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Well I decided to try to put together a little something to help out newbies and keep them from damaging their engines by listening to advise from those who have had good luck with Under Drive Pulleys (UDPs).
So first of all I would like to say that I do not totally condemn UDPs because there can be some advantages when done correctly. Misconception #1 is that UDPs are not balanced. The chances are that most UDPs are actually very well balanced. The problem does not lie within the UDP itself. Misconception #2 is that harmonic balancers are not neccessary. The problem is that when you remove the harmonic balancer without balancing the rotating assembly and then install a UDP you could end up with a balance that is far enough out to do some serious damage to your engine -if not completely destroy it.
There are many that will say that they have never had problems with a UDP and have never balanced their engines. But every KL has a very unique set of weighted rods. No two sets are the same from the factory. Some are very close to balanced and some are quite a bit off. I have balanced my own rod sets for years now and also done things like lightening and beam polishing. I have done rod sets down to within less than a half gram of difference between heaviest and lightest. Factory rods are not this close at all! This is what the Harmonic balancer is for.
The harmonic balancer is really a vibrational damper as well as a balancer. It balances out the engine by offsetting the vibrations (crank flex) that is induced by the differing weights of the rod/piston sets. In stock a stock KL I have found as much as 3 or 4 grams of difference between the lightest and the heaviest rods of a matched set. This can be hell on bearings or your oil pump if the damper is replaced with a non-balancing UDP. It does not matter how well your UDP is made or how well it is balanced - it will still not get rid of those oscilations. Your engine may last for quite some time like this but it will defintitely last a shorter time than with a HB. The unbalanced assembly might last years like that or it might just go out in a week with a big bang!
I have put together a couple of GIF animations below so as to hopefully demonstrate the idea. The first is with the normal harmonic balancer. The balancer opposes the harmonic vibrations in two ways. One way is the resistance to applied force (vibration/crank flex) due to gyroscopic inertia. If you have ever spun a bicycle wheel and tried to turn it at 90 deg to the axis of rotation you would find that it can be difficult depending upon the rotational speed. The other component of the HB balancing is the rubber barrier which absorbs (dampens) much of the crankshaft vibrations and nulls them out.
If you are familiar with how Bose headphones with sound canceling work then you know that they sample the noise with a special microphone and then take the recorded noise and compare it to typical noise and then they produce an output which is 180 deg out of phase to the incoming noise sample which effectively cancels out the target noise.
This is somewhat how a harmonic balancer helps to reduce the crank vibrations that are induced by mis-matched rod/piston weights. They do not totally get rid of the vibrations but they reduce them significantly. Here is the simulated HB and the simulated UDP version is below that. Both are assuming that the rotating assembly has not been machine shop balanced and both are exaggerated simulations for demonstration purposes.
Here is the HB version:
And here is the UDP version:
The moral of the story is that if you want to use a UDP then do yourself and your engine a favor and get the rotating assembly balanced so that you get more out of your engine. And if you do not care if it puts more wear on your bearings or pump then don't worry about the balancing. After all - there are some protesters out there who will say that they have never had a problem with UDPs. Wikipedia has an article on UDPs and I would suggest that it is worth reading. But it isn't anything that I didn't already know.
P.S. A well balanced engine makes more power by not losing it to parasitic vibrational induced drag.
So first of all I would like to say that I do not totally condemn UDPs because there can be some advantages when done correctly. Misconception #1 is that UDPs are not balanced. The chances are that most UDPs are actually very well balanced. The problem does not lie within the UDP itself. Misconception #2 is that harmonic balancers are not neccessary. The problem is that when you remove the harmonic balancer without balancing the rotating assembly and then install a UDP you could end up with a balance that is far enough out to do some serious damage to your engine -if not completely destroy it.
There are many that will say that they have never had problems with a UDP and have never balanced their engines. But every KL has a very unique set of weighted rods. No two sets are the same from the factory. Some are very close to balanced and some are quite a bit off. I have balanced my own rod sets for years now and also done things like lightening and beam polishing. I have done rod sets down to within less than a half gram of difference between heaviest and lightest. Factory rods are not this close at all! This is what the Harmonic balancer is for.
The harmonic balancer is really a vibrational damper as well as a balancer. It balances out the engine by offsetting the vibrations (crank flex) that is induced by the differing weights of the rod/piston sets. In stock a stock KL I have found as much as 3 or 4 grams of difference between the lightest and the heaviest rods of a matched set. This can be hell on bearings or your oil pump if the damper is replaced with a non-balancing UDP. It does not matter how well your UDP is made or how well it is balanced - it will still not get rid of those oscilations. Your engine may last for quite some time like this but it will defintitely last a shorter time than with a HB. The unbalanced assembly might last years like that or it might just go out in a week with a big bang!
I have put together a couple of GIF animations below so as to hopefully demonstrate the idea. The first is with the normal harmonic balancer. The balancer opposes the harmonic vibrations in two ways. One way is the resistance to applied force (vibration/crank flex) due to gyroscopic inertia. If you have ever spun a bicycle wheel and tried to turn it at 90 deg to the axis of rotation you would find that it can be difficult depending upon the rotational speed. The other component of the HB balancing is the rubber barrier which absorbs (dampens) much of the crankshaft vibrations and nulls them out.
If you are familiar with how Bose headphones with sound canceling work then you know that they sample the noise with a special microphone and then take the recorded noise and compare it to typical noise and then they produce an output which is 180 deg out of phase to the incoming noise sample which effectively cancels out the target noise.
This is somewhat how a harmonic balancer helps to reduce the crank vibrations that are induced by mis-matched rod/piston weights. They do not totally get rid of the vibrations but they reduce them significantly. Here is the simulated HB and the simulated UDP version is below that. Both are assuming that the rotating assembly has not been machine shop balanced and both are exaggerated simulations for demonstration purposes.
Here is the HB version:
And here is the UDP version:
The moral of the story is that if you want to use a UDP then do yourself and your engine a favor and get the rotating assembly balanced so that you get more out of your engine. And if you do not care if it puts more wear on your bearings or pump then don't worry about the balancing. After all - there are some protesters out there who will say that they have never had a problem with UDPs. Wikipedia has an article on UDPs and I would suggest that it is worth reading. But it isn't anything that I didn't already know.
P.S. A well balanced engine makes more power by not losing it to parasitic vibrational induced drag.
a dead pulley
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