Is LWFW safe after LN Bearing retro fit?
03-14-2010, 10:52 PM
#17
Rennlist Member
Yes the M96 cranks are not forged but the big issue that was explained to me was that the weakness is exacerbated by applying reverse pressure on the crank (which it cannot take at all).
Think of driver who habitually chirps the tires on a downshift and you see the light. He is sending a jolt right into the crank which eventually breaks.
Think of driver who habitually chirps the tires on a downshift and you see the light. He is sending a jolt right into the crank which eventually breaks.
03-14-2010, 11:36 PM
#18
Nordschleife Master
Yes the M96 cranks are not forged but the big issue that was explained to me was that the weakness is exacerbated by applying reverse pressure on the crank (which it cannot take at all).
Think of driver who habitually chirps the tires on a downshift and you see the light. He is sending a jolt right into the crank which eventually breaks.
Think of driver who habitually chirps the tires on a downshift and you see the light. He is sending a jolt right into the crank which eventually breaks.
03-15-2010, 11:23 AM
#19
Rennlist Member
I would have to wonder about that concept since chirping either way would create that issue. I'd be impressed that the Porsche Engineering Department was able to design a crank that took a rotational force in only one direction. If the crank is shearing, it doesn't matter which way (IMHO, which doesn't mean much).
03-15-2010, 06:46 PM
#20
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A light just came on.
If you play with the dual-mass flywheel (even an old tired one), the main thing you notice is that there is some rotation allowed between the crankshaft mating surface and the clutch/pressure plate surface. When you hit the end of the travel (in either direction, meaning accelerating or decelerating), you hit one of the elastomers that reduces the shock of clutch engagement.
I always thought it was to make life easier on the transmission, as it inherently reduces the peak torque the transmission will see in an upshift, but now I see it can also reduce the BACK TORQUE transmitted to the crankshaft by the drivetrain when doing a harse downshift.
It can easily be such "torque spikes" which cause metal parts to fail in a spectacular manner...
If you play with the dual-mass flywheel (even an old tired one), the main thing you notice is that there is some rotation allowed between the crankshaft mating surface and the clutch/pressure plate surface. When you hit the end of the travel (in either direction, meaning accelerating or decelerating), you hit one of the elastomers that reduces the shock of clutch engagement.
I always thought it was to make life easier on the transmission, as it inherently reduces the peak torque the transmission will see in an upshift, but now I see it can also reduce the BACK TORQUE transmitted to the crankshaft by the drivetrain when doing a harse downshift.
It can easily be such "torque spikes" which cause metal parts to fail in a spectacular manner...
03-16-2010, 01:09 AM
#24
Burning Brakes
03-16-2010, 12:49 PM
#25
I don't know guys a good 20 pounds of the dual mass flywheel is ridigd and directly bolted to the crank,
the top portion which provides the friction surface for the disc is allowed to articulate to provide dampening on the trans side. I think the proposition that a light flywheel is going to be be your unraveling is a wash, the Boxster I took apart to build a spec car had a broken crank with a dual mass and I haven't heard of any of the Boxster spec guys complaining of probems with the LWFL. Dramatic picture though in the eariler thread but I have seen pictures like that in V/8 and 4 clyinder cars that have been driven out of the rpm range.
great post regards
the top portion which provides the friction surface for the disc is allowed to articulate to provide dampening on the trans side. I think the proposition that a light flywheel is going to be be your unraveling is a wash, the Boxster I took apart to build a spec car had a broken crank with a dual mass and I haven't heard of any of the Boxster spec guys complaining of probems with the LWFL. Dramatic picture though in the eariler thread but I have seen pictures like that in V/8 and 4 clyinder cars that have been driven out of the rpm range.
great post regards
03-16-2010, 07:16 PM
#26
Burning Brakes
The Boxster Spec engines are 2.5 liter. These have the shortest stroke (72mm) of all M96 engines and therefore are by design the strongest crankshafts with the most rod journal overlap in comparison to their centerline.
This makes these cranks the hardest to break, where the larger engines, like the 3.6 liter with more than 82mm of stroke have much different overlap and are therefore weaker by design. The main journal sizes were increased when the crank went to 82.8mm from 78mm but not at the same ratio to that of the stroke increase.
So not only do the bigger engines make more power, but they also have 20+mm more stroke with only a .120 difference in main bearing diameter.
This makes these cranks the hardest to break, where the larger engines, like the 3.6 liter with more than 82mm of stroke have much different overlap and are therefore weaker by design. The main journal sizes were increased when the crank went to 82.8mm from 78mm but not at the same ratio to that of the stroke increase.
So not only do the bigger engines make more power, but they also have 20+mm more stroke with only a .120 difference in main bearing diameter.
03-16-2010, 07:23 PM
#27
Burning Brakes
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A LWFW is a good mod to employ in an engine that is used extensively as a track car. However the M96 has no crank damper (until the 3.8L)...so the FW provides this function. If you research crank dampers for other engines you may see more on the subject such as; Loss of power, harmonic vibrations, bearing life, ignition issues... faster revving, but some loss of power is actually common.
If a light weight flywheel, (even a perfectly balanced one from a top notch manufacturer) is bolted to an engine without dynamically balancing the entire assembly ... you may be playing with fire. Or you might be ok and the engine may not experience any failures. On the other hand.... if the minute changes in balance don't 'line up' in your favor.. you may get what Jake has posted above. The Porsche crank is made from powder... powdered metal. It's not a forged GT3 crank. Caveat Emptor.
I have experience in this area on non-porsche engines. Not trashing the LWFW idea, just saying it needs to be employed correctly. Via full dynamic balancing...w/ clutch also.
It's interesting to see Pac996 describe the 996 shifting like lightning... I think my 2 gearboxes have both shifted like a truck. I'm on shifter console type #3 and gearbox #2 in my attempts to get better shifting. Hmmm. Spring is in the air... getting track fever already
If a light weight flywheel, (even a perfectly balanced one from a top notch manufacturer) is bolted to an engine without dynamically balancing the entire assembly ... you may be playing with fire. Or you might be ok and the engine may not experience any failures. On the other hand.... if the minute changes in balance don't 'line up' in your favor.. you may get what Jake has posted above. The Porsche crank is made from powder... powdered metal. It's not a forged GT3 crank. Caveat Emptor.
I have experience in this area on non-porsche engines. Not trashing the LWFW idea, just saying it needs to be employed correctly. Via full dynamic balancing...w/ clutch also.
It's interesting to see Pac996 describe the 996 shifting like lightning... I think my 2 gearboxes have both shifted like a truck. I'm on shifter console type #3 and gearbox #2 in my attempts to get better shifting. Hmmm. Spring is in the air... getting track fever already
03-16-2010, 07:51 PM
#29
Burning Brakes
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