Another Project started (GT30R, fuel managenent etc)
10-04-2006, 01:59 PM
#301
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Markus,
Con-rods under normal circumstances are only stressed in the length direction (compression and extension) with no bending forces.
Comparatively small bending forces in the transverse (engine) direction will occur if you spin a bearing and a somewhat larger bending forces will occur if there is a valve-piston collision (broken belt on an interference engine), since the piston does not hit the intake and exhaust valves simultaneously.
In structural (civil) engineering there is something called “buckle strength”, which relates to columns under compression. If you compress a column lengthwise, it will at some force (depending on the geometry of the column) buckle (bend) in a random direction if it is a homogenous cylindrical rod. Con-rods have H- and I-profiles and are dimensioned very differently between manufacturers. If the compressional forces are too high it will bend in the direction where the con-rod has the weakest bending strength. Judging by the 951’s con-rod dimensions and my experience with a slightly bent (but still functioning) con-rod, it is easier to bend it in the (engine’s) longitudinal direction, than in its transverse direction.
Once a con-rod has been even slightly bent, it has lost a lot of its strength.
I don’t think the compression pressure can cause this. We just need one 951 engine with high HP that doesn’t have to constantly change con-rods to prove that and it seems, that ST’s is a good example.
I can see three possible causes for your bent rods. The less likely is piston-valve collision and a more likely (partial) hydrolock, defined as the quench area being filled up with liquid (unburned gasoline) both twisting the piston and putting a very high force on the con-rod. This mechanism btw, seems to have an equal chance of bending a rod and/or cracking a cylinder.
If you installed bent con-rods, then maybe your compression pressure was enough to further bend and eventually break the weakened rod(s).
Laust
Con-rods under normal circumstances are only stressed in the length direction (compression and extension) with no bending forces.
Comparatively small bending forces in the transverse (engine) direction will occur if you spin a bearing and a somewhat larger bending forces will occur if there is a valve-piston collision (broken belt on an interference engine), since the piston does not hit the intake and exhaust valves simultaneously.
In structural (civil) engineering there is something called “buckle strength”, which relates to columns under compression. If you compress a column lengthwise, it will at some force (depending on the geometry of the column) buckle (bend) in a random direction if it is a homogenous cylindrical rod. Con-rods have H- and I-profiles and are dimensioned very differently between manufacturers. If the compressional forces are too high it will bend in the direction where the con-rod has the weakest bending strength. Judging by the 951’s con-rod dimensions and my experience with a slightly bent (but still functioning) con-rod, it is easier to bend it in the (engine’s) longitudinal direction, than in its transverse direction.
Once a con-rod has been even slightly bent, it has lost a lot of its strength.
I don’t think the compression pressure can cause this. We just need one 951 engine with high HP that doesn’t have to constantly change con-rods to prove that and it seems, that ST’s is a good example.
I can see three possible causes for your bent rods. The less likely is piston-valve collision and a more likely (partial) hydrolock, defined as the quench area being filled up with liquid (unburned gasoline) both twisting the piston and putting a very high force on the con-rod. This mechanism btw, seems to have an equal chance of bending a rod and/or cracking a cylinder.
If you installed bent con-rods, then maybe your compression pressure was enough to further bend and eventually break the weakened rod(s).
Laust
