Whats the highest spark advance a stock L98 likes?
 

Recently I have spent some time looking at the spark tables of various chips I have.

The main ones I have looked at is ANYJ9304 (Thats the original chip from my IROC), The ARAP chip in its stock form and a Fastchip Stage II.

From what I can see after comparing these three chips is that the sock IROC chip never goes higher than approx 40 degrees in the main spark table, while both the ARAP and the Fastchip goes as high as 47 degrees in some parts of the spark table. (Thats the one I used before I started getting into doing my own).

So, my qustions is, whats the highest you other guys run on a stock L98?

I have no problems with pinging even with the ARAP chip, I get a few knock counts when I crank it, but after that I don't get any even if I do a WOT run down the track....

I know there is a difference between the way the octane ratings are calculated in US/Europe, anyway I run 98 octane gas.

I am going down to the track later today if it doesn't start to rain, and will bring the laptop and get some logfiles.

First time I am going to dive deeper into this interesting subject, so any suggestions and/or comments are more than welcome!!!

The BLM/INT numbers are very close to 128/128 at part throttle, but a little lean in idle.

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ZaphodB a.k.a Carl Andersson - zaphodb@faxancruisers.org
'89 Camaro IROC-Z 5.7L TPI - Mods under construction :-)
Custom ARAP Chip Under Development/180 t-stat, Edelbrock TES Headers,
SLP Airfoil, K&N Filters, Accel 8mm wires, Relocated MAT, AFPR @ 45 PSI

The trick is running the min amount of timing (consistant with best HP).

With the AL Vette heads I made best power with 28d of total timing. Lots of guys start with so much timing that they are past best power before they even start. Then try and make up for it with too much fuel.

In cruise you want a fair amount of timing, and all the more so if running a lean cruise mode.

As you approach 42d you also get VERY prone to cross firing. There is 45d between posts on the distributor, that is a mechanical limit. Depending on your initial timing you can get into the high 40s but I'd strongly suggest looking for power in the 34d (total @ WOT) and less range.

Also, if running a cold thermostat, you can use less timing in cruise mode.

Also, not all detonation is detectable by the stock Knock Sensor, and some is inaudible. If you plugs have a gray hue to them your border line detonation, or into trace detonation. Little specks and your seriously into it.

If I'm not mistaken there is a limit to calc spark advance in your bin anyway, so anything more then 42d (maybe it's 45) is just ignored anyway.

With DIS, and an extreammely lean cruise mode I've gone to 56d timing and 17:1 AFr and there is just nothing to be found there. A car will take X amount of HP to go any specific distance, so there is a point of diminishing returns.

I hope this doesn't sound stupid, but what else do I need to take in account to calculate the total timing?

I have got the base timing set at 6 degrees...

I think you are right about the timing upper limit, but I haven't looked at the disassembly of the code yet!

And as soon as the weather get better I'll go get some datalogging of a few different chips, I have just made small changes to...

(Lowering the timing from the stock ARAP code).

Would you suggest flattening the timing figures at WOT to say... 18 for a total timing of 28 all across the board ? I noticed in GM's bins that they go high and low a lot. Does total timing need to kick in around 3000 RPM or so or higher ?

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92 Z28 5.7
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Trick Flow 23° heads, Comp Cams XR270HR-10 cam, Accel intake base + ported TPI,
TES headers, Flowmaster catback, MSD ignition, #24 SVO injectors, 1LE aluminum driveshaft.

Click here to see pics of my car !

I too have noticed that on most of the stock GM Bins for the L98, 40* seems to be the maximum on part throttle cruising. My stock eprom for my SD only attained 40* under the most ideal of part throttle cruise.

The best octane that I can readily get where I live is 94. I have done a variety of tests and I am finding using spark plugs of one range colder with 94 octane gives me the least number of incidences of knock. I have also been playing with the spark advance to find a happy compromise between performance, driveability, responsiveness and economy.

For MAX fuel economy I ran some fairly lean AF ratios:

30 KPA 17.6
40 KPA 17.2
50 KPA 16.8
60 KPA 16.4 to a max of 74 KPA.

For these AF ratios, I was running Spark Advance of 47.2* (30-50 kpa) to 45.3-44.0* (60-74 kpa). I encountered virtually no incidences of knock with these low AF ratios and Spark Advances. In fact, I found I had to run Spark Advances of at least this level or my throttle response became "mushy" due to the lean AF ratios. Higher Spark Advance restored some of the "crispness" back into my throttle response. I was getting fuel economy of 28-30 US Mpgs (or 12-12.5 Km/Litre) with these AF ratios and Spark Advance.

However, I found that my engine would develop a "surge/bucking" with these lean AF ratios. It was most pronounced at the lower kpa loads and smoothened out at higher loads. I have since reduced the leaness to a range of:

30 KPA 17.2
40 KPA 16.6
50 KPA 16.4
60 KPA 16.2 to 74 KPA

And reduced my Highway Mode Spark Advance a couple of degrees. The "surging/bucking" has basically vanished and my gas mileage only suffered about 1-2 MPG. I now get fuel economy of 26-28 US MPGs (11.3-12 Km/Litre).

I am currently doing testing on Spark Advance and WOT, but that is another topic.

[This message has been edited by Glenn91L98GTA (edited August 14, 2001).]

Okay... The thing that got me spooked was that so many people have told me that the stock tables in the ARAP bin are a bit too aggresive on timing, but I have not had any problems running with them, in fact the car runs very good, and gets decent mileage (Not as good as yours yet, but you have spent more time working on that than I have.... I get like 23-24 mpg at best, but maybe not so bad for a MAF L98?)

So lowering the advance in the main spark tables might be a bad idea if I run highway fuel mode?

Or can I lower it to lets say 45 degrees maximum maybe?

If I get some decent weather this weekend I will spend some time with the laptop scanning the results of some chips I have prepared, but it has been raining here a lot lately.
Especially when I plan to do this kinda stuff.... https://www.thirdgen.org/messgboard/frown.gif https://www.thirdgen.org/messgboard/frown.gif