[QUOTE=aws;6445636]Weissach911
Quote:
|
dilivar studs allowed the heads to blow off the cylinders far enough to blow the CE head gasket out of the groove and pinch it between the cylinder and the head.
|
What I don't understand about this description is why a steel stud will survive when a Dilavar stud will allow this problem to occur.
Both studs would have a similar yield strength (Within 20%) and a similar preload so why does one stud fail so badly and the other survive so well?
If I understand correctly to allow the CE ring to escape from the groove the head would need to lift by several millimetres.
If we assume that 4mm would be a 'good' value then the stud would need to elongate by significantly more than its yield point and therefore the stresses would need to be in excess of 120ksi for the Dilavar stud.
A peak cylinder pressure of 2000psi would generate a force of about 62ksi in each stud. If we add this to the existing preload of around 50ksi then I would agree that this is far too close for comfort. Add a bit more load for expansion and you could well be in trouble.
The steel stud, assumimg you are using a higher strength material will see exactly the same loadings but the additional stress casued by differences in expansion will also result in a situation where the studs are very, very close to yield.
Add a detonation to the equation and I could well understand problems occuring.
It is also interesting that Porsche EB welded the heads onto some of their Turbo race engines but I am not sure how this would reduce stud loading.
If we now consider the 4mm displacement needed to allow the ring out of the groove then the stud would have to extend by this amount.
The yield extension of both steel and Dilavar studs will be about 0.3mm so the studs would have to stretch significantly.
Peak cylinder pressure has a duration of about 25 degrees of crank angle and at say 6000rpm then the peak force is maintained for about 42 milliseconds
If the material had to extend by 4mm during this time the 'impact rate' would be about around 7 metres per second.
I don't know of any steels that will transmit plastic waves at this velocity and the only conclusion is that the studs would have to snap.
I am afraid to say I find that unless explanations of problems of this type are backed up by some engineering justification they are just too big a leap of faith.
I am sad to say that the 'argument' about practical experience versus theory has been going on since the start of Scientific Experiment.
All of the real engineering progress we have made in the last two hundred years has been as a reult of improved understanding and better mathematical modelling so please lets not throw it all away over a few studs.
I think it is always important to try to justify claims that one product is good and another is bad and i have no vested interest in either type of stud.