EDIT: The title is supposed to be Timing VS. AFR

I've been messing with AE for a week or so now and finnally decided to move on to something else before I pulled all of my hair out.

I've gotten my WOT AFR to around 12.5 at lower revs and leans out to 12.8 by 5000 rpm and stays there up to my 5800 shift point. I've been running 35 deg of total timing from 3200 on up. I decided to wack about 4 deg out just to see how it would respond. It responded all right, my AFR droped by about .75 across the board and my AFR stabilized nicely where it used to fluctuate about 2 tenths. I also noticed my oil pressure stabilized. It had been fluctuating by about 5 psi at WOT for a while now, I just assumed my tolerances where getting a little loose from 60,000 miles of abuse.

My question is, what does this mean? Did my AFR richen up becuase more unburnt fuel is exiting the combustion chamber because I'm starting the burn later. Or is this a classic case of covering up a too rich condition with too much timing?

 

Bingo.................

By running *too* much timing, you put ALOT of extra heat into the piston dome, and that goes to immediately heating up the oil flying around in the crankcase.

The key, is
*running the min amount of timing consistant with max performance*.
**NOT** running on threshold detonation, unless you need that 1 HP, and have the money to pay for it.

 

Keep in mind that the VE varies with the timing. If you pull lots of timing, the VE plummits and this causes the actual VE to differ from whats in the table.

 

BMonte,

That would be my question. What happened to your MAP readings at WOT when you pulled timing out?

 

Mine would rise up slightly when I pulled too much timing. I would suspect something similar happpens to him.

 

To Bruce's point, I have seen mine either flatten or increase slightly in the higher rpm levels when I leaned out the WOT mixture.

 

Pretty much anything that effects the VE will cause teh MAP to change as the airflow through the TB(s) changes. It can be a good indicator of which way the tune is going.

 

Dim,

I guess the point was that in my case leaning the mixture out tended to maintain the same level or minimize the drop in MAP kPa between torque and HP peaks.

 

What did you lean the mixture out to? Decreasing MAP at WOT implies greater VE, and possibly power.

 

Dim,


I was running 12.4:1 AFR at WOT. The MAP would drop 2-3 kPa between 4000rpm and 5400rpm. When I leaned the mixture to 12.8:1, the MAP remained constant across the same rpm range and overall was a bit higher than when AFR was 12.4. I would conjecture that torque wasn't dropping off as quickly as before and in turn extended the HP peak. Trax had shared he's had similar results.

 

If the numerical value of the MAP is greater at the leaner AFRs, then that would point to the VE, and likely torque, decreasing.

 

Only way to know for sure is to hop on a dyno. But the MAP readings and VE are inversly proportional.

 

haven't paid attention to the map, but you guys got me thinkin. I'll have to take a look, and then try leaning it back out to 12.5-12.8.