This seems as good a place as any to bring up my observations on this subject, so here goes a very wordy commentary.
Doug, take a look at your crankshaft where the flat surfaces are machined that drive the oil pump. Since the crankshaft should always turn the oil pump (instead of the oil pump turning the crank), there should only be significant marks on two ends of the crankshaft flat surfaces, not on all four. If there are four distinct marks, then I suspect a crankshaft torsional vibration is a possible cause of the oil pump to shatter, as the pump should never turn the crank so you should not see real significant marks on any more than two spots on the crank flats. My wording may be poor, but if you look at the parts and think about it, then it should make sense.
What can cause the torsional vibration (or twisting)? Well, all crankshafts in engines have some amount of this twisting due to the combustion pressure pushing the pistons down and that causes a twisting motion in the crank. Since there are distinct intervals of this going on depending on the number of cylinders, the crankshaft rpm is continuously increasing and decreasing at different rates at different points along the length of the crankshaft itself. This is very minor, so you don't notice it, but the crankshaft feels it and over time it can break or break things that are attached to it. The flywheel and clutch (or torque converter in an automatic) dampens out this twisting at the rear of the crankshaft, but the front is just flopping around, so a torsional vibration damper is installed on the front to control it.
The production crank damper is tuned to reduce this vibration at specific rpms where it is worst and is then deemed appropriate for the production engine. Things that affect the ability of the production crank damper to do its job are:
- changing the crankshaft material (like from a GT cast crank to Cobra forged steel or aftermarket billet),
- changing the stroke (like using a 4.6L crank damper on a 5.4L crank or the opposite),
- running to a higher rpm than production fuel shut-off (since the production crank damper would not have been designed for it and there may be bad vibrations up there that are uncontrolled),
- dramatically increasing the input loads from combustion (each impulse into the crankshaft is much greater, therefore creating more twisting).
In your particular case, you certainly did the last one by using nitrous. But, I'm not aware of people having problems such as yours from nitrous use alone, so this effect may be minor.
However, there are a couple other things that can have a major effect on crank torsional vibrations in general and these are:
- aftermarket superchargers
- aftermarket crankshaft dampers.
You are not using an aftermarket supercharger, but I suspect you are using an aftermarket damper (instead of the "piggyback" style, which can still allow use of the production damper). I really don't know if aftermarket crankshaft dampers are properly tuned, but from what I've seen over the past several years when someone breaks a production oil pump, that does seem to be a common link, so I suspect they may not be tuned for how you are using it.
The other major player is aftermarket superchargers, particularly centrifugal types. I am not aware of any problems related to this with any kit that can be purchased and installed properly, so don't take this as a condemnation of those kits at all. From personal experience, those kits work great. But, when a person gets greedy for more power (face it, we all do, that's why we modify our Mustangs) and doesn't do all of the upgrades properly, problems can result. For example, I broke 3 production oil pumps in my '96 Mustang GT several years ago. The standard
Vortech kit was great. Running a smaller pulley for more boost was even better. Swapping to a T-trim was fantastic. But then the oil pumps started shattering. I was lucky enough to see when they would break. It was when I slammed the throttle shut at high rpm at the end of a ¼ mile pass. All 3 broke under the same condition, and with the configuration the car was in at the time, it would only take about 20 passes for it to happen. After lots of looking at the hardware each time and discussions with people having more knowledge, it appeared to be fairly obvious that it was a crankshaft torsional problem. But why was it happening with the T-trim and not the standard S-trim blower? Well, the thought was that the supercharger bypass was not large enough (I was foolishly still using the little plastic bypass from the standard Vortech kit instead of upgrading to the bigger bypass) and the compressor was going into surge. This was being fed back through the supercharger belt and down to the crank, producing rather violent torsional vibes. Unfortunately, as much as I was having fun with breaking parts and learning, I decided to do the PI headswap thing and run naturally aspirated at that point, so I never went back to completely prove/disprove the theory.
But based on personal experience, some logic and seeing what other people have written about their oil pumps breaking, it almost always seems to be caused by one of the reasons listed (aftermarket crank damper, running higher rpm than stock fuel shut-off with the production crank damper, or using a centrifugal supercharger and a bypass that is too small).
The aftermarket billet oil pump gears are stronger than the production pieces, but that is why they don't typically break, not because they address the basic reason for the breakage to begin with. I suspect the production pumps should be okay for anyone who stays with minor mods or even major mods if the above concerns are addressed.
By the way, I suspect Al's (Boss 330) pump breakage was likely due to running the engine way above the rpm range the Navigator crank damper was designed for. Or if he was running a 4.6L Cobra damper then it was not tuned for the 5.4L crank torsional characteristics. However, I applaud him greatly for what he's done pioneering the 5.4L 4V performance!
These are just my thoughts, but it may give people some insight as to what is going on (or a good nap if they fell asleep reading it). - EDO
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