Sorry the pics of the graphs won't paste.
I have looked a lot at these blown engine issues. In the end it was all under our noses. The rods are plenty strong for this application and I have compared them dimensionally to both EVO X and SRT4 rods. They are no lighter and although the bolts are a mm smaller in diameter, the DISI engine was not designed for high RPM and we are not breaking rod bolts. I have looked at binding issues, hydrolock etc ... nothing was concrete.
So then I looked at the rod construction and noticed the small end is tapered. Interesting. Diesel engines taper the small end so that the bearing surface is lower so that the rod takes a high load. The rod I have seems to have some good wear on the bottom of the bearing caused by cylinder pressure. Interesting. Perhaps poor oiling. Perhaps too much pressure.
I then looked at how similar cars produced power and as it turns out no EVO or SRT4 motor makes as much torque as we do at such low RPM. Period.
The way this Mazda is calibrated is more like a truck than a performance car. We don't need more than 300 ft lbs of torque to make well over 400whp but Mazda didn't want that for this motor.
Everyone has been looking at hp gains for this car and have completely ignored the torque and where it is at its maximum and that it doesn't carry into the RPM band.
Further torque is the one that is DIRECTLY proportional to cylinder pressure.
Case and point, look at the tq and hp on this 450whp evo
Now look at Darksun's dyno. Again look at the torque he made.
Take a look at this recent DISI dyno. 375 ft lbs at 3000 RPM.
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Cylinder pressure is what causes the "force" that is then back calculated as torque by a dyno. The gases that cause the cylinder pressure as a result of combustion expand at the same rate no matter how quickly you engine is spinning. This is the reason that you advance spark timing - so that the flame front occurs at the appropriate time to provide as much force to the downward moving piston as possible.
Now, as RPM rises, the piston moves much faster. So then amount of TIME it is subject to the pressure of the expanding gases is less per each power stroke. The reason cars make torque during a certain part of the RPM band is because there is a relationship between how you can control the burn rate (which depends on air/gas inflow, mixing, lighting mixture) with respect to piston speed
So when you are making 300ft lbs of torque at 2500 RPM, the motor sustains the force required to makes that torque for longer each power stroke than if you made the torque at 4500 RPM. Let's not even talk about the fact that a dyno takes an average force applied which favors high RPM to low RPM. It's pretty damn hard to make the torque we do at such a low RPM. The principle is similar to lugging.
So our engines making 300 ft lbs of torque at 2500RPM and only 280whp are under a lot more mechanical stress than an F/I honda motor making 300 ftlbs of torque at 5000RPM and 400whp at 7500RPM.
Once RPM increases too much, inertial loads of the rod and piston break rod bolts. The counterpart is extreme torque at low RPM. Oil pushed out of bearings and extreme connecting rod loads are the result. This load applied again and again eventually results in bent rods and holes through blocks.
Once you open up the flow path to this car it makes MUCH more torque at an even lower RPM instead of breathing better at higher RPMs.
Have you seen the rods on a diesel? Here is an example of a TDI (yes diesel) dyno. Look familiar?
Here is an example of a 1.9 TDI rod and piston that makes 170 ft lbs of torque and 100hp from the factory. The rods are bombproof compared to us because THEY WERE DESIGNED to take high torque at low RPM.
Nuff said. I don't think anyone should be wondering why we break rods anymore.
Solution? There are many factors that affect how an engine produces torque and power. In order for the stock bottom end to stay in one piece, change the stock tune so that you don't hit 21psi at 2800 RPM. Help the motor breathe and move the torque curve further in RPM. Not only will the engine survive longer, but it will also make more whp. And everyone will be a hero, not just whoosh, who certainly didn't make the power he did by chance.
EDIT: This is an addition further explaining the low load rod breaking phenomenon:
I know that people are concerned with the part throttle blow-ups. These are certainly explainable - and I think the short article below from Hot Rod magazine clearly explains how fatigue and material impurity can lead to brittle fractures when components are under high stress. Brittle fractures start at a point of imperfection or impurity in the metal and propagate through. This is why some guys last longer stock and some don't. When a brittle fracture starts to happen, you won't feel anything until that fracture has grown enough to cause the rod to snap. The fracture will most likely let go under vacuum or when engine RPM changes (gear changes) since the rod is being "pulled apart" at that point if you will.
Cylinder pressure, high RPM, etc all play a part in exploiting a material flaw if it exists in the first place and bring it out of the wood work faster.
If you look at MS3 rods, you clearly see that the surface finish is not smooth (machined) like it is in aftermarket forged rods.
Stock Forged Steel
Original-equipment forged steel rods are the next step up the strength and reliability ladder. Detroit-sourced OE-forged rods begin life as bars of carbon steel that are passed through a rolling die. The rolling process compacts the molecular structure and establishes a uniform, longitudinal grain flow. The bars are then heated to a plasticized state, inserted into a female die, and pressed into the near-final shape while a punch locates and knocks out the big end bore. In doing this, the grain flow at the big end is redirected in a circular pattern, like wood fibers surrounding a knot, and excellent compressive/tensile strength results. Finally the rod is put through a trimmer (that leaves the characteristic thick parting line on the beam), the big end is severed and machined to create the cap, bolt surfaces are spot-faced, then final machining and sizing take place.
But there are some drawbacks. When the forging hammer hits the hot bar, heat transfers from the bar to the hammer causing a phenomenon called de-carb (decarburization). Here, trace amounts of the carbon in the steel migrate to the surface resulting in a rough finish full of what metalurgists call inclusions. An inclusion is described as anything that interrupts the surface of the metal, or a lack of cleanliness (impurities) in the material. The effect of a surface inclusion can be likened to a nick in a coat hanger. Bend it enough times and the wire will fail, usually right at the nick. The rough surface caused by de-carb affects the surface to a depth of 0.005 to 0.030 inch and is packed with inclusions that are a breeding ground for cracks. The old hot rodders trick of grinding and polishing the beams is a valid solution to this problem, though far too labor-intensive to ever be considered by Detroit.
When it comes to inclusions caused by impurities, Detroits need to control costs can result in purchases of bulk steel that may (or may not) contain contaminants such as silicon that are not detected during manufacture. Such impurities can interrupt the grain boundaries between the parent molecules and lead to a fracture minutes or years after the rod is first installed in an engine. Its a matter of luck and what kind of abuse the flawed rod is subjected to.
Stock forged steel rods are an economical choice that should be able to handle one horsepower per cubic inch with quality fasteners, and as much as twice the factory-rated output if the beams are polished.
