PontiacBonnevilleClub.com

Now that I have KR nailed down (for now) in my HPTuner experience there are 2 topics I want to discuss...together:

Timing advance and its relationship to AFR.

First the basics....

A lot of people just spew out an accepted fact that 11.7-11.8 AFR is the target for PE mode. I followed this claim without questioning the logic, but I'm at a stage of tuning where I need to ask why. Why?

There are three tables that deal with the brunt of the commanded AFR and by default the AFR decreases over time AND rpm while you mash on the gas pedal. This can cause the AFR to decrease down to as low as 10.5. So why am I leaning this condition all the way back up to 11.8? I noticed that after I made a table adjustment that prohibits my AFR from falling below 11.7 my timing also decreased slightly.

Lets pull up a WOT timing cell from the High Octane table....4800rpm by 140kPA = 11.40659 degrees of timing.

By default this was 10.0 and I used to get about 10 without KR, but after the AFR changes it would reach 8.0-8.5 at best. I increased this recently to 11.4xxxx and now I see 9.0-9.5 degrees of timing at WOT. Fair enough, its an improvement, but why aren't I getting ~11.4? The only table I believe that could have any effect on this would be "AFR Correction" table, which simply adjusts the timing according to the current AFR. I left this table stock and at 11.7 AFR it adds 3 and at 10.7 it adds 6 degrees. Well 11.4+3=14.4, so something else must be adjusting the outcome.

There is a table called Max Torque Timing vs RPM vs Cyl Air, which HPT captions as:

"Theoretical spark advance that delivers maximum toque. PCM torque calculation routines reference this table when estimating current engine torque output relative to theoretical maximum. We do not recommend modifying this table."

K sound advice, I won't. However, I noticed that the values in this table are slightly lower than the ones in the High Octane table...yet higher than the Low Octane table.

In short the butt dyno is stronger after my adjustments, but I'm not seeing the numbers I'm commanding. Just looking for the cause of my timing woes.

The AFR is accepted because that is where a good target is to make safe power. Now by safe power i mean that there is enough fuel to make a good burn with out any wasted

as the saying goes, the leaner AFR the more power you can make (to a point). You really want to lean the car out as much as possible with out KR, and this happens to be the 11.7-11.8 number. With the force inducted there is the cylinder pressure that makes the fuel ignite sooner. The less fuel in the cylinder at the time the easier it can burn.

As for the timing, there are many factors that play into the timing role, and remember not all of the tables in the PCM are unlocked, the ones that we can see are only a fraction of whats actually in there

1993 sle wrote:The AFR is accepted because that is where a good target is to make safe power. Now by safe power i mean that there is enough fuel to make a good burn with out any wasted

as the saying goes, the leaner AFR the more power you can make (to a point). You really want to lean the car out as much as possible with out KR, and this happens to be the 11.7-11.8 number. With the force inducted there is the cylinder pressure that makes the fuel ignite sooner. The less fuel in the cylinder at the time the easier it can burn.

As for the timing, there are many factors that play into the timing role, and remember not all of the tables in the PCM are unlocked, the ones that we can see are only a fraction of whats actually in there

Thank you for the clarification, I appreciate it.

Here is an interesting article regarding AFR vs. Timing: http://www.innovatemotorsports.com/resources/rich.php

Allmachtige wrote:A lot of people just spew out an accepted fact that 11.7-11.8 AFR is the target for PE mode. I followed this claim without questioning the logic, but I'm at a stage of tuning where I need to ask why. Why?

It's not spewing, decades of industry experience with power adders dictates that 11.7 to 11.8:1 is often the sweet spot, this holds true with the L67 as well.

Allmachtige wrote:There are three tables that deal with the brunt of the commanded AFR and by default the AFR decreases over time AND rpm while you mash on the gas pedal. This can cause the AFR to decrease down to as low as 10.5.

If you had a wideband O2 sensor you would not agree that the AFR changes under WOT. The engine certainly gets hot very quickly, but the AFR does not change.

Allmachtige wrote:There is a table called Max Torque Timing vs RPM vs Cyl Air, which HPT captions as:

"Theoretical spark advance that delivers maximum toque. PCM torque calculation routines reference this table when estimating current engine torque output relative to theoretical maximum. We do not recommend modifying this table."

Most of the people I 'grew up' tuning with would not agree with HPT's statement above. In fact, most people found that if they changed the timing in the High Octane Spark table they would only see half the amount intended, unless they add the same amount to the Max Torque Timing (Mean Best Torque in PT).

Cheers,

Foghorn wrote: Most of the people I 'grew up' tuning with would not agree with HPT's statement above. In fact, most people found that if they changed the timing in the High Octane Spark table they would only see half the amount intended, unless they add the same amount to the Max Torque Timing (Mean Best Torque in PT).

I read that they are relational tables and changes should be made to both at the same time if you alter the High Octane table. Am I thinking correctly?

If so would I see larger increases to timing if I added, oh lets say, 0.4 to a specific cell(s) in both the Mean Best and High Octane tables?

Here is the write up from Clubgp about timing. Not a lot to do with the AFR though. http://www.clubgp.com/newforum/tm.asp?m ... =1&smode=1
Good read on how to advance your timing. I'm agree with above. If you tune with a wideband you will notice that the AFR isn't changing like it says with the scanner. That is just the commanded AFR from the PCM. Not actual. Basically it sounds like you are battling the increased volume=dec timing issue. Get a hold of that and you should be able to raise your timing a little more. Have you tried Eddies scan table data thingy. I don't know if that works with HP tuners or not or just PT. Just a thought.

SuperSLE wrote:Here is the write up from Clubgp about timing. Not a lot to do with the AFR though. http://www.clubgp.com/newforum/tm.asp?m ... =1&smode=1
Good read on how to advance your timing. I'm agree with above. If you tune with a wideband you will notice that the AFR isn't changing like it says with the scanner. That is just the commanded AFR from the PCM. Not actual. Basically it sounds like you are battling the increased volume=dec timing issue. Get a hold of that and you should be able to raise your timing a little more. Have you tried Eddies scan table data thingy. I don't know if that works with HP tuners or not or just PT. Just a though.

Eddie's table thingy seems broke when it comes to timing. The "SHAZAM" function only modifies various cells on the far left of the High Octane table.