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Old 04-25-2011, 11:20 PM   #1 (permalink)
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SGII Interesting Information - KR "Knock Retard" Gauge

While looking through the provided XGauge codes on the SGII for my car, I came across the "knock retard" value and plugged it in, just for kicks and giggles. From what I understand, this gauge is supposed to report the degrees of ignition retard from the current timing due to detected detonation by the knock sensor. I didn't think this would provide much valuable information, as it initially just read 0 most of the time.

I later noticed, however, that when accelerating in the higher gears this gauge started reporting knock retard at above 24 in. Hg manifold pressure at 2400 RPM, and above 22 in. Hg after the torque converter lockup clutch engaged at 1500 RPM. I began using this gauge as a reference when accelerating, maintaining the highest possible manifold pressure without causing spark knock.

I had no idea that there was any spark knock going on in the engine, I have never heard it before. Apparently, this is the peak cylinder pressure that the 87 octane/E10 can handle in my engine with the default timing values. I guess I can run all the way up to 29.9 in Hg at 100% load, but it seems that this comes at the expense of retarding ignition timing, which I'm guessing is a cause of the loss of efficiency that I have seen with heavy engine loading in the past. Not to mention that there is detonation occurring that is damaging to the engine. This number would usually go up to about 10 or 20 under moderate acceleration, but I watched it go up to around 60 when passing. So this means that on 87 octane/E10 I can only expect a useful load of 74% from my engine.

I will experiment with some higher octane fuel when prices come down a little, or next time I am near the gas station that sells midgrade for the same price as regular . This is an interesting find, it will definitely influence how I drive from now on. I am wondering if anybody else has had any experience with this value and might be able to enlighten me as to whether or not this is what is actually occurring.

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Old 04-26-2011, 11:52 AM   #2 (permalink)
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...very interesting parameter.
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Old 04-26-2011, 06:58 PM   #3 (permalink)
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If you can keep the timing from retarding it should translate into higher MPG. I have read on other sites where they are disconnecting the knock sensor to improve MPG.


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Old 04-26-2011, 08:43 PM   #4 (permalink)
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...sounds like "maybe" that knock-sensor signal could be used in conjunction with MAF and/or MAP to approximate a BSFC boundary?
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Old 04-27-2011, 06:34 AM   #5 (permalink)
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I 'learned' last yr when driving to texas from ca, that when I use gas below the 91 octane that is required for my 02 Q45, the " ign " function reports less than the optimal number. And the result is less mpg. It also corresponds to a higher 'load'.

On my trip this week, I tracked ing, load, hp, fia, fwt, mpg, mph, to get a better idea of the value in running 91 octane.
My conclusion is that IF your vehicle say to use 91, then your engine will perform at optimal conditions. However, IF your use less than 91, the ecm will 'retard' the optimal perameters. If you weren't running a scangauge or closely tracking your mpg, you probably would not notice the difference until you push the car.
For example: driving fully loaded, or diving thru inclines, or driving in 90+ degree w/ a/c on.
When I ran less than 91, the 'ing' function would not report more than 29. with 91 gas the 'ign' function reports 32-36

My gut feeling though is that if your car doesn't require it, then using it probably WONT show any improvement. But that is just my guess. If your car hits 31-36 with regular gas, the 'ign' fuction can't run at 40.
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Old 04-27-2011, 07:18 AM   #6 (permalink)
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Quote:
Originally Posted by Old Tele man View Post
...sounds like "maybe" that knock-sensor signal could be used in conjunction with MAF and/or MAP to approximate a BSFC boundary?
I think this definitely denotes a BSFC boundary, though it would be entirely dependent on the fuel. I believe this boundary would change significantly with a higher grade fuel.

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Originally Posted by nemo View Post
If you can keep the timing from retarding it should translate into higher MPG. I have read on other sites where they are disconnecting the knock sensor to improve MPG.
That's my thinking as well. I have a feeling that the KR value is less than optimum timing for the load/speed, when it comes into effect.

Disconnecting the knock sensor??? If you want to replace pistons and head gaskets perhaps... Detonation is nothing to mess around with. If the sensor is picking up a knock, then there is already detonation occurring in the engine.

Quote:
Originally Posted by mcrews View Post
My gut feeling though is that if your car doesn't require it, then using it probably WONT show any improvement. But that is just my guess. If your car hits 31-36 with regular gas, the 'ign' fuction can't run at 40.
As far as I know, my car is made for 87. It has a compression ratio of 9.6:1, less than some of the newer cars that are made for regular fuel. My IGN usually runs at 29-32 highway cruise under normal load with regular fuel. It seems that the effects of low-grade fuel occur under moderate- to high-load conditions, with as low as 20% throttle opening, well within the range that I normally accelerate. So my thinking is that higher-grade fuel might benefit my engine by allowing for higher loads and more throttle opening during acceleration.
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Old 04-27-2011, 10:21 AM   #7 (permalink)
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you may be right.
I only can vouch for what happened with me.
I would be a difficult a-b-a. you would have to run almost empty, the take on 2-3 gallons and then drive the same distance.
but certainly doable.

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