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]

Originally Posted by grahambehan

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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