Maybe I’m just lost in the terminology but I could use some help in understanding this. I am sure it’s a silly question to most. :bb

Stock setup is 6° BTDC. Some members may take it to 8°, 10° or even more BTDC to gain performance on their particular motor until they start getting some knocks. Now this is where I get screwed up. If there is a knock the ECM (or ESC) will advance the timing to prevent the knock. Isn’t advancing firing of the spark even more BTDC? In which case moving it from 6° to 10° would only help prevent knocking?

The more I think about the more confused and the more my head hurts. Maybe I have been setting my wrong all the time. :confused: :confused: :confused:

Thanks, Sam

 

I think they're trying to say that it's best to keep it advanced to 8 or 10 degrees for performance reasons and IF there's a problem with knocking, the sensor will take care of it.

There are no dumb questions, only dumb people! :D
I'd rather ask 'dumb' questions than make bad assumptions.

 

Oooops - forgot to continue the thought.....:rolleyes:

...IF there's a problem with knocking, the sensor will take care of it for that moment that the problem existed. When the problem is gone the timing goes back to 8 to 10 degrees.

 

Thanks Bill!
Still some confusion though. If the ESC advances the timing (more before TDC) to prevent the knock, how would setting the base time from 6° to say 12° BTDC start to cause some knock that the ESC would would have to compensate for? Am I reading other posts wrong or just out confusing myself :D

 

I'm not sure where you got the info from, but when the ECM detects knock it retards timing to get rid of it.

Now, you might be thinking of the ESC test in which once per key on, the ECM will advance timing until knock is heard to verify that the knock sensor is functioning. This happens under a certain condition which is described in the factory service manual. Something like when the car is warmed up and the TPS is beyond a certain position, then the check will occur. If no knock is detected a code will set.

During regular driving though, anytime knock is detected, the ECM will pull timing out to make it stop.

The timing tables in the chip are based off the base timing that is set which is 6 degrees stock. If you bump timing up 2 degrees, the chip still assumes that timing is at 6, and in effect you advance the entire timing table 2 degrees which similates a performance chip in one aspect. This is a cheap way to get a small performance gain assuming that you have no knock conditions present at 6 degrees. If your car does knock regularly at 6 degrees, then advancing the base timing more will actually hurt performance because you'll be getting more knock signals and the ECM will be retarding timing even more often.


Make sense now???!!! :)

 

I know my head is like a rock today :crazy: My car runs fine. Just trying to get it straight in my rock. Pretty sure I know how it works, if there is knock it will want to fire before the knock occurs.

I think it's the termenology that I'm bss aackwards

In regards to TDC, what is

Advanced:
Retarded (me?):
To pull timing:
Bump timing up:

Thanks for bearing with me!

Sam

Or is my conception of a knock all wrong, I was thinking of it as pre-detonation. Am I wrong on that?


[Modified by 87SAM, 9:48 AM 4/5/2002]

 

I have written many times on this but here goes again in short form. You timing degrees are relative to TDC. 6*BTDC is just what is says. The spark is sent including time lag of spark getting to spark plugs 6* of the crank before the piston reaches TDC. In other words it gets a head start sort-of-speak. OK now comes the performance part with regards to spark advance. There is something that must be understood with spark advance before continuing. The closer to WOT (this is a very loose assumpion) the more air and fuel you should be pumping into your cylinders. With that said, this also creates a more *volatile* charge. Therefore the kernal *flame* will travel across the cylinder much quicker. Conversly at highway cruising and only part throttle you are not cramming air and fuel in as much therefore the charge is not as volatile. I this case the flame caused by the spark will NOT travel across the cylinder as fast as the before mentioned synerio. OK try to digest and understand what I just wrote and next I will let you know what you are trying to do with your spark advance.
OK lets get started. In either case the piston is moving upward towards the cylinder head. While this is happenind it is compressing the air and fuel. At a certain point the spark will fire (your desired spark advance). This will start the process of the flame traveling across the cylinder. As this happens the air and fuel is burned and converted to heat and pressure. You want the spark advance to burn all the air and fuel and also reach maximum pressure at the same time the piston reached TDC. In other words it is all a *timing* thing. :) OK now as stated before at WOT the charge is much more potent therefor not as much timing is needed to get to max pressure. At cruise you will need more timing advance since the charge will take longer to burn.
If you have the timing tables off then you will either start the flame too soon and cause KNOCK or too late and loose power since the piston is then moving downward away from the head and relieving the pressure. When you start the flame to soon then you have to drive the piston into these gases that expanding and trying to force it back in the direction it is coming from. Not a real good situation. Hence the pinging and engine self destruction. Your car is equiped with the knock sensor that *hears* this before any harm can be done and pulls out timing (lets the piston get closer to the head before starting the flame)
All in all, the name of the game is to get the perfect timing were the spark allows the pressures to become max at just the point of the piston reaching TDC. Thus maximum force is applied to the piston which can only go one direction, back down. This will give you max performance. There is only one way to do this since other factors also apply such as load, octane fuel, air temp, etc. The way is burning your own chips. I took my below setup from high 12-13s to low 12s. You would not have believed the difference. All too many times people go with way too much advance and actually hurt performance. My advance at WOT is only 29.2*. The rule of thumb is run as little advance as possible that gives you max performance. The only way to find were max performance is at the track or with a stop watch or some other timing device. If you use SOTP you will case your tail.
Hope this explains what you are looking to learn. I tried to be breif and not get into a lot of details but it was still pretty long. Later!


[Modified by ski_dwn_it, 5:27 PM 4/5/2002]

 

1) The timing set before something. The timing is aways set advanced (before) TDC. If the timing spec is 6* before TDC (BTDC) and you re-set it to 8* BTDC, you have advanced the timing.
2) The timing moved in the opposite direction. In the above situation, if you re-set the timing to 4* BTDC, you have retarded it.
3) To pull timing, is jargon or slang meaning to reduce the amount of advance or retard the timing from it's present setting.
4) Bump timing up is more jargon, meaning to further advance the timing.

From earlier posts i understood you to think that the EST advances timing when it hears knock. That is just backwards. As tuners looking for the last ounce of power, we advance the timing. This may allow (or cause) knock. The EST retards the timing to eliminate the knock. Maybe more later. Good luck.

 

No, you've got it backwards. No it doesn't want to fire before knock occurs. The computer makes the plugs fire after, or later in the stroke, from where the knock occurred. If knock occured at 38 degrees, the plug will be commanded to fire later, say, at 34 degrees, which is later in the stroke.

First knock and detonation are the same thing and the terms are used interchangably.

Second BTDC (Before Top Dead Center) refers to the position of the piston in the block. Piston position is measured in degrees, just like time is measured in second, minutes, hours, etc.

6 degreed BTDC means the spark will jump the plug gap when the piston is 6 degrees before it reaches the top of its stroke, which is as far upwards as it can travel. The upper-most travel of the piston is called Top Dead Center

Now, when knock is detected by the knock sensor a signal is sent to the car's computer and the computer then retards the timing so the plug won't fire as soon - so it takes timing out, meaning spark will occur later not earlier.

So once this kicks in, the piston must now travel farther up toward TDC before the spark will jump the plug gap. This later firing stops the knock.

So say you'e buzzing along at 2000 RPMs and nail the throttle. Timing advances to where it's programmed to be, but for some reason (bad gas, hot engine, low octane gas, etc.) detonation/knock is detected by the knock sensor.

Now at that point lets say you timing is running at 42 degrees (just an arbitrary number chosen for illustration purposes) when the knock is detected. The computer gets the signal from the knock sensor that things ain't right so the computer lowers the timing in about 4 degree increments to 38. It keeps lowering the timing until the knock sensor no longer reports that knock is occurring.

So from 42 it goes to 38, then to 34, maybe even to 30 degrees BTDC.

Once the computer no longer receives info that knock is occurring, it begins to slowly add timing back, but at a much slower rate than it took it out.

The computer's pretty smart but it isn't a genius, so it always takes out more timing than is actually needed to quell knock.

For example, suppose it would only take one degree LESS timing to quell knock, the computer will still take out, say, 4 degrees, which is 3 degrees more than is actually needed to cure the problem.

That's why having your timing set correctly is so important. If you rely on the computer to pull out timing to quell knock it really kills power in the process; This is because the computer always takes out too much and adds it back too slowly.

Such is life.

Hope this helps.

Jake

 

DING DONG, the bell has rung! :smash:

Whew! I wanna thank you guys for the education and sticken with me on this!!! Part of my problem was the jargon but you guys gave me the picture as to what is really going on. My conception what normally causes the knock and what is done to correct it was way off.

Don't want to sound like I'm exaggerating but I really appreciate. I'll be saving this post to a file!

Thanks again, Sam

 

If peak cylinder pressure occurs at TDC you're going to break pistons, bend/break rods, push the crank out of the bottom of the block. Peak cylinder pressure should occur around 15 degrees AFTER top dead center.