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So basically I see a lot of contradiction of opinion. Whats best? Go light weight or not?
Light weight only or light weight + underdrive or just underdrive?
Under drive will reduce the effectiveness of your accessories (alternator spins too slow etc). If it's literally just light weight, then yeah go for it.
never remove a harmonic balancer and replace it with a solid pulley, all this does is introduce harmonics in the engine that can accelerate crank shaft failure.
light weight accessory pulleys yeah sure they are less critical, and as said above changing to an under drive ratio will free up horse power as the cost of the accessories effectiveness, ie and alternator may not provide as much current at a given engine rpm since it is now turning slower.
Yeah the question was relating to harmonics. Most oem main pulleys seem have some sort of damping rubber type insert. This is where the yes or no comes in....some people say yeah no problem others say never.
My preference has been to let the damper do it's job and leave it alone. Most solid aluminum "lightweight" pulleys do very little damping, but from what I've seen, that's sometimes not an issue. Seems some motors benefit from the damping more than others, because I've seen guys run the solid lightweight pulleys for years of track days and daily driving without major issue.
I don't run the light weight solid pulleys because to me, the performance difference is barely tangible and not worth the risk. You'd feel even a slightly lightened flywheel much more, and I usually end up with one eventually some way or another.
If you really want to have slightly less inertia there, but want to keep your damping, check out Fluidampr. They have an interesting product that it supposed to do both. Instead of an elastomer, it uses a floating inertia ring laser sealed in a housing with some silicone shear fluid. Supposedly you get really good damping, but the floating ring makes the effective weight about 2/3 in transients. I just swapped one of those out on a boxer engine that was previously running a solid lightweight pulley and I could swear the thing was noticeably quieter at idle afterwards (not exactly a blind study). Only real downside I know of is that they are expensive.
I ran a solid alloy pulley on our 4G63 drag engines but would NOT recommend for a street or circuit application. As stated above, removing the harmonic dampener can cause parts failure in a short time period. I recall tuning a turbo Honda B18C that shattered the oil pump gears during a dyno pull as a result of harmonics introduced by a solid alloy pulley. The issue is that the harmonics tend to be focussed at specific rpm ranges, so if you're spending a lot of time in these ranges then damage can be done quite quickly. With the drag engines we were always accelerating through the rev range rather than running in steady state so it's less of an issue.
If you do want an under drive pulley then use a quality aftermarket product from the likes of ATI etc.