Destroying the dogma: lightweight crankpulleys

[Polygon]

This has been a very good read. I've never been a fan of light weight pulleys. I don't see the point in such a risk for the marginal benefits.

Now, I have to ask, can we still get the OEM harmonic dampener beyond buying them used?

[jba3]

This is all quite above my head, so hopefully I can ask a simple question. I have an underdrive pulley on my car. I've had it for about 35k, and IIRC from the receipts, it was installed about 40k before I bought the car. It's currently at 135k miles. I also have a lightweight flywheel (I think). Does that mean it's somewhat "balanced"? I am running the stock 9Bs with the normal bolt-ons, at 14psi. I have not had any issues (replaced a starter and alternator) in those 35k miles.

When I pull the motor for the 120k service, should I go with an OEM one instead, or is the underdrive acceptable since I have the lightweight flywheel as well?

[J. Fast]

Just so happens I came acorss a harmoic balancing issue today and figured I post up.

This is a stock harmonic balancer and timing sprocket for an EVO.





This is the same stock harmonic balancer for an Evo on a modified engine.




The two side by side...



You can see where the harmoic frequency resonated thru the crankshaft and caused the sprocket to dig into the harmonic dampener.

A stock harmonic damper will not protect your engine. The natural frequency is on a different Hz rating with different weight on the crankpins. If you have a modified engine make sure you doublecheck your stackup and dynamic balance. This is a perfect example of why a stock dampener should not be used in conjunction with a modified engine...

Jeremy

Jeremy

[pos-vr4]

well, thats to say if the 6g72 had a better option we could. as it is the OE damper only neutralizes a very small range anyway, and after a change of internals its range of operation is even smaller.

What do you know about the "modified" Evo engine?? was it rebalanced after the internals were changed?

[i3igpete]

modified in what way? this would only be a valid comparison if both sprocket/pulleys were out of engines with the same power output, but different reciprocating assemblies. if the power has been upped in the "modified" engine, this is not a valid comparison. the increase in journal torques would WAYYYY overshadow the slight under/overtuning of the pulley.

[J. Fast]

modified in what way? this would only be a valid comparison if both sprocket/pulleys were out of engines with the same power output, but different reciprocating assemblies. if the power has been upped in the "modified" engine, this is not a valid comparison. the increase in journal torques would WAYYYY overshadow the slight under/overtuning of the pulley.

Both vehicles are stock ecu, stock everything including stock compression 8.8:1. The only difference between the two is Wiseco pistons, Eagle rods, TOGA main and rod bearings, and Arias wrist pins. Not that it pertains to the discussion but the dampener that's dog shiet is also owned by a 20 yr old... so who -knews'.

Jeremy

[pos-vr4]

Both vehicles are stock ecu, stock everything including stock compression 8.8:1. The only difference between the two is Wiseco pistons, Eagle rods, TOGA main and rod bearings, and Arias wrist pins. Not that it pertains to the discussion but the dampener that's dog shiet is also owned by a 20 yr old... so who -knews'.

Jeremy

Did both engines still have the balance shafts? if the modded car had them deleted that is the kind of harmonic damage one would expect to see.
The change of internal parts would require a rebalance of the lower end.

That IS a big difference in rotating mass for sure. On my 3.1 that i just had balanced had a total bob weight of 780 grams lighter than a OE engine, that is almost 2 lbs difference over a stock engine at 7 k rpm that is tons of out of balance mass.

[i3igpete]

balance shafts do act on the crank harmonic but not in the way you'd expect. they affect the harmonic balance due to their rotational inertia but their main purpose, to cancel engine vibration with centrifugal forces, does not actually affect harmonics of the crank. removing them would make a HUGE difference since they act as high-inertia crank hubs.

[Trab]

The damaged one probably wasn't tightened properly.

Hmmm, my lightweight one looked just like the one pictured above with the crank cog groves in it.

[SkortchGT]

i've always thought it was not a good idea to replace your stock pulley with an aftermarket undampened pulley. horsepower gain is minimal, just not worth it, especially when a crank is absolutely meant to have a dampener on it.