GM TBI 7427 ECU What does this do --- “Altitude Spark Bias"

**tplasek** · 08-30-2023

I have a very mild SBC 383 in a Jeep using a 7427 ECU. It has been used primarily off-road all over Texas, Oklahoma and Arkansas without issue. We just returned from a trip to the San Juan Mountains in Colorado ranging from 9,500 ft to 12,500 ft and experienced severe overheating issues not related to the radiator, fan or thermostat. The exhaust and floor boards seemed to be much hotter than normal while leisurely cruising down dirt roads at 20 mph. All over heating problems went away after we returned to Texas.

In a 7427 ECU, how does “Altitude Spark Bias” and “Altitude Spark Advance Correction vs. Baro vs. Vacuum” affect the timing advance with increasing elevation? If I change the Altitude Spark Bias value, the values in the corresponding “Altitude Spark Advance Correction vs. Baro vs. Vacuum” also change.

Early in my tuning I was chasing down a spark retard issue and discovered my actual spark, as measured on the distributor, did not match my LT1 timing table in the ECU. I was missing 10 degrees everywhere even with the base timing set correctly. Once I set Altitude Spark Bias to 0, my missing ten degrees instantly returned. I suspect this may have been related to me, unknowingly at the time, zero-ing out the values in the table “Altitude Spark Advance Correction vs. Baro vs. Vacuum” but leaving the Altitude Spark Bias set to 9.8. However, in Colorado I had both, the Altitude Spark Bias and the “Altitude Spark Advance Correction vs. Baro vs. Vacuum”, all set to zero. Could this have retarded the timing at high elevations? Enough to cause hot exhaust and severe overheating?

**tplasek** · 08-30-2023

UPDATE: I just remembered I collected a 30 minute datalog while in the San Juan Mountains. After reviewing, now I'm VERY confused.

1. I completely forgot being at 10,000 ft only represents around 65 kPa at WOT, which is atmospheric pressure. This pushed all my cruising cells in the timing table drastically to the left. I see Map values as low as single digits when going downhill riding low gear. This caught me off guard but I can understand why this is happening.
2. What I don’t understand is the ECU is adding 10 degrees of timing to my timing table across the board when at 10,000 ft elevation. I can read the instantaneous Map and RPM values from the datalog and cross plot this to the correct timing table cell. Each and every time, the datalog shows 10 degrees more sadvance compared to the value in the timing table. No knock counts or knock retard but I am cruising around at 1700 rpms with a MAP of 40 kPa and a recorded timing value of 45 deg. Good or bad, where is the extra 10 degrees of timing at 10,000ft coming from?

**dave w** · 08-30-2023

I not able to answer your timing questions.

Perhaps the attached $OD data log, with WBO2 information, from a daily driver in Flagstaff AZ (approximately 7000 feet elevation) will be helpful.

**tplasek** · 08-31-2023

Originally Posted by dave w

I not able to answer your timing questions.

Perhaps the attached $OD data log, with WBO2 information, from a daily driver in Flagstaff AZ (approximately 7000 feet elevation) will be helpful.

Dave, Thank you for sharing the datalog. I see many similarities between the datalog you provided and mine. Do you by any chance have the timing table that was in use when the datalog was recorded? I would love to match the timing values as recorded in the datalog to the values in the timing table. Do they match? Is there any additional timing being commanded everywhere in the datalog due to the elevation? The more I read, I hear 1 deg of additional advance per 1000 ft of elevation is a general rule of thumb. However, I’d really like to know what table in my bin is commanding this.

**dave w** · 08-31-2023

Originally Posted by tplasek

Dave, Thank you for sharing the datalog. I see many similarities between the datalog you provided and mine. Do you by any chance have the timing table that was in use when the datalog was recorded? I would love to match the timing values as recorded in the datalog to the values in the timing table. Do they match? Is there any additional timing being commanded everywhere in the datalog due to the elevation? The more I read, I hear 1 deg of additional advance per 1000 ft of elevation is a general rule of thumb. However, I’d really like to know what table in my bin is commanding this.

The timing table is from the attached Excel file +/- a degree or so here and there.

http://www.gearhead-efi.com/Fuel-Inj...-Throttle-ONLY!

Vortec Timing.JPG

**tplasek** · 08-31-2023

Dave, Thank you for providing the timing table and the screenshot of your altitude timing values. Some things I take away from the information you provided.

1: TunerPro does not record the actual timing value as being commanded by the ECU. It records a value ten degrees higher compared to the value set in the timing table. Maybe TunerPro does not take into account the 9.8 degrees of Spark Bias? Below are captures of the recorded avg timing values from the data log you provided to me. The recorded values are all ten degrees higher compared to what is in the Vortec timing table.

Daves Spreadsheet.jpg AVG Timing from Datalog.jpg

2:If Tunerpro records timing values 10 degrees higher compared to commanded timing, my timing in Colorado looks to match my spark table and was probably NOT the cause of my overheating. I ordered a dial back timing light to confirm.

3:I'm sure you are aware, the values in the “Altitude Spark Advance Correction vs. Baro vs. Vacuum” table will change depending on what you have the "Altitude Spark Bias" set to. When the ASB is set to 9.8 degrees, the values in the ASACvsB table are all very close to 0. When you change the ASB to 0, all values in the table increase by 9.8. You left the the 9.8 values in the table after setting the ASB to 0. I did differently, and zero'd the table after setting the ASB to 0. I still don't know when the values in the “Altitude Spark Advance Correction vs. Baro vs. Vacuum” are in play, but I now don't believe this was contributing to my overheating issues.

ASB.JPG

**dave w** · 09-01-2023

I'm seeing different SA numbers from the data log.

Attached is the .adx file I used to playback / export the data log.

SA.jpg

**tplasek** · 09-01-2023

Dave, Thank you for helping out with this. When I use the $0D ADX file you provided above, the timing values in TunerPro and your spreadsheet match the values you show. As a test, I re-loaded the ADX I have been using since the beginning of my tuning and the extra 10 degrees of timing came back. I'm fairly confident I downloaded the ADX file from TunerPro's website. Maybe I somehow corrupted the ADX file along the way.

Thank's again!j

fixed timing.jpg