I've often heard about the failure of the no. 7 piston in Gen3 engines. What is the most common failure mode, and why is this particular cylinder so susceptible?

 

Most/all the failures are on forced induction motors. IMHO the #7 cylinder is furthest from the throttle body either runs hotter and/or leaner than the others. When monitoring A/F ratios there usually is a difference in the reading from individual cylinders and left/right banks. Measuring A/F at the collector before the cats is normal but not optimal if the delta is big enough to cause failures.
We found the Dodge engines had big A/F differences at the front/right ( TB is in the rear on the trucks ) when using the extra 9th injector for turbos and superchargers.

 

Yeah, like they say, "Search is your friend". It's apparently a characteristic of the stock intake manifold which creates a non-uniform pressure distribution, with the no. 7 cylinder getting too much air and leaning out.

SO, if a person has an FI system that replaces the stock manifold with one of a different design (e.g. a Radix) then this should not be an issue, correct?

 

Theoretically. GM and Dodge have spent 10000 times more than magnuson in research on even distribution. (maybe not 10000 but a least 1000, the radix after all is expensive as hell, so I imagine they are tyring to make up some research costs there) Of course with different intentions...FI and NA. I would gamble that even with the radix there is quiet a variation between cylinders afr. Maybe magnuson will chime in with specs of individual cylinder tests or something.

 

If you look inside the intake manifold, it is a straight shot to the back of it. Air travels the path of least resistance which is to the back of the manifold. Both 7 and 8 go out most of the time because they tend to run leaner than the other cylinders.

 

It's a straight shot to the back but the air has to go up through a 3"x4" hole in the top of the manifold to reach the individual runners...it's kind of a screwy looking setup if you cut the manifold apart and look at it. the air to the front cylinders has to turn 180 degrees to get there...not the best setup IMO

 

my #7 went after it tried to compress water.

 

oops....I also discovered the hard way once that water is NOT compressable !!

 

hehe..i tried 10 or 11 time and no..water is not compressable..i just got lucky

 

I had my 8.1 injectors flow tested before I used them. I put the highest flower on #7 at the rail. Figured why the hell not. Got it done cheap and hopefully #7 will live on in my turd.