The Why’s and Why Not’s of Doing Anything
“A Simplified Explanation of Why Things Come Loose and Stuff Breaks”
The failures these Porsche 6 engines are having are not limited to cars that have had changes or any sort of upgrade. Many failures have happened to completely stock engines. Reasons have been given as to why. Believe them or not but the failures occurred and resulted in damage.
Anything that is subjected to high frequency vibrations and excitements will fail. The closer these forces come to the part’s natural frequency the more damage will occur until final and complete destruction. I have seen bridges (videos) completely collapse due to high frequency oscillations. These are the same forces a crankshaft goes through in its use. Those “noodle” things my kids played with when they were small are a good example of what a crankshaft goes through. Hold one end, (this being the flywheel end) and shake it up and down. The other end, (the crankshaft pulley end) will wiggle wildly. This is what happens to crankshafts. The end opposite to the greatest load does all of the movement. The dual mass flywheels fitted to these engines mask the problem somewhat by lowering the magnitude of the forces but, as seen in some stock engines, the harmonic excitements are still causing failures. Another way of looking at this is to get hold of a 2” diameter bar of steel and hold one end and hit the other end with a hammer as hard as you can. Feel anything?
That is a somewhat basic overview of what is happening but what are the causes? Internal combustion engines (IC) are very inefficient machines. They could be called pulse generators too. Lets take the Porsche 6 Cylinder Boxster engine as the example. It has 6 cylinders that create an internal explosion every 120° based upon the crankshaft design. Add in some hypothetical’s. Let’s say the firing or ignition point is in the mid 20°’s BTDC (25) and the exhaust cam lobe is symmetrical with an exhaust valve opening (EVO) of 123° ATDC, with seat to seat duration of 254° and a lobe centerline LCL of 110°. This means the piston has maximum of 148° of crankshaft rotation to do work. This is a typical example how an engine will create power or force upon the piston for about 140°-150° of crankshaft rotation after the initial firing point. So some of the piston motion happens without creating any torque at all.
The next firing point is 120° after the first. The first piston in the cycle is moving down the cylinder producing work and turning this work into rotational crankshaft motion. Then the next cylinder to fire, fires, but momentarily stops the piston from rising under compression. It’s this stop start pulsing effect that twists the crankshafts back and forth at very high frequencies. Unlike an electric motor that produces torque for every degree of every revolution it goes through.
A good example of an IC engine and its inefficiencies is the Harley Davidson V twin. The sound it makes is based upon the firing sequence. 0°- 315°-405°-0°. The two firing points are only 90° apart and then there is the long wait (315°) until it fires again. Ever seen a bike pull up beside you at a light and see the rider’s hand shake holding onto the throttle and hand brake. This is the shake this engine creates from the crankshaft design. In comparison to the Porsche 6 that fires every 120° (even firing) the Harley is what is called odd firing. These engines shake violently with incredible back and forth rocking motions. We broke the dyno drive shaft on the engine dyno when we were involved in a V Twin engine project.
IC engines due to the piston motion generate different types of forces we have to control. Horizontal and vertical shaking, back and forth or fore and aft rocking (4 Cylinders especially) and the excitements that cause most of the destruction.
The Porsche 6 creates 6 even pulses every 720°, one every 120°. What that does is create 3 twisting forces upon the crankshaft every revolution of the crankshaft. These twisting forces have opposite forces trying to unwind the crankshaft as well. The magnitude of these forces is based on the length of the crankshaft, crankshaft stroke, (pin overlap), its torsion stiffness, the mass of the bob weight attached (counterweight) and the movements of inertia of the rotating parts attached to the crankshaft (rods, pistons, and their parts). These can be divided into two parts again, those that are truly rotating and those that are reciprocating in their motion. The frequency is how often these excitements happen, measured in hertz, “Hz.”
For every engine revolution these Porsche 6 engines create 3 torque pulses actually known as 3 orders or “3rd order” excitement. The frequency is a simple mathematical solution. For example, the GT3 engine has a max RPM limit of 8800 RPM, the excitement frequency calculation would then be 3 x 8800/60 = 440 Hz. This excitement is happening 440 times per second every engine revolution. It’s these very high speed excitements that cause the parts to loosen or to self destruct. The closer these high speed frequencies come to the part’s own natural frequency the part will loosen, fail or could self destruct. Back in the days of the 962C race engines, which these later engines have morphed from, I saw broken crankshafts, broken cam gear drives, and cylinder failures among the many failures that occurred. In my youth when all I could afford was a VW Beetle we would hot-rod these and regularly broke the crankshafts.
There is a misconception that these Porsche 6 engines are naturally balanced. They are better balanced than a Flat plane V8 engine, but do not get confused with dynamic balancing and this discussion. Two completely different conditions. Dynamic balancing is a process done when building an engine to ensure that the rotating and reciprocating parts are counter balanced around the center of gravity or the center of the actual engine rotation (crankshaft) by adding counter weights to offset these outward forces. Pistons, pins, rings and clips are all weighed and equalized as are the connecting rods. These weights are all calculated and by percentage are counter balanced by the bob weights added to the crankshaft. Each counter weight on the crankshaft is then equalized to ensure the crankshaft will rotate around its axis symmetrically and not in a rocking motion. Sometimes the counterweights are off set but that is not relevant to this discussion. Balancing RPM is quite low as once the components are balanced at low speed they will remain balanced at higher speeds. Porsche 6 engines do not carry a lot of counter weight on their cranks because of the equally offset paired piston assemblies, unlike a V8 engine for example. This is where the misconception of naturally balanced comes from. However, dynamic balancing is performed without the engine running; therefore, it is absent of the torque pulses that induce the forces upon the pistons under compression.
Not to get too technical but when these torque pulses are measured as a function of crankshaft position they will show a positive peak and a negative peak. It’s this negative peak that we are most interested in as these negative peaks contain the complex harmonic orders that cause the problems. Just to make things even more understandable, the positive peaks are assembled into groups of peaks and averaged out and become what is known as “Mean Torque” values which is what an engine dyno measures. Engines that have more cylinders with more firing events per revolution typically have lower negative torque pulses as the time between firing events is less. These engines typically have lower harmonics and always appear to run smoother.
To help absorb the twisting of the crankshaft and the springing effect they go through at high frequencies, dampers or absorption units, are fitted to the pulley end of the crankshaft. These units typically have a single resonate frequency which are designed or tuned by the MMOI of the material used to absorb and counter the twisting effects. More modern units have elastomers O rings that can now have a single but wider band (RPM) effect. It is this type of damper we supply to the Porsche customer. These O rings come in different durometers that can be tuned to specific inertia weight to eliminate torsional crankshaft twisting.
The discussion of the actual torsion twisting is very complex and beyond this discussion. The testing that was performed and the fitment of the damper removed the twisting of these crankshafts down to tenths of 1 degrees of movement. Prior we saw upwards of 2 – 3 degrees of twisting. Since fitting these units we have not had one failure reported and in two occasions the damper had an immediate positive effect on the running of the engine. One engine, a Turbo 3.6L, ran over 212 MPH and another just broke the world record for a 997TT car. Two good results showing that fitting these dampers had a positive effect and not a negative one.
Let’s talk about some of the failures seen. Crankshaft pulley’s coming loose, camshaft actuator bolts coming loose and oil pump scavenge gears breaking. We now understand why, now let’s look at these individual part failures.
Starting with the crankshaft pulley bolt. It’s directly connected to the crankshaft and at the end of the crankshaft that sees the highest forces and movement.
The oil pump gears are made from sintered metal or commonly called powered metal. OEM’s use this material as it’s cheap to make parts from. The oil pump is connected to the crankshaft via the 1st motion shaft commonly called in the Porsche world as the intermediate shaft. They are connected by a combination of straight or tapered spur gears. The excitements or harmonics created travel through the gears and down the pump driveshaft and end up in the oil pump at the weakest parts, the gears. Remember that “noodle” reference I made previously? Wear patterns on these 1st motion spur gears show that the drive side of the gear is not the only one coming into contact with its pair.
Now let’s talk about the camshaft actuators. These seem to be the more common failure. They are held together by five 6.0mm bolts. It does need to be said that not all engines have failed. That has to be considered and a lot of the ones that have not failed are probably driven differently and at speeds either above or below where the Porsche 6 has the most problems.
GT3 Cup engines are a good example of this, but they are still affected. The RPM that causes the most destruction is below the RPM that most Cup engines are run at. Although they still generate the same harmonics and excitements. Another way of putting it is the difference between highway driving and race track driving. A race engine is typically running through a wider RPM range and doing it a lot quicker time wise. The crankshaft doesn’t spend anywhere near the time at the critical frequencies as a street engine does. With that said, it still creates the same harmonics but the time is less to cause damage seen in street driven engines.
Now let’s consider a critical factor in how a race engine makes torque. The cylinder head and the valve train assembly, intake system, cylinder head ports, valves, camshafts and their control and behavior are all absolutely critical in making power. The excitements that are created within the crankshaft, go through the 1st motion shaft gears, up the camshaft drive chains and into the valve train. The street engine has the actuators that have the bolt problem. The cup engines have fixed camshaft drive gears but still get excited. Now we have valve train issues and have to try to control the excitements coming up through the chains and into the valve train. We see uneven wear patterns on upper valve spring retainer faces, cam followers, timing chain stretch resulting in chain guide wear, valve seats, valve margins and possibly could contribute to the accelerated wear of the valve guides. These forces induce uncontrollable valve spring surge which we try to control by adding more nose pressure or springs with a higher spring rate. All of which rob power. These engines are typically “timed” by using “close enough” setting tooling, but accuracy is all over the place and in race engines we are always looking at the power losses that we “giveaway.” We expect the cam timing or valve timing, as it should be called, to be exactly as we set it up in assembly when the engine is at 8800 RPM. Instead it’s all over the place.
These may be small losses individually, but when added together per engine revolution add up to quite a lot of power losses. Making horsepower is hard enough without giving it away by not looking at what is going on. Some of the advantages we have gained back are in the “loss column” not in the “look at what we have done” column. It’s one thing trying to make more power by invention, it’s another recovering the power losses we lose in just turning the engine over.
This is a simple overview of what is actually going on inside your engine when it’s running. A lot more than many understand or consider. Consideration of all consequences have to be evaluated and taken into account when building these engines. Even stock engines fail as OEM’s are forced to make parts lighter to meet ever tightening Federal Fuel Economy Standards.
Unfortunately in this business, the quest for these huge power numbers becomes the driving force without any sort of understanding or engineered application. It’s easy to throw parts at an engine and “hot rod” them to make more power. It’s another to understand what you are doing and the dangers that are involved in this type of work.