M182 Intake Backfire - Rod Bend

[Sean]

To me it feels more like a problem with your combination and what you are trying to do rather than an ECU problem as such.

Firstly, valve spring pressure. You say you have 155 pounds on the seat. I have done some basic maths , to get an idea of where you're at. Allowing for valve margin and stem diameter I got around 3.338 square inches of valve area on the back of the inlet valve. At 24 psi, thats about 83.5 pounds force on the back of the valve, and at 36 pounds (during backfire) its around 125 pounds. These are back of the napkin kinds of numbers, but they give in the ballpark of 72 pounds on the seat at 24 psi and 30 pounds at 36 psi.

Secondly, the low timing. I get that you are trying to go easy on the equipment with by having the low timing , but with those low numbers you will be generating LOTS of egt, potentially a stratospheric number at 24 pounds and 13,8 degrees. As a side note do you have inconel exhaust valves, as I'd be concerned about a stainless valve deforming at the egt's I'd expect to see? Then, when the shift occurs you're pulling timing further, so total is 11,5 degrees after tdc. At this point there will be even more exhaust heat, and fire in the header pipe.

My assumption is that a backfire in itself with a valve open won't bend a rod, but lighting off a cylinder at totally the wrong spot certainly could. One hypothesis is that the residual fire, and heat, in the exhaust is causing a backfire on the overlap. Inlet valve is starting to open while while there's still all the fire and heat, and it flashes up in to the plenum. Pressure spike / flash in the plenum burps an inlet valve open and lights a cylinder off, or lights off a cylinder just as the inlet valve is closing. If a closed valve sees a local pressure of 46.5 psi (due to harmonics / pressure waves) , that's effectively 0 pounds on the seat , Of course there are other dynamic factors to consider, such as valve float, which is certainly possible.

There may also be a scenario where the exhaust valves themselves are that hot that they are lighting the inlet off on the overlap. Other versions could be the exhausts are deformed from the heat and not sealing, or the ex valves are starting to seize in the guides, again leading to not sealing.

I'm not sure that the hydraulic locking a cylinder theory is valid, I'd expect that if it hosed in enough fuel to hydraulic you'd see a misfire in the lambda trace (after the spikes where it has backfired), whereas it seems to just go very rich, which seems correct.

Final observation, Brisk 12 is equivalent to a NGK 8 in heat range I believe? At the power levels you're aiming for, I'd be at a 10 in NGK. My colleague disagrees, he'd be at a 9.

Hopefully this may give you some things to contemplate on. Best regards, Sean.

[ckassen]

Sean - I appreciate the comments and I believe all are valid and are as good of theories as hydro locking.

Some additional data, the car did experience a backfire early in this process with 16.8 deg of timing (separate early log). It doesn’t mean the hot exhaust isn’t causing an issue, but is an additional data point. I also find it funny that 50% ignition cut does not result in a backfire, but a 60% does (every other power cycle is good, but if you stack two ignition cuts back to back you are not).

Heads did not look good and had several weak springs. Builder actually found one broken spring (very close to the base/end, but still). Unfortunately did not confirm which cylinder. They replaced all the springs due to several being weak, valve guides were also worn past spec. Valves themselves were within spec.

In addition, we removed the aggressive blower cam and went ahead and installed the lower duration turbo cam. While not a guarantee, the custom blower cam was never ran on a spintron and may have been exacerbating some issues. We also increased seat pressure to 175lb.

Brisk RR12S would be equivalent to NGK8. While you could go colder, there are many shops in the 1500whp+ E85, 30psi range running the same plug. If I get some signs of detonation, will definitely consider cooling it down further.

Will let you guys know once the car is back on the road if the issue is addressed.