Hey guys,

On a stock chip, say for years 85-87 whats the highest advance they came with? And is it in addition to the base? like 40 in the chip, 6 base = 46 advanced

Because my car at 3000 rpms is running 40 degrees advanced only 6*
This is baaaaaaaaaaaaaaad. Doesn't seem right either, did the 85 have some really horible spark tables, or is something wrong?
thanks!

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1985 IROC-Z
TPI 305, 700R4 with shift kit, richmond 3.73's, auburn pro series posi, crane compucam 2030, ported plenum, gutted air boxes, all the little insignificant stuff.

 

Regarding timing, I cannot give you too much specific information on your particular eprom. However, I can give you some observations about "commonalities and uniqueness" between the 1989 $6E MAF BIN and 1990-92 $8D SD BIN. This will hopefully guide you in being able to determine what the values mean in your BIN if someone has the specific value.

Most of the GM Bins that I have seen, do not have too much spark in the various tables in PE/WOT, but fair amount if at part-throttle in a nice flat cruise on the highway with a functioning EGR.

Between the various GM BINs for the F-body $8D and $6E that I have looked at (excluding the ARAP BIN for the $6E) few stock GM BINs go much above 38-40* for Total Spark Advance. You only get this on a steady highway cruise on a flat road with a functioning EGR and CCP at 100% Duty Cycle. This is something common I have noticed on varous GM eproms.

Unrelated, but another commonality (except the ARAP BIN again) most stock GM Bins seem "error" on the rich side. Some worst than others.

However, the method the $6E and $8D eproms use to calculate Total Timing and how certain tables operate (e.g. Main Spark Table), are different.

With SD, the Main Spark Table appears to only affect the relative timing. On the $6E, it appears to be more of a "cap" of your Total Spark at various RPM/Load ranges.

With SD, and no extra spark from Highway Mode, PE or Cold Start, your Relative Timing is the same as your Main Spark Table and you must add the Base to get your "displayed" Total Spark Advance. E.G. My base is 6* and at 2,500 rpm @ 40 Kpa of load having 33* would display 39* Total Spark Advance and 33* Relative Timing.

Of course, if your REAL Spark Advance of your Distributor is not actually 6*, your REAL Total Spark Advance will differ by a similar amount. If your REAL SA is 10*, then your REAL Total Spark Advance would be 43* (33* + 10*), though your DISPLAYED Total Spark Advance per your scan tool will remain 39*.

Then there is how Highway Mode, PE, Cold Start and Knock Retard affects the Relative Timing and the DISPLAYED Total Spark Advance. This is a whole other subject again, that I will not address right now.

With the $6E MAF BIN, the Main Spark Table behaves differently. It appears the Main Spark Table also includes the base component and you are affecting the Total Spark Advance (excluding PE, HM or CS). I say this from the few experiments I did on a friend's 89 MAF car and what I observed when I ran my Diacom+ on his car.

Using the same example as before, 6* Base and 33* Main Spark, the $6E seem to display 33* Total Spark Advance and 27* Relative. While the SD car displayed 39* Total Spark and 33* Relative.

Also, the $8D SD has a "Max Spark Advance" constant that limits the Maximum Total Spark advance, but the $6E MAF does not appear to have a similar "Max Spark Advance" constant to "cap" the maximum amount of Spark Advance. I suspect your earlier MAF Bin is similar to the $6E MAF.

[This message has been edited by Glenn91L98GTA (edited August 01, 2001).]

 

So Is it possible that with my 85 ecm, being older and by far the slowest ever that the ecm had not switched to WOT when I punched the throttle and climbed to 4500 where it started knocking like crazy?

with all my sensors being in decent shape I can't understand why it would knock so hard, and not retard after the first knock.
thanks

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1985 IROC-Z
TPI 305, 700R4 with shift kit, richmond 3.73's, auburn pro series posi, crane compucam 2030, ported plenum, gutted air boxes, all the little insignificant stuff.

 

In WOT, the key is running as little timing as consistant with max performance. There are lots of fast cars running less then 30d timing, and some less then 20 (turbo'd).

While *slow* that is in electrical terms. No human can detect the difference in cpu speeds (in gm ecms). Most good drivers, can barely feel .1sec, in throttle response. The best of the F1 guys can just barely feel .01. The GM ecms figure cals generally at 12.5 msecs. So unless your just out of the level of being a F1 driver, you **might** notice it.

 

Im just trying to find out why the thing kocks

I can't feel a difference in 20 hp

(jk)

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1985 IROC-Z
TPI 305, 700R4 with shift kit, richmond 3.73's, auburn pro series posi, crane compucam 2030, ported plenum, gutted air boxes, all the little insignificant stuff.

 

Have you thought about other things that aren't prom related. I seem to remember you said this car is your daily driver and such. (Actually I think you said that you can't have the car down for the possible three days it may take to do an ECM swap.) This might mean the car has quite a few miles. Which means you may have quite a bit of carbon build-up on your pistons and chambers which raises compression. You could also have a high rate of oil consumption (which newer GM heads are quite susceptible to) which lowers the octane rating. You could also have a slipped damper which makes reading the timing against the pointer useless as it may be advanced or retarded by a few degrees. Naturally, you could have all these things happening to you at the same time and that may explain why the ecm can't correct for the knock. Than again, maybe the prom is screwed!

 

Haha, thanks for the optimism

But the head/carbon thing is very possible, even probable

But I know the dampener is fine and other things

Im doing the swap, i finally found a local ecm and harness, i think the harness will make the job even quicker.
thanks for the post with all the avenues

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1985 IROC-Z
TPI 305, 700R4 with shift kit, richmond 3.73's, auburn pro series posi, crane compucam 2030, ported plenum, gutted air boxes, all the little insignificant stuff.

 

As others have said, you can have up to 40 degrees of timing not at WOT, although my stock 305 bin also had up to 40 degrees of timing -at- WOT, but that was only if I hit 5000 rpm. Otherwise, my PE timing was in the mid to upper 30's with no knock, but this was with the small-chamber inefficient 305 heads and the peanut cam.

Glenn's right on about the $6E/$8D in how they're calculated. The only difference is that the $6E spark table includes the base timing, while the $8D spark table doesn't, just like he said. The $6E also has a max spark advance, usually capped at 42 degrees from most of the bins I've seen, but I think that value is 42 degrees added to the base timing from what I read in the GMECM archives the other day.

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Greg Westphal
'87 IROC 355TPI/A4

 

You are correct Greg. The Max Spark Advance just wasn't defined in the version of $6E that I have from TunerCat. When I double checked the "hac", I noticed it immediately. Also the hac documents it as "Relative" as you said.