What breaks the rods in forced indution motors like Stock48's stock lq4? I know that detonation typically breaks the pistons but do the rods just give out from the pressure? Does cylinder pressure cause this or is it just the torque of the engine stretching the metal until it fails?

 

Cylinder pressure. The downward force becomes more then the rod can take and then it bends them in a "S" shape shortening them and then they fail. Stock48 has some pics of his "S" rods, lol.

 

So is cylinder pressure always a by-product of more horsepower? What I'm asking is if I make 700rwhp on 16psi is that the same cylinder pressure as 700rwhp with better heads, cam and 12psi? I know compression affects the cylinder pressure but I don't completely understand how boost affects it.

 

No they are not the same. The hp gained from a different cam and heads is from more efficient air flow of the engine. Youve made it a much more efficient air pump so it doesnt ahve to work so hard to make the same hp. With the higher boost less efficient setup youre having to cram more air in there to net the same hp whereas with the beter heads air is flowing out of the engine more efficiently. Cylinder pressure is directly correlated to tq, this is why your timing at peak tq is reduced, peak tq is where you are most likely to see detonation due to high cylinder pressures. Higher hp and lower tq will save rods(not so easy on pistons though due to added heat).

 

Cylinder pressure at detonation... >mass of Fuel + >mass oxidizer (ignited at stoich)= more pressure (tq)
and pre-ignition makes it exponential on the rods.

 

Ok, what if your torque is higher in the rpm band, can you then run more torque before the rods go? At higher rpm's you dont need as much cylinder pressure to make the same amount of torque, right?