It was posted earlier that the problems associated with cylinder 7 going lean may be due to manifold design and firing order. While that may be partly true, there is one element that was ignored, at least in the responses to the thread. And that is the MAP sensor.

In wot, the MAF sensor or the MAP sensor determine AF and other things. The MAP sensor senses pressure or vacuum in the manifold and then relays the info to the computer, and the computer makes changes to timing and AF based on the MAP's output. The MAP sensor has a diaghram that is sensitive to pressure changes, as in actual steady increases in boost or decreases like vacuum, but it also can react to turbulence associated with bends that are common in the ductwork where ever there is a bend of 90 degrees or even 45. Well, I believe even though I havent seen the LSx manifold internally that there are curves or bends in the runners between the throttle body and the head. I think a combination of firing order, the bends in the runner tracts and the characteristics of air under boosted conditions may be producing turbulence in the manifold that fools the MAP into altering AF and may be the culprit of the dreaded #7 doom.

One way to know for sure would be to data log the MAP sensor to see if it fluctuates under boost and these fluctuations do not follow the steady increase of boost as measured on a boost gauge. Has anyone done this?

 

I don't think the MAP has anything to do with it. I think it's a comdination of the intake design, firing order, and coolant travel.

 

The MAF is in full control of fueling during PE, AFAIK.

 

This appears to be limited to tunes having the stock MAF and intake, as I haven't heard of anyone lunching #7, having a SD MAF-less setup and a FAST intake.

 

The MAP can override the MAF during open loop and does. Also, the MAF can be "fooled" by airflow that is altered by a bend in the intake tract that it placed within approximately 7 inches of the MAF. This is according to Greg Bannish in his new book.

So you modify your intake tract and place the MAF closer to the Throttle body, but you have a bend over the radiator that is too close to the MAF and the MAF and MAP may be working together to cause the dreaded #7 leanout.

 

I log the map since I run 2bar constantly and don't see anything unusual. You would think that an SD tune would see more problems which I haven't seen.

Arnel

 

On most closed loop cars the MAP is maxed out with 1lb of boost so I doubt that is the problem. What I do see is alot of bad tunes.Alot of people run too much timing or the fuel system can't keep up at high street loads.I bet those 2 are probly the cause of 95% of the failures.
I'm lucky enough to say that we've never lost #7 or #8 piston on any car we tuned. We did however crack liners in 2 blocks probly from too much power or heat in the cylinders

 

Off topic. This book would be a great book if they replaced the "Advanced" text in the title to "Begining". If I had seen the book beforehand, I wouldn't have bought it.

 

I thought maybe it might be the tune being too close to the edge but I'm not a tuner. Thanks.

 

Not to mention on the LS2s the MAF is directly in front of the TB. I'm sure GM did more research on that than anyone of us could possibly think of.