It's all in the tune!

There is so much nonsense about blown engines spewed forth on the Lightning boards that I felt compelled to write this down for quick reference.

1. The stock rods are not "weak."

2.  It's detonation, not boost, that kills Lightning engines.

The rods do not break because of the compressive loads from combustion -- they break because of DETONATION.

Even mild detonation causes forces an order of magnitude greater than even huge boost.

"Knock is virtually always the cause of failure in a supercharged engine."  "The pressure spike caused by the [detonation] explosion can reach several thousand PSI, and pressure rise is rapid enough to be considered an impact load. These temperatures and pressures are almost ten times higher than those accompanying controlled combustion. . . No metals in existence today, no forged pistons, no special head gaskets can withstand sustained detonation."

Corky Bell, Supercharged: The Design, Testing, and Installation of Supercharger Systems, pp. 27, 178.

The load on a rod is a mixture of tensile loads at the top of the compression and exhaust strokes and compressive loads during the power stroke. Yet on the power stroke, the compressive load of combustion helps to mitigate the tensile loads, which are unchecked during the exhaust stroke. Thus, the tensile loads on a con rod are greatest when at TDC on the exhaust stroke, where it is purely a function of piston weight, RPM, and stroke -- not boost.  See Bell, pp. 27, 178.

Bottom line: At 14 PSIG, peak rod pressure is only up about 20% over an NA motor.  See Bell, pp. 21-24.  With detonation, peak rod pressure is up about 1,000%.  Controlled combustion just pushes the piston in the butt while it's already on the way down.  Detonation kicks it in the face while it's still on the way up.

So, the oft-repeated 16 PSIG "safe limit" for boost on a stock block has little to do with normal combustion loads and everything to do with the fact that each extra PSIG increases the tendency for detonation.

As well stated by PWR_WHLS on NLOC:

"I’ll be clearer on this – RPM’s KILL a motor. Even a 500rpm increase from stock will do this! Tensile stresses in a rod increase by a squared function of rpm. The centrifugal force due to rpm is the critical load here. Increasing the rpm from 5400 to 5900, a 9% increase in rpm, increases the tensile stresses by over 23%. Do you think you have a 25% tensile margin in your rods?

Boost adds compressive stresses, which actually improve the fatigue capability of the rods. If you’re worried about higher boost levels, back the shift point rpm down a couple of hundred. Remember, HP is a function of torque and RPM. Leave it to the rice boys to get their HP from RPM. Their small mass motors can handle these loads. Let’s use the increased torque from boost to build our HP and keep the rpm down to keep the motors together. The area under the torque curve is what truly matters.

If you want to read more, Corky Bell has a couple of great books out that explain this extremely well without getting too technical. His classic is on turbocharging, and very similar information can be found in his new supercharging book Supercharged! Design, Testing and Installation of Supercharger Systems."

And as added by transmission guru Gregg Evans:

"Something I haven't seen mentioned is what it's doing to the transmission. On most of the shifts, I don't think it's too terribly important to go up to 5600, but the 1-2 won't take it too long, it's about at the limit of what it can handle stock, (hence the Torque Reduction program) and a few hundred RPMs will kill the intermediate plates before too long."