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The Silent Killer
This is a term I am sure you have heard before, but probably not heard it used in the context of engines. Engine failures are not uncommon; all of us have either been the victim of a failure or know someone who has. Most often it is your typical failure, one we have seen or heard of before. Recently, failures that result from a different kind of cause are becoming more common. The Porsche GT3 engine has had quite a few harmonic failures as evidenced by the volume of emails I have received and our own unfortunate experiences. Just to put this in context, all engines suffer from this phenomenon.
Engines create a continual series of vibrations known as harmonics. Each time the engine ignites the fuel mixture and there is combustion, the forces from these combustion’s act upon the crankshaft. The forces generated from these combustion’s force the pistons downward which, in turn, rotate the crankshaft. This is known as tangential torques. The rotating and reciprocating masses create torque pulses within the crankshaft each time it rotates.
The Porsche GT3 engine seems to be having its fair share of vibrational failures. We have seen crank pulley bolts and flywheels come loose, cam actuators come apart and in our own particular case we had an engine that broke the scavenge gears inside the oil pump. I know of other GT3 engines that have had similar failures. These failures are not new to the Porsche Boxer engine.
Harmonics are not new. For years the Porsche Boxer engine has been thought of as a neutrally balanced engine with each pair of opposite pistons always in the same position and the forces are counter balanced or cancelled out. Not so. As engine internals have become lighter and lighter to meet mileage requirements the issue of balance and harmonics has become more real.
So how did we come up with the specifications of the new damper? First we had to understand the cause and then we needed to find the RPM range and the magnitude of the pulses. This was done by fitting up special test equipment at each end of an engine along with high speed impulse sensors and inputting this information into some very powerful software. Some of the areas of analysis that can be computed are, angular vibration velocities measured degrees per sec, angular vibration displacement measured degrees, torsion angle orders, energy generated at certain frequencies related to engine speed, and the energy generated at specific orders at their critical engine speeds. Essentially, by running the engine at different RPM’s under load we can measure the deflection of the crankshaft by the tooth count at each end of the engine. We know the degree difference between each tooth on these wheels so the deflection of the crankshaft can be calculated by the stroke or offset of the crankshaft and the degrees of deflection measured. We are extremely grateful for the assistance of an outside company who provided the test equipment and the software. This information is then supplied to the damper manufacturer who can then calculate the size and weight of the damper along with the size of the elastomer required. These new dampers are able to be rebuilt so anyone using one in a racing application can periodically rebuild it. The factory 997 RS pulley is located on the crankshaft by a very small amount of engagement 0.160”, with no interference at all, a small alignment dowel pin and held in place by the pulley bolt. We always see the end of the crankshaft nose showing signs of pulley movement. Earlier I stated where the pulley bolts have come loose. Crankshaft nose end moves, pulley moves, bolt comes loose. This new damper increases the engagement onto the crankshaft nose to over 0.500” almost all the way back to the front crankshaft seal. We have also included interference to the crankshaft nose diameter. The damper is located by the same dowel pin and the original pulley bolt. The outside diameter of the damper is engraved the same as the factory Pulley is. The OEM, TDC, and Timing marks are retained along with full 360 degree increments.
Added benefits can also be had in simple performance gains. A “straight” crankshaft, an engine with less “internal vibration” will always produce more torque. Would a race version of the GT3 engine gain torque if fitted with a damper? Absolutely, it’s an engine first, a Porsche engine second. A small amount of additional torque can make the difference. Removing all of the vibration from the timing chain, keeping the camshaft timing consistent with the designed position will always result in more torque. By adding a crankshaft damper designed especially for these engines to “match” the low speed frequencies causing the current failures, the counter acting absorbs ion of the damper will counter the crankshaft twist and eliminate the failures. Now changes to the engines displacement with longer stroke crankshafts, longer or shorter rods, etc. can be done. This added mass at the correct end of the crankshaft will allow owners to remove the stock heavy mass dual flywheels and change to the single mass flywheels. The greatest dimensional change in the crankshaft happens at the pulley end of the crankshaft. Even if no other changes are done to the engine the owner can now have confidence that the known parts to fail should not!