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Thread: Tuning 85 TPI - Need spark tuning advice

  1. #1
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    Tuning 85 TPI - Need spark tuning advice

    Sorry but the msg editor is shoving everything into one block. Hi everyone, I have a 1985 C4 TPI L98 that I am tuning. 870 ECU, AEM wideband wired into a Moates APU1 for datalogging. The engine is lightly modified, light intake cleanup and porting, 1.6 rockers, stock cam, long tube headers, no cats, dual 2.5" exhaust, descreened MAF. No smog pump, EGR system removed/disabled on chip. This is my first try at tuning, so I know the basics, but don't have much experience. Timing is stock at 6 degI have been datalogging and tuning the MAF tables to keep the AFR around 14-15. I leaned out tables 5 and 6 to get the WOT AFR up to 13, which I have read is a good value. I'm not sure if the descreened MAF has made this necessary. It was 11-12 AFR at WOT before. I thought that my engine would be leaner at WOT with better air flow from my mods, but I've read that stock WOT AFR is very rich.With my light mods, I am was thinking I would just have to tune the MAF tables and the spark advance vs rpm vs load table. The power enrichment spark vs AFR table is blank. I don't see a knock count value in my available datalogging variables in 1F.xdf. But I do have a knock flag value along with the other error code flags. I was going to increase the spark advance vs rpm vs load table values slightly and see if I could feel a difference, but I want to make sure I can tell when the engine is knocking, before it becomes severe. Are there any other values that you think i should play with to try to increase performance? Thanks

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    Quote Originally Posted by BlueCorvette View Post
    Sorry but the msg editor is shoving everything into one block. Hi everyone, I have a 1985 C4 TPI L98 that I am tuning. 870 ECU, AEM wideband wired into a Moates APU1 for datalogging. The engine is lightly modified, light intake cleanup and porting, 1.6 rockers, stock cam, long tube headers, no cats, dual 2.5" exhaust, descreened MAF. No smog pump, EGR system removed/disabled on chip. This is my first try at tuning, so I know the basics, but don't have much experience. Timing is stock at 6 degI have been datalogging and tuning the MAF tables to keep the AFR around 14-15. I leaned out tables 5 and 6 to get the WOT AFR up to 13, which I have read is a good value. I'm not sure if the descreened MAF has made this necessary. It was 11-12 AFR at WOT before. I thought that my engine would be leaner at WOT with better air flow from my mods, but I've read that stock WOT AFR is very rich.With my light mods, I am was thinking I would just have to tune the MAF tables and the spark advance vs rpm vs load table. The power enrichment spark vs AFR table is blank. I don't see a knock count value in my available datalogging variables in 1F.xdf. But I do have a knock flag value along with the other error code flags. I was going to increase the spark advance vs rpm vs load table values slightly and see if I could feel a difference, but I want to make sure I can tell when the engine is knocking, before it becomes severe. Are there any other values that you think i should play with to try to increase performance? Thanks
    Does this engine still have cast-iron 76cc smog heads on it? I know I had a similar engine in a truck. It had a very mild 204/214 @ .050 cam in it. With the old carb HEI setup it liked about 18 initial timing. Full centrifical advance of 20 all in by about 2,600 rpm and 10 @ 10 in/hg of vacuum advance. Vacuum advance started about 4 in/hg. That gave a total timing of 38 at 2,600 rpm and a cruise timing of about 48 running about 70 mph. Idle timing was about 28 at 800 rpm. It was a 3 speed manual truck. Those iron heads gave a low static compression ratio, combined with the terrible chamber design a slow burn. It really liked the timing coming in early.If I looked at the timing map I could revise it similarly and send it to you. But I need a good log of your LV8 values vs RPM and TPS.
    Last edited by Fast355; 01-21-2017 at 09:27 AM.

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    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?

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    Quote Originally Posted by BlueCorvette View Post
    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.
    Last edited by Fast355; 01-21-2017 at 01:54 PM.

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    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.

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    Quote Originally Posted by BlueCorvette View Post
    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.

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    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.

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    Quote Originally Posted by BlueCorvette View Post
    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.
    Last edited by Fast355; 01-22-2017 at 02:01 PM.

  9. #9
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    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.
    Last edited by Fast355; 01-22-2017 at 02:58 PM.

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    I don't see a stoichiometric scalar. Is that what I am looking for? Yes I heard that around 36 was good for TPI so your map makes sense. Thanks, I'll take a look at that bin. I think some of the names are different depending on the tool. Is PE multiplier a tunerpro RT name?

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    I see %Change To Fuel/Air Ratio At WOT Vs. RPM and %Change to Fuel/Air Ratio At WOT Vs. Coolant Temp. tables. I like your idea about not changing the MAF tables for WOT AFR. I think I will restore the last two MAF tables to stock, and try to tune these to get the AFR to 12-13.

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    Quote Originally Posted by BlueCorvette View Post
    I see %Change To Fuel/Air Ratio At WOT Vs. RPM and %Change to Fuel/Air Ratio At WOT Vs. Coolant Temp. tables. I like your idea about not changing the MAF tables for WOT AFR. I think I will restore the last two MAF tables to stock, and try to tune these to get the AFR to 12-13.
    The stoichiometric value is a constant, not a table.

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