When ever I bump the timing up and test the eprom I get an ESC error code. It only happens when max timing should be. Am I maxing out my timing advance or what's going on?
In the constants, should I turn down the "Main spark bias" from 20 to something like 10? What will this do to the overall timing?
What the heck is "Coolant spark advance bias"? The value is also 20 but I don't understand what this constant is for.

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, Jon (350 TBI!)
91 Red My website

 

I've been messing around with a 16147060 hack and found a Max timing of 42 degrees. I don't know if you're hitting this, but just be aware that it is, apparently, the hard coded limit to spark advance.

The only thing I know about main spark bias is what TunerCat says: "The Main Spark Bias is subtracted from the spark advance in order to provide greater range to certain spark advance management functions. A global change to spark advance may be made by changing this value. Increasing the value will reduce the final spark advance by the amount of bias increase and vice versa."

I don't have a clue right now what the coolant spark advance bias is.

I like all the things we can modify, I just wish I knew what they all did.


------------------
1991 Chevy C2500HD 5.7L HD. 16147060 ECM
Mods: Crane PowerMax 2030 cam 204I 214E @ .050 and 423I 446E lift, Jacobs Pro Street Ignition System. Using DataMaster and TunerCat with $85 ECM files.

 

Found out more about this while researching spark advance.

From http://www.diy-efi.org/gmecm/ecm_inf...ion/spk747.txt

"0x014: Warm spark bias. This value is subtracted from the Total SA. It's purpose is to bias the main SA table, so that negative advance values can be in the table as a positive number."

"0x015: Cold spark bias. This value is subtracted from the Total SA. It's purpose is to bias the Coolant compensation spark table. Again, to allow for a negative advance in this table."

"0x019: Maximum spark (2 bytes). This value is the highest total advance that can be programmed into the distributor. Any Calculated Total SA above this value gets clipped to this value. It's purpose is due to the physical limitations of the distributor. You can only advance the timing so far until you are sparking on the previous cylinder. With eight spark firings per distributor revolution, there are only 45 degrees between each set of plug wire terminals on the cap. GM uses the value of 42 degrees as the maximum advance. This always [sic] a safety margin to direct the spark to the proper terminal."



------------------
1991 Chevy C2500HD 5.7L HD. 16147060 ECM
Mods: Crane PowerMax 2030 cam 204I 214E @ .050 and 423I 446E lift, Jacobs Pro Street Ignition System. Using DataMaster and TunerCat with $85 ECM files.

 

Yes, but the timing is measure in Crankshaft degrees. Since the crank turns twice for every revolution of the camshaft/distributor, the 45* distributor is equivalent to 90* crankshaft or spark timing.

90* is the maximum you can set the spark to in the eprom. With highway mode and a functioning EGR I routinely run 47.2*. You actually need that extra spark advance because the mixture is leaned and diluted with exhaust gas, so the mixture is not that volatile and the extra spark helps.

But that is getting close to the maximum that you would want to run and not experience driveability problems. When Highway Mode is not invoked, I limit myself to 42* max, and that is with the EGR. I would back it off a bit further if I didn't have an EGR.

 

Thanks, this helps a bunch.

------------------
, Jon (350 TBI!)
91 Red My website

 

I really doubt your at 47d timing with a distributor. You'd be cross firing, and 5-7 given any excuse would be missing.

There are two limits on dissy cars, one for the large cap, and one max for the small cap.

DIS is a whole nutter matter.
Might splain why dissies are disappearing, and things like eDIST will become so popular.

Now with an eDIST and stuff you can run over 50d, but that is really like for some ultra low CR engine. Ya, like a turbo motor <G>...

Course this might be ignoring some other stuff.......

 

What you can wind up doing is messing the ESC test up.

The EST test will assume that the test will force the engine to knock when it does the test. So if a lil too rich or some combo there of, the engine might not pass the ESC test, so it sets an ESC code

Some programs will prohibit PE timing if the engine fails the ESC drill.

 

Yes, Bruce, I am running 47.2* while in Highway Mode. Remember the 47* is base on the crankshaft which is 90* between cylinders. There is 720* for a complete operating cycle based on crankshaft degrees (720*/8=90*), while the distributor only makes 1 revolution during that same interval. Hence the 45* between 5 and 7 is really 90* based on the crankshaft (and the spark timing). All timing is based and measured on crankshaft degrees.

 

Come on Glenn, if you want to be condesending, we'll just end the discussion now.

OK, so some how you have the spark taking the path of greatest resistance in the cap rotor air gap. At 45.000000000001d BTDC the air gap will be shortest to the last cylinder then the one your trying to fire.

You might think you have 47d in the chip, but unless you've redone the code, the timing with stop at 42 or 45d.
Your scanner will even report the calc'd value.

Or you might be using a 0 bias, and setting the timing up from there to get an alleged 47d but, the ecm ain't gonna calc over the limit set in the program, and sparks take the path of least resistance.

The above ref dissy code, DIS is different.

 

So I think what is being said is that 45* at the crank means 22.5* at the distributor (which is half the distance between the contacts in the cap, right?), and 90* at the crank is 45* at the distributor. 47.2* BTDC at the crank is then 23.6* BDTC at the dist. Therefore, the rotor is closer to the last contact it passed (by 1.1*) while it is on its way to the contact connected to the cylinder you want. Thus, spark would hit the last contact it past before it would hit the one you want. Or you would have a bit of spark at both.

The 42* limit is hard coded in my code. It might be different in others.

------------------
1991 Chevy C2500HD 5.7L HD. 16147060 ECM
Mods: Crane PowerMax 2030 cam 204I 214E @ .050 and 423I 446E lift, Jacobs Pro Street Ignition System. Using DataMaster and TunerCat with $85 ECM files.

[This message has been edited by davidjon_99 (edited October 20, 2001).]

 

Okay, I was thinking about this again tonight when I found out that Yanks don't play until tomorrow night! Duh...
If the max timing is say 42 degrees in the distrib, what says you can't run 48 degrees by moving the base timing to 6. Wouldn't that make 48 total degrees advance? I understand the whole idea about electricity and path of least resistance but if you mechanically make it so the distrib is already advanced, can't you then run 42 ontop of whatever your base is set too?

 

This discussion proceeds on the assumption that the distributor is indexed so that at 0* advance the rotor tip is PERFECTLY aligned with the contact at the cap and there is NO advance biased into the assembly.

I am not saying that this is or is not the case - however I am inclined to believe that when Glenn says that he IS running 47.5* advance - that is what he is getting.

This leads me to believe that G..M. has done the smart thing and biased the distributor so that if you are at 0* spark advance the rotor tip is already passed the contact at the cap.

Considering that the engine spends a vast majority of its time at spark advance levels over 20* - it would be logical to index a distributor with this 20* advance built in. This would of course be only 10* of distributor motion.

Like I said - I do not know if this is the case - but it sure seems to fit the bill.

Grumpy: your posts get difficult to read with all the "dis" and "dissy"
And I don't really think Glenn was being condescending, he just knows what he has done. Chill man.

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Karl Hunter
Hunter Motorsports
Vancouver, B.C. CANADA

 

There are ways to trick the program so your actual timing is past 42* BTDC. What the program does is calculate the spark advance from the spark table, spark bias, initial spark advance setting, power enrichment spark advance, etc. that are in the prom and then it clips it at 42* BTDC if the calculated value is greater than 42*. That means that if it calculates a spark advance of 47.5*, it stops it at 42* BTDC. Depending how the ECM's program is written, you might be told by your scan tool that you are running an advance greater than 42*. If the person programming the ecm did it correctly (IMHO) what you would see from the ALDL data would be the actual spark advance the ECM is using, not just the calculated spark advance. Remember, the prom assumes you are telling it the true value for base timing. Yes, you can fool it by telling the program that your base timing is 0* BTDC and then set the mechanical advance to 6* BTDC. However, the law of nature can never be broken. You'd have some or all of your spark hitting the spark plug of the wrong cylinder when you got to a total timing (mechanical and programmed) equal to or greater than 45* BTDC.

[EDIT} Oops! Glenn is correct below. The above is wrong.

------------------
1991 Chevy C2500HD 5.7L HD. 16147060 ECM
Mods: Crane PowerMax 2030 cam 204I 214E @ .050 and 423I 446E lift, Jacobs Pro Street Ignition System. Using DataMaster and TunerCat with $85 ECM files.

[This message has been edited by davidjon_99 (edited November 03, 2001).]

 

David, this is not how the rotor/distributor works. The ECM will advance the position of the rotor relative from it's base setting via the module in the distributor. Much like the way a mechanical distributor uses weights, springs and vacuum.

Only when the rotor is positioned over the correct terminal in the distributor will the electrical connection be made to fire the spark plug. When you have the 45* of advance, the rotor is not "half way between one terminal and another" as your post implies. The rotor has been advanced from it's base setting by the amount of relative timing the ECM commanded the module in the distributor to advance it.

Th 42* constraint is a constant on SD cars (and $6E MAF) that can be adjusted.

The real issue the potential for cross-firing as Bruce said, and other driveability issues as I said. If the spark advance is too far advance (and no cross-firing is occuring), you start to make the cylinder "work against you" as the cylinder is ready to release all of its engery before the cylinder has even hit TDC.

These higher spark advances are only allowable because of the EGR. The EGR's adding of less volatile mixtures makes the mixture burn slowly. Without the additional advance (that is why the ECM stock works somewhere between 38-42*), the engine would feel sluggish when the EGR was functioning. That is also why all these high spark advances only occur during light part throttle.

When you invoke Highway Mode, the mixture is even less volatile (if you have a functioning EGR). The throttle feels very sluggish, even at the stock/max 42* setting. Increasing the advance beyond 42* will increase the throttle response. I can feel it when it is advanced beyond 42*; there is a night/day difference in how my throttle response feels at 42* vs 45+*. The car feels like a dog at 42* - it NEEDS the extra advance if you have Highway Mode invoked and a fairly lean AF ratio.

But once I start to exceed 47*, then the mixture starts to fire too soon and the piston works against you. I noticed that my gas mileage actually starts to decrease if I advance it much further than 47*.

 

Actually - Glenn, the rotor does not move in that fashion. It is fixed to the distributor shaft - unlike that of a mechanical distributor. That is why I suspect that the rotor position is indexed so that when at 0* spark advance the rotor is already passed the terminal in the cap. This is the only way you could be running 47* and not get crossfiring and/or misfiring. I have taken these distributors apart enough times to know that if that rotor is moving in relation to the shaft - something is very wrong.

------------------
Karl Hunter
Hunter Motorsports
Vancouver, B.C. CANADA

 

Oops, I don't know what I was thinking, but you are very correct.

 

Glenn, I have to agree with Karl in that the rotor is fixed in an ecm controlled dist. The non-ecm controlled ones do move the rotor but also move the pick-up with vacuum advance and that's how you get more advance, too. I also believe the rotor can't be indexed at zero otherwise 45 degrees of spark would be impossible.

 

Thanks Karl for clearing that up for me.

 

No David, I was wrong. According to Karl, the distributor DOES fire the spark before or after the actual positioning of the rotor to the terminal on computer controlled cars.

In talking to Karl, he stated that there is some built in advance to the rotor's position - enough to generally fire the spark when the rotor is close to the terminal (with normal spark advance specs) so it is not an issue. But if you get into too long of spark advance, the possibility of cross-firing is a highly possible.

 

1 & 4 are 45° apart. 2 & 3 are 22.5° apart.

2 and 3 are the limit because the air gap becomes too large. Thus the 42° limit in the prom. Really its 42 + whatever initial timing you have.

Is that correct?

 

You all have it wrong.The computer controlled dist has nothing to do with the rotor.If you have ever seen the inside of one there is a round magnetic disc below the rotor(I forgot the name).It has 8 contact magnets that the ignition module picks up.If the the dist was set at 0 with advance wire off,then the #1 magnet is lined up w/ the module.Then the advance would be relitive to the module and magnet.IF there is a stock mechanical advance inside the dist its easy to tell.The magnetic disc on the dist shaft will be clocked X amount before the rotor contact point.

 

Uh - yeah thats what I said. and that is the case indeed. I checked.

------------------
Karl Hunter
Hunter Motorsports
Vancouver, B.C. CANADA

 

Uh cool dude

 

Brought this back from the dead!

What are the limits for small cap and large cap?

 

It all depends on the tolerances of the caps and rotors . A dizzy is a dizzy, 8 cylinders has only 45 degrees between each post. It's safe to say that max timing on a dizzy should be around 42 to 43 degrees. If your car needs more than that then it should be converted to CNP or DIS.
8 degrees of mechanical advance plus 42 commanded electronically equals 50 degrees BTDC!

 

Any one care to post the old and new SBC firing orders?.

 

18436572 for old, not sure on the new

 

For the LS1 #4 and 7 are swapped: 1-8-7-2-6-5-4-3

From the '747 hac:

RBob.