Tuning 85 TPI - Need spark tuning advice

[BlueCorvette]

Yes I have the stock iron heads, and yes the stock cam is mild.I found a Spark Advance vs Rpm vs Load Table example in a forum for a 1985 C4 TPI like mine. The post said it would ping in over 80 deg air temp but he could race at lower temps with 95+ octane gas. He mentions 10:1 compression and distributor timing is 4 deg, but I see it as an example of the absolute max advance values. I think that his retarding the distributor timing by two degrees forces the ECU to add an extra 2 degrees to the final, not sure about that. Using a spreadsheet, I calculated the cell by cell delta values for the my stock map vs the max map. Then I added 1/4 of the delta to each corresponding cell to create a work Spark Advance vs Rpm vs Load Table. For example, if the max table was adding 8 degree over stock, I added 2. For 3200 rpm X 144 load the stock value is 34.10 while the max value is 42.90. So that would be a stock advance of 6 deg + 34.10 deg = 40.10 deg spark advance. The new advance would be 6 deg + 34.10 deg + 2.2 deg = 36.3. Not a big difference but I was being conservative for my first spark map change. I datalogged a few 2nd/3rd gear full throttle runs to 5000 rpm, and didn't notice a difference in my butt-meter. The most important thing I was testing was to see if the knock flag had been set in any of the data points in my logging. I didn't see any. I have seen other error flags get set in my datalogging so I have some confidence that the knock flag will show up as a warning. So I was adding 1-3 deg of advance across the table and nothing happened. Anybody surprised? Why won't this thing let me do line breaks?

[Fast355]

Yes I have the stock iron heads, and yes the stock cam is mild.I found a Spark Advance vs Rpm vs Load Table example in a forum for a 1985 C4 TPI like mine. The post said it would ping in over 80 deg air temp but he could race at lower temps with 95+ octane gas. He mentions 10:1 compression and distributor timing is 4 deg, but I see it as an example of the absolute max advance values. I think that his retarding the distributor timing by two degrees forces the ECU to add an extra 2 degrees to the final, not sure about that. Using a spreadsheet, I calculated the cell by cell delta values for the my stock map vs the max map. Then I added 1/4 of the delta to each corresponding cell to create a work Spark Advance vs Rpm vs Load Table. For example, if the max table was adding 8 degree over stock, I added 2. For 3200 rpm X 144 load the stock value is 34.10 while the max value is 42.90. So that would be a stock advance of 6 deg + 34.10 deg = 40.10 deg spark advance. The new advance would be 6 deg + 34.10 deg + 2.2 deg = 36.3. Not a big difference but I was being conservative for my first spark map change. I datalogged a few 2nd/3rd gear full throttle runs to 5000 rpm, and didn't notice a difference in my butt-meter. The most important thing I was testing was to see if the knock flag had been set in any of the data points in my logging. I didn't see any. I have seen other error flags get set in my datalogging so I have some confidence that the knock flag will show up as a warning. So I was adding 1-3 deg of advance across the table and nothing happened. Anybody surprised? Why won't this thing let me do line breaks?

So long as the timing is set at the factory 6* BTDC the initial value is not added to the spark advance. Setting at 4° would retard the timing 2°. Setting at 8° would advance it 2°. There is a setting in the chip that compensates for the initial timing value. Look at the Main/PE/Coolant/IAT spark timing tables.

[BlueCorvette]

You are thinking of the 165 / $32 tables. My 870 / $1F just has Spark Advance vs Rpm vs Load, Power Enrichment Spark Vs. AFR (blank stock), Highway Mode Spark Advance Vs. Load, and Time Out Spark Vs. Coolant Temp. So, unfortunately it looks like its just Spark Advance vs Rpm vs Load to tweak for general/max performance. The 870 is so slow and limited, I was thinking of switching to the 165. It would make datalogging/tuning so much easier, but I didn't want to mess with that until I had the car running well.

[Fast355]

You are thinking of the 165 / $32 tables. My 870 / $1F just has Spark Advance vs Rpm vs Load, Power Enrichment Spark Vs. AFR (blank stock), Highway Mode Spark Advance Vs. Load, and Time Out Spark Vs. Coolant Temp. So, unfortunately it looks like its just Spark Advance vs Rpm vs Load to tweak for general/max performance. The 870 is so slow and limited, I was thinking of switching to the 165. It would make datalogging/tuning so much easier, but I didn't want to mess with that until I had the car running well.

I looked at my $1F .XDF and a .BIN for a 1985 305 Camaro (Actually a Hypertech Chip read) I have on my tuning laptop. Definately has a Coolant Spark Compensation table as well as a MAT compensation table. However only the Coolant Spark table has values in this 305 calibration. Also the tune has a PE spark table that is used and adds up to 3* of timing depending on the air/fuel ratio.

[BlueCorvette]

Yes you are right, I missed the Coolant spark table. That would adjust spark uniformly across a RPM for a load range. Interesting that it will add 10 deg at the highest load in very cold weather. That seems like a lot. The mixture at that temp must burn so slowly that they can throw a 10 deg advance at it with no problem. I still don't see anything that looks like IAT? I like that the tuner was using the PE spark table to add for richer AFR. That seems like a more refined way to do it. Except I don't understand how the ECU could use that table with a narrowband O2 sensor? The table has precise AFR ranges.

[Fast355]

Yes you are right, I missed the Coolant spark table. That would adjust spark uniformly across a RPM for a load range. Interesting that it will add 10 deg at the highest load in very cold weather. That seems like a lot. The mixture at that temp must burn so slowly that they can throw a 10 deg advance at it with no problem. I still don't see anything that looks like IAT? I like that the tuner was using the PE spark table to add for richer AFR. That seems like a more refined way to do it. Except I don't understand how the ECU could use that table with a narrowband O2 sensor? The table has precise AFR ranges.

I have a $1F and $1F Updated .XDF that both have MAT (basically IAT) based spark compensation tables in them. As for the PE AFR Spark, it is done off commanded air/fuel ratio. When you are in open loop the mixture is commanded richer than 14.7:1. In PE the PE adders modify the commanded air/fuel ratio from 14.7:1 or whatever stoichiometric is set at. Should set Stoichiometric at 14.2:1 if you are running fuel that has 10% Ethanol in it. On my definition the MAT compensation is labeled Spark Advance Correction vs MAT. In very hot weather it can be helpful to remove 4-6* of timing when the MAT is elevated to help keep you out of detonation.

[Fast355]

Just in case you are curious I had a 1985 Camaro TPI setup I tuned using the 1985 305 TPI Computer a few years ago for someone I knew. It was on a 290 HP GM 350 performance parts engine in a Monte Carlo SS. I just copied the spark table into a 1985 700r4 Corvette's calibration, along with my PE Fuel multiplier and Command closed loop at 14.2:1. WOT air/fuel ratio is programmed for 12.5:1. Total timing is 36* from 2,800-4,000 rpm, 37* at 4,400 rpm, 38* at 4,800 rpm. I would only try this timing map if your distributor is set at the stock 6* BTDC. I would put your stock MAF table values in place for now. You should lean out the air/fuel ratio using the PE Multiplier rather than the MAF tables and once you command the air/fuel ratio leaner, fine trim the WOT with the MAF Tables.