51]

Thx Hib. No “burst knock” in the LT5 cal. Think that came w the LSx motors.
When u were datalogging, how stock was Barney?

[rkreigh]

the LT5 seems to have fantastic knock resistance. I enjoyed Marc's presentation about the knock events and how they drastically affect timing and very slowly return timing so the car feels very lazy


When I got the LSV, Larry left the race chip in it and it had 31 deg timing and ran around 12 to 1 compression and ran just fine on pump 93 gas. I ran some high octane unleaded at the track and saw no improvement and indeed no knock issues data logging with Ease.


Cam timing of course helps as the cyl pressure doesn't spike up at lower rpms and the high rpms as Graham mentioned are pretty forgiving


I got a bit "knock paranoid" from my old days of buick turbos as we "drove over the crank" every now and then from knocking the bottom end clear through the oil pan.


I'm very impressed at the efficient combustion chamber and knock resistance of the lt5. It allows much higher compression ratios on stock pump gas than other engines.

[Hib Halverson]

Thx Hib. No “burst knock” in the LT5 cal. Think that came w the LSx motors.Copy that. I was wondering about that because back in my Barney days, I was not a calibrator.
When u were datalogging, how stock was Barney?

Auto Masters Street Skinner engine with Marc Haibeck cal.

I mixed 91 oct gas with Rocket Brand 100-unleaded to get about 95-oct R+M/2.

[XfireZ51]

Bringing this one up again because I continue to play around w it. A couple of questions come to mind, ideally for the likes of Graham and/Marc. But please chime in if u have 'played" around w this as well.

I have attached a pic w the stock Knock Retard values for the 92 LT-5.

My questions are:

1. Given that many of us have modified our motors, drivetrain, clutch etc., as Graham has stated, we have introduced other "noise" for the knock sensor to detect. How much from stock values would it be prudent to reduce the sensitivity of the sensor and still retain a level of protection from knock?
I believe Marc feels comfortable w modifying the attack and decay rates by +/- 50% depending on the parameter.

2. In the pic I have attached, the parameter for SA v MAT is displayed.
Just to clarify, this is showing that beginning at a MAT of 43C, the ECM is
retarding timing by 3*, correct?

The pic also shows that MAX Retard in PE for the 405hp motors is 4.92*
while the 375hp motors may retard timing by a minimum of ~7* in PE.
What changed to bring that about? Would it be ok to use the 93-95 settings
for the 375hp motors?

[Marc Haibeck]

Yes Dominic, the spark advance is retarded by 1 degree when the air horn is at 89 degrees F. And -3 degrees when the air horn is at 110 degrees F. This can take the edge off power on the dyno or at the drag strip. At peak power one degree of spark advance can change the HP by 10 on a 500 hp engine.

To avoid this loss, the table can modified. I recommend relocating the Manifold Air Temperature sensor out in front of the radiator to avoid the negative effect.

When the sensor is relocated the table values are a good feature. I think that it is good for engine safety to retard the timing when the ambient air is 90 degrees or more.

Long ago someone from GM told the group at the Gathering in BG that the sensor was placed in the compromised location to satisfy a requirement to be able to test the engine with all of the sensors that will be used with that engine. Rather than rely on a sensor located on the chassis that is installed at a different time and place.

[XfireZ51]

Very true on the SA v MAT table. That explains why we perceive that the LT-5 LOVES cold air and delivers snappier performance. Under normal operating conditions, the MAT will typically show an air temp of 70-90C. If its located, as mine is downstream of the radiator but ahead of the airbridge, it never reaches higher than 45C or so. If that table is not modified, u would end up w an extra 3* SA which could create a situation (ie knock) where the knock sensor invokes the ECM to retard timing.

Marc, so I assume when u relocate the MAT sensor and basically convert it to an IAT (Intake Air Temp) sensor, you also modify the MAT/CTS Blend table for fueling.(see attached).
Once the MAT is relocated and used as an IAT, the ECM will likely NEVER see the air temps where the stock calibration assumes normal operating conditions to be. So they need to be modified to operate correctly with the "new normal" relocation of the MAT/IAT sensor brings about.
Some would make the argument that u would compensate for the MAT relocation using the VE tables, but that would be incorrect. If you datalog the motor during varying outdoor ambient temps, u would find that what may have been an optimum tune in say temps of 70F, are not in 50F or 90F. When u modify the VE table to achieve a particular BLM or AFR, the modification will only be valid for those particular conditions found during ur datalog. In fact, if you sit there idling, intake air temps rise, what was a good BLM when u first started idling will go likely rich as temps rise if the table was kept stock. The MAT Compensation table is used to modify the VE in order to maintain a stable BLM regardless of intake air temps.

Perhaps Graham could chime in on whether the MAT Comp table is active for both Closed and Open loop operation. I would think it would be for both.

[Hib Halverson]

Very true on the SA v MAT table. That explains why we perceive that the LT-5 LOVES cold air and delivers snappier performance. (snip

Any performance engine loves cold air and delivers "snappier" performance when it has it.